Download - Programming paradigms

A.V.C.COLLEGE OF ENGINEERING

MANNAMPANDAL, MAYILADUTHURAI-609 305

COURSE MATERIAL

FOR THE SUBJECT OF

PROGRAMMING PARADIGMS

S UB NAME : CS2305 PROGRAMMINGPARADIGMS

SE M : VI

D E P AR T M E NT : COMPUTER SCIENCE AND ENGINEERING

ACAD E MIC Y E A R : 2013-2013

NAME O F TH E F AC U LT Y : PARVATHI.M

D ES I G NA T I O N : Asst.Professor

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A.V.C College of Engineering

Department of Computer Science & Engineering

2013 Odd Semester

SYLLABUS

CS2305 PROGRAMMING PARADIGMS L T P C

3 0 0 3

AIM:

To understand the concepts of object-oriented, event driven, and concurrent

programming paradigms and develop skills in using these paradigms using Java.

UNIT I OBJECT-ORIENTED PROGRAMMING – FUNDAMENTALS 9

Review of OOP - Objects and classes in Java – defining classes – methods - access

specifiers – static members – constructors – finalize method – Arrays – Strings -

Packages – JavaDoc comments

UNIT II OBJECT-ORIENTED PROGRAMMING – INHERITANCE 10

Inheritance – class hierarchy – polymorphism – dynamic binding – final keyword –

abstract classes – the Object class – Reflection – interfaces – object cloning – inner

classes – proxies

UNIT III EVENT-DRIVEN PROGRAMMING 10

Graphics programming – Frame – Components – working with 2D shapes – Using color,

fonts, and images - Basics of event handling – event handlers – adapter classes –

actions – mouse events – AWT event hierarchy – introduction to Swing – Model-View-

Controller design pattern – buttons – layout management – Swing Components

UNIT IV GENERIC PROGRAMMING 8

Motivation for generic programming – generic classes – generic methods – generic code

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and virtual machine – inheritance and generics – reflection and generics – exceptions –

exception hierarchy – throwing and catching exceptions – Stack Trace Elements -

assertions – logging

UNIT V CONCURRENT PROGRAMMING 8

Multi-threaded programming – interrupting threads – thread states – thread properties –

thread synchronization – thread-safe Collections – Executors – synchronizers – threads

and event-driven programming

TOTAL=45 PERIODS

TEXT BOOK:

1. Cay S. Horstmann and Gary Cornell, “Core Java: Volume I – Fundamentals”, Eighth

Edition, Sun Microsystems Press, 2008.

REFERENCES:

1. K. Arnold and J. Gosling, “The JAVA programming language”, Third edition, Pearson

Education, 2000.

2. Timothy Budd, “Understanding Object-oriented programming with Java”, Updated

Edition, Pearson Education, 2000.

3. C. Thomas Wu, “An introduction to Object-oriented programming with Java”, Fourth

Edition, Tata McGraw-Hill Publishing company Ltd., 2006.

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CS2305 PROGRAMMING PARADIGMSnit No Title

I Object-oriented Programming – Fundamentals

II Object-oriented Programming – Inheritance

III Event-Driven Programming

IV Generic Programming

V Concurrent Programming

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CS2305 PROGRAMMING PARADIGMS

UNIT I

OBJECT-ORIENTED PROGRAMMING – FUNDAMENTALS

1) Review of OOP Concepts a) Encapsulation b) Inheritance c) Polymorphism

2) Objects and Classes in Java 3) Language Basics

a) Comments b) Data Types c) Variables d) Statements e) Functions

4) Defining Classes 5) Methods 6) Access Specifiers 7) Static Members 8) Constructors

a) Parameterized Constructors b) this Keyword c) Garbage Collection

9) Finalize Method 10) Arrays

a) One Dimensional Arrays b) Multi Dimensional Arrays

11) Strings 12) Packages 13) JavaDOC Comments 14) References

1. Review OOP conceptsThe object oriented paradigm is built on the foundation laid by the structured

programming concepts. The fundamental change in OOP is that a program is designed around the data

being operated upon rather upon the operations themselves. Data and its functions are encapsulated into a single entity. OOP facilitates creating reusable code that can eventually save a lot of work. Polymorphism permits to create multiple definitions for functions. Inheritance permits to derive new classes from old ones.

Benefits of object oriented programming Data security is enforced. Inheritance saves time. User defined data types can be easily constructed.

© A.V.C College of Engineering


Inheritance emphasizes inventions of new data types. Large complexity in the software development can be easily managed.

Java HistoryJava is a general-purpose object oriented programming language developed by Sun

Microsystems of USA in 1991. Originally called ―oak‖ by James Gosling, one of the inventors ifthe language. This goal had a strong impact on the development team to make the languagesimple, portable, highly reliable and powerful language. Java also adds some new features.While C++ is a superset of C. Java is neither a superset nor a subset of C or C++.

C++

C Java

Basic concepts of Object oriented programming All Java programs are object oriented. Computer programs consist of two elements: code

and data. In process oriented model code acting on data. Procedural languages such as C employ

this model. To manage increasing complexity object-oriented Programming approach was conceived.

Object-oriented programming organizes a program around its data (that is, objects) and aset of well-defined interfaces to that data. An object-oriented program can becharacterized as data controlling access to code.

AbstractionAn essential element of object-oriented programming is abstraction. Abstraction refers to

the act of representing essential features without including the background details orexplanations.

For example, people do not think of a car as a set of tens of thousands of individual parts.They think of it as a well-defined object with its own unique behavior. This abstraction allowspeople to use a car to drive to the grocery store without being overwhelmed by the complexity ofthe parts that form the car. They can ignore the details of how the engine, transmission, andbraking systems work. Instead they are free to utilize the object as a whole.

The Three OOP PrinciplesAll object-oriented programming languages provide mechanisms to implement the

object-oriented model. They are Encapsulation Inheritance Polymorphism



a) EncapsulationEncapsulation is the mechanism that binds together code and the data it manipulates, and

keeps both safe from outside interference and misuse.For example, shifting gears does not turn on the headlights in car, because it encapsulates

the information about all process.In Java the basis of encapsulation is the class. A class defines the structure and behavior

(data and code) that will be shared by a set of objects. For this reason, objects are sometimesreferred to as instances of a class. Thus, a class is a logical construct and an object has physicalreality.

b) InheritanceInheritance is the process by which one object acquires the properties of another object.

This is important because it supports the concept of hierarchical classification. For exampleBird

Attributes:

Feathers

Lay eggs

Flying

bird

Attributes:

-----------

----------

Non flying

bird

Attributes:

-----------

-----------

The bird 'robin ' is a part of the class 'flying bird' which is again a part of the class 'bird'.The concept of inheritance provides the idea of reusability.

c) PolymorphismPolymorphism means the ability to take more than one form. Subclasses of a class can

define their own unique behaviors and yet share some of the same functionality of the parentclass.

Shape

Draw()

Circle Object

Draw()

Box Object

Draw()

Triangle Object

Draw()

2. Class and ObjectClass defines a new data type. Once defined, this new type can be used to create objects

of that type. Thus, a class is a template for an object, and an object is an instance of a class.

ClassA class is a blueprint or prototype from which objects are created. The entire set of data

and code of an object can be made of a user defined data type with the help of a class. We cancreate any number of objects for the same class.



For example the construction of building requires a plan. Without plan we can't constructbuilding. Here plan acts as class. Building acts as object. Using the same plan we can build anynumber of buildings. In the same way we can create any number of objects for the same class.Class tells the object’s state and behavior. Basically class tells what the object should have.

ObjectObject is an instance of class. Object is real world entity which has state, identity and

behavior. It may represent a person, a place, a bank account, a table of data or any item that theprogram has to handle.

JAVA Program structureA file containing Java source code is considered a compilation unit. Such a compilation

unit contains a set of classes and, optionally, a package definition to group related classestogether. Classes contain data and method members that specify the state and behavior of theobjects in your program. Java programs come in two flavors:

1. Standalone applications that have no initial context such as a pre-existing main window2. Applets for WWW programming

Execution begins when you launch the java virtual machine and tell it to find a particular class.After finding the class, java executes the enclosed main() method with signature:

public static void main(String[] args){

………...}

Program ExecutionCreate a source code file using a text editor. The source code is then compiled using the

java compiler javac. Each java program is converted into one or more class files. The content ofthe class file is a set of instructions called bytecode to be executed by Java Virtual Machine(JVM).Java introduces bytecode to create platform independent program. JVM is an interpreterfor bytecode which accepts java bytecode and produces result.

Java Source Code

Java Compiler (Javac)

Java Byte Code (.class)

Java Virtual Machine

Java program Output



3. Language Basics

a) CommentsJava comments make your code easy to understand, modify and use. Java supports three

different types of comment styles.1.Everything between initial slash –asterisk and ending asterisk-slash is ignored by the java

compiler.(/*……*/)2.Double slash (//)mark is ignored by java compiler.3.Everything between initial slash – asterisk - asterisk and ending asterisk-slash is ignored

by the java compiler and another program called JAVADOC.EXE that ships with theJDK uses these comments to construct HTML documentation files that describe yourpackages, classes and methods as well as all the variables used. (/**……*/)

b) Data TypesJava defines eight simple (or elemental) types of data: byte, short, int, long, char, float,

double, and boolean. These can be put in four groups: Integers - This group includes byte, short, int, and long, which are for whole valued

signed numbers. Floating -point numbers- This group includes float and double, which represent numbers

with fractional precision. Characters - This group includes char, which represents symbols in a character set, like

letters and numbers. Boolean - This group includes boolean, which is a special type for representing true/false

values.

c) VariablesVariables are locations in memory in which values can be stored. They have a name, a

type, and a value. Before you can use a variable, you have to declare it. After it is declared, youcan then assign values to it.

Java actually has three kinds of variables: instance variables, class variables, and local

variables. Instance variables are used to define attributes or the state for a particular object. Class variables are similar to instance variables, except their values apply to all

that class’s instances (and to the class itself) rather than having different valuesfor each object.

Local variables are declared and used inside method definitions, for example, forindex counters in loops, as temporary variables, or to hold values that you needonly inside the method definition itself.

Although all three kinds of variables are declared in much the same ways, class andinstance variables are accessed and assigned in slightly different ways from local variables.

Scope© A.V.C College of Engineering


Java has no global variables. All variables must be declared within either a class scope or a function scope.

Declaring VariablesFor variable declarations, Java syntax is similar to C and C++ syntax. For native types,

the syntax may be summarized as follows: One word type name. A list of variable names, separated by commas. An optional initializer following each variable name. Terminated by semicolon.

The following are examples of variable declarations: This line of Java code declares a 32-bit integer variable named i:

int i; This line declares two 32-bit integer variables named i and j:

int i , j; This line declares i and j as above and initializes them:

int i=3, j=4;

d) StatementsJava, like C, is an expression-based language. Expressions must always appear in the

context of statements. A statement consists of an expression followed by a terminating semicolon(;).

Statements can be made up of zero or more expressions, provided that their combinationmakes syntactic sense. Expressions (except for the return value of void methods) have a valueand a type. A terminating semicolon (e.g., 27;) turns it into a legal statement even though it doesnot do anything useful. A simple statement consists of an expression and a semicolon. Thesemicolon is a required terminator.

4. Defining ClassThe class is at the core of Java. It is the logical construct upon which the entire Java

language is built because it defines the shape and nature of an object. As such, the class formsthe basis for object-oriented programming in Java. The entire set of data and code of an objectcan be made of a user defined data type with the help of a class. Thus, a class is a template for anobject, and an object is an instance of a class.

The General Form of a ClassThe class contains data and the code that operates on that data. A class is declared by use

of the class keyword. The data, or variables, defined within a class are called instance variables.The code is contained within methods. Collectively, the methods and variables defined within aclass are called members of the class. The general form of a class definition is shown here:

class classname{

type instance-variable1;type instance-variable2;



// ...type instance-variableN;type methodname1(parameter-list){

// body of method}type methodname2(parameter-list) {

// body of method}// ...type methodnameN(parameter-list) {

// body of method}

}Here is a class called Box that defines three instance variables: width, height, and depth.Currently, Box does not contain any methods (but some will be added soon).

class Box{

double width;double height;double depth;

}

Instantiating a class

Class declaration only creates a template; it does not create an actual object. To create aBox object, you will use a statement like the following:

Box mybox = new Box(); // create a Box object called mybox The new operator allocates space for the new object, calls a class constructor and returns

a reference to the object. It should be noted that the class and its constructor have the same name.

After this statement executes, mybox will be an instance of Box. Each time you create aninstance of a class, you are creating an object that contains its own copy of each instance variabledefined by the class. Thus, every Box object will contain its own copies of the instance variableswidth, height, and depth. To access these variables, you will use the dot (.) operator. The dotoperator links the name of the object with the name of an instance variable. For example, toassign the width variable of mybox the value 100, you would use the following statement:

mybox.width = 100;This statement tells the compiler to assign the copy of width that is contained within the

mybox object the value of 100.

5. MethodsClasses usually consist of two things: instance variables and methods.

This is the general form of a method:Type name (parameter-list)



{// body of method

}

Type specifies the type of data returned by the method. This can be any valid type,including class types that you create. If the method does not return a value, its return type mustbe void.

The name of the method is specified by name. This can be any legal identifier other thanthose already used by other items within the current scope.

The parameter-list is a sequence of type and identifier pairs separated by commas.Parameters are essentially variables that receive the value of the arguments passed to the methodwhen it is called. If the method has no parameters, then the parameter list will be empty.

Methods that have a return type other than void return a value to the calling routine usingthe following form of the return statement:

return value;Here, value is the value returned.

Program includes a method inside the box class.class Box{

double width;double height;double depth;// display volume of a boxvoid volume() {

System.out.print("Volume is ");System.out.println(width * height * depth);

}}

class BoxDemo3{

public static void main(String args[]) {Box mybox1 = new Box();Box mybox2 = new Box();// assign values to mybox1's instance variablesmybox1.width = 10;mybox1.height = 20;mybox1.depth = 15;/* assign different values to mybox2's instance variables */mybox2.width = 3;mybox2.height = 6;mybox2.depth = 9;// display volume of first boxmybox1.volume();// display volume of second box



mybox2.volume();}

}This program generates the following output.THE JAVA LANGUAGEVolume is 3000.0Volume is 162.0

The mybox1.volume() invokes the volume( ) method on mybox1. That is, it callsvolume( ) relative to the mybox1 object, using the object’s name followed by the dot operator.Thus, the call to mybox1.volume( ) displays the volume of the box defined by mybox1, and thecall to mybox2.volume( ) displays the volume of the box defined by mybox2. Each timevolume( ) is invoked, it displays the volume for the specified box.

6. Access specifiersEncapsulation provides another important attribute: access control. Through

encapsulation, you can control what parts of a program can access the members of a class. Bycontrolling access, you can prevent misuse.

How a member can be accessed is determined by the access specifier that modifies itsdeclaration. Java supplies a rich set of access specifiers. Some aspects of access control arerelated mostly to inheritance or packages. (A package is, essentially, a grouping of classes.)

Java’s access specifiers are Public Private Protected default access level.

PublicWhen a member of a class is modified by the public specifier, then that member can be

accessed by any other code. So main( ) has always been preceded by the public specifier.Classes can be declared public. A public class is accessible by any other Java class. A

class that is not declared public has package access, which means that only classes within thesame package may access it. Class members labeled public are accessible to all other classes.

PrivateWhen a member of a class is specified as private, then that member can only be accessed

by other members of its class.Class members can be declared public or private to enforce proper encapsulation. Certain

data elements or methods that are necessary for the class’s internal behavior should be protectedfrom access by outside classes. These should be declared private. A class member that has beendeclared as private will remain private to its class. It is not accessible by any code outside itsclass, including subclasses.

ProtectedAny member that is declared protected is accessible only by a class that is derived from

the current class or is in the same package as the containing class.



default access levelWhen no access specifier is used, then by default the member of a class is public within

its own package, but cannot be accessed outside of its package.

7. Static MembersStatic Variables

If you define a field as static, then there is only one such field per class. In contrast, eachobject has its own copy of all instance fields. For example, let's suppose we want to assign aunique identification number to each employee. We add an instance field id and a static fieldnextId to the Employee class:

class Employee{...private int id;private static int nextId = 1;}

Now, every employee object has its own id field, but there is only one nextId field that is sharedamong all instances of the class. Let's put it another way. If there are one thousand objects of theEmployee class, then there are one thousand instance fields id, one for each object. But there is asingle static field nextId. Even if there are no employee objects, the static field nextId is present.It belongs to the class, not to any individual object.

Constants

Static variables are quite rare. However, static constants are more common. For example,the Math class defines a static constant:

public class Math{...public static final double PI = 3.14159265358979323846;



...}

You can access this constant in your programs as Math.PI. If the keyword static had beenomitted, then PI would have been an instance field of the Math class. That is, you would need anobject of the Math class to access PI, and every object would have its own copy of PI. Anotherstatic constant that you have used many times is System.out. It is declared in the System class as:

public class System{...public static final PrintStream out = . . .;...}

As we mentioned several times, it is never a good idea to have public fields because everyonecan modify them. However, public constants (that is, final fields) are ok. Since out has beendeclared as final, you cannot reassign another print stream to it:

out = new PrintStream(. . .); // ERROR--out is final

Static MethodsStatic methods are methods that do not operate on objects. For example, the pow method

of the Math class is a static method. The expression:Math.pow(x, y)

Computes the power xy. It does not use any Math object to carry out its task. In other words, ithas no implicit parameter. In other words, you can think of static methods as methods that don'thave a this parameter. Because static methods don't operate on objects, you cannot accessinstance fields from a static method. But static methods can access the static fields in their class.Here is an example of such a static method:

public static int getNextId(){return nextId; // returns static field}

To call this method, you supply the name of the class:int n = Employee.getNextId();

Could you have omitted the keyword static for this method? Yes, but then you would need tohave an object reference of type Employee to invoke the method. You use static methods in twosituations:

1. When a method doesn't need to access the object state because all neededparameters are supplied as explicit parameters (example: Math.pow)

2. When a method only needs to access static fields of the class (example:Employee.getNextId)

8. ConstructorsA constructor is a special method whose purpose is to construct and initialize objects.

Constructors always have the same name as the class name.The constructor is automatically called immediately after the object is created, before the

new operator completes.Box mybox1 = new Box();



new Box( ) is calling the Box( ) constructor. When you do not explicitly define aconstructor for a class, then Java creates a default constructor for the class. The defaultconstructor automatically initializes all instance variables to zero.

There is an important difference between constructors and other methods: A constructorcan only be called in conjunction with the new operator. You can't apply a constructor to anexisting object to reset the instance fields.

Features of constructor A constructor has the same name as the class. A class can have more than one constructor. A constructor may take zero, one, or more parameters. A constructor has no return value. A constructor is always called with the new operator.

a) Parameterized ConstructorsAdd parameters to the constructor.class Box{

double width;double height;double depth;// This is the constructor for Box.Box(double w, double h, double d) {

width = w;height = h;depth = d;

}// compute and return volumedouble volume() {

return width * height * depth;}

}class BoxDemo7{

public static void main(String args[]) {// declare, allocate, and initialize Box objectsBox mybox1 = new Box(10, 20, 15);double vol;// get volume of first boxvol = mybox1.volume();System.out.println("Volume is " + vol);

}}

The output from this program is shown here:Volume is 3000.0



Object is initialized as specified in the parameters to its constructor.Box mybox1 = new Box(10, 20, 15);

The values 10, 20, and 15 are passed to the Box( ) constructor when new creates the object.Thus, mybox1’s copy of width, height, and depth will contain the values 10, 20, and 15,respectively.

b) “this” Keywordthis can be used inside any method to refer to the current object. this keyword has two

meanings: to denote a reference to the implicit parameter to call another constructor of the same class.

// A redundant use of this.Box(double w, double h, double d){

this.width = w;this.height = h;this.depth = d;

}

c) Garbage CollectionThe purpose of garbage collection is to identify and discard objects that are no longer

needed by a program so that their resources can be reclaimed and reused. A Java object is subjectto garbage collection when it becomes unreachable to the program in which it is used.

9. Finalizer methodFinalizer methods are like the opposite of constructor methods; whereas a constructor

method is used to initialize an object, finalizer methods are called just before the object isgarbage collected and its memory reclaimed. To create a finalizer method, include a method withthe following signature in your class definition:

void finalize() {...}Inside the body of that finalize() method, include any cleaning up you want to do for that

object. Before you start using finalizer methods extensively in your Java programs, however, beaware that finalizer methods have several very important restrictions. First of all, the finalizermethod is not guaranteed to be called until the object’s memory is actually reclaimed, which maybe some time after you’ve removed all references to that object.

However, calling finalize() does not trigger an object to be garbage-collected. Onlyremoving all references to an object will cause it to be marked for deleting, and even then, Javamay or may not call the finalize() method itself—regardless of whether or not you’ve alreadycalled it. Finalizer methods are best used for optimizing the removal of an object—for example,by removing references to other objects, by cleaning up things that object may have touched, orfor other optional behaviors that may make it easier for that object to be removed. In most cases,you may not need to use finalize() at all.


Some object-oriented programming languages, notably C++, have explicit destructormethods for any cleanup code that may be needed when an object is no longer used. The mostcommon activity in a destructor is reclaiming the memory set aside for objects. Since Java doesautomatic garbage collection, manual memory reclamation is not needed, and Java does notsupport destructors. Of course, some objects utilize a resource other than memory, such as a fileor a handle to another object that uses system resources. In this case, it is important that theresource be reclaimed and recycled when it is no longer needed. You can add a finalize methodto any class. The finalize method will be called before the garbage collector sweeps away theobject. In practice, do not rely on the finalize method for recycling any resources that are in shortsupply—you simply cannot know when this method will be called. If a resource needs to beclosed as soon as you have finished using it, you need to manage it manually. Add a disposemethod that you call to clean up what needs cleaning. Just as importantly, if a class you use has adispose method, you will want to call it when you are done with the object. In particular, if yourclass has an instance field that has a dispose method, provide a dispose method that disposes ofthe instance fields.

10. ArraysAn array is a group of like-typed variables that are referred to by a common name. Arrays

of any type can be created and may have one or more dimensions. A specific element in an arrayis accessed by its index. Arrays offer a convenient means of grouping related information.

Arrays are ordered collections of identical objects that can be accessed via an index. Javaarrays have their own behavior that is encapsulated into their definition when the compilercreates them. Since arrays are objects, array variables behave like class-type variables. It isimportant to distinguish between the array variable and the array instance to which it refers.

Declaring an array only declares the array variable. To instantiate the array, the new operator must be used with the [] operator to

enclose the array size. The array size can be any integer expression.The following code declares an array variable and initializes it to null:char data[] = null;

Array ConstantsThe declaration of an array must include its type, but not its size. An array initializer is

either a new expression that evaluates to the correct array type, or a list of initial element valuesenclosed in { }. The following array constant provides initial values for its elements:

int[] int_array = {1, 3, 4, 15, 0};

Using ArraysArray elements can be accessed using an index in square brackets [n], where the index

must be an integer. Arrays start with index number 0, not 1. The index can be thought of as anoffset from the beginning of the array. The index may not be less than zero or greater than thedeclared size. If the array is declared size n, the index must be in the range 0 to n-1. Any attemptto index an array with an illegal value will cause an exception to be thrown. All arrays have adata field called length that indicates its size.

int ia[] = new int[100];for (int i = 0; i < ia.length; i++){


ia[i] = i * i;}

Copying Array ElementsThe library method System.arraycopy() is useful for copying a number of elements from

one array to another. The method can be used on any type of array and is declared as follows:

public static void arraycopy (Object src, int src_position, Object dst, int dst_position, intlength)

The method copies elements from the given source array, beginning at the specifiedposition to the destination array at the specified position. It copies the number of elementsspecified by the length argument. The destination array must already be allocated. Any type ofarray may be copied. If range exceeds bounds of either array, a run-time error results.

Arrays in Java are in between a class and a data type although they are implemented as aclass internally by Java.

char data[] = null;char[] data = null;int length = 60;char[] data = new char [length];

a) One-Dimensional ArraysA one-dimensional array is a list of like-typed variables. Array is created by following steps.1. Create an array variable of the desired type. The general form of a one dimensional array

declaration istype var-name[ ];

2. Allocate memory for array.Method1: The general form of new as it applies to one-dimensional arrays appears asfollows:

array-var = new type[size];The elements in the array allocated by new will automatically be initialized to zero.Method2: Arrays can be initialized when they are declared. The process is much thesame as that used to initialize the simple types. An array initializer is a list of comma-separated expressions surrounded by curly braces. The commas separate the values of thearray elements. The array will automatically be created large enough to hold the numberof elements you specify in the array initializer. There is no need to use new. For example,to store the number of days in each month, the following code creates an initialized arrayof integers:// An improved version of the previous program.class AutoArray {

public static void main(String args[]) {int month_days[] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31,30, 31 };System.out.println("April has " + month_days[3] + " days.");

}}



b) Multidimensional ArraysIn Java, multidimensional arrays are actually arrays of arrays. These, as you might

expect, look and act like regular multidimensional arrays. However, as you will see, there are acouple of subtle differences. To declare a multidimensional array variable, specify eachadditional index using another set of square brackets. For example, the following declares a two-dimensional array variable called twoD.

int twoD[][] = new int[4][5];This allocates a 4 by 5 array and assigns it to twoD. Internally this matrix is implemented as anarray of arrays of int.

There is a second form that may be used to declare an array:type[ ] var-name;

Here, the square brackets follow the type specifier, and not the name of the array variable. Forexample, the following two declarations are equivalent:

int al[] = new int[3];int[] a2 = new int[3];

The following declarations are also equivalent

11. String objectsThe Java String class (java.lang.String) is a class of object that represents a character

array of arbitrary length. While this external class can be used to handle string objects, Javaintegrates internal, built-in strings into the language.


An important attribute of the String class is that once a string object is constructed, itsvalue cannot change (note that it is the value of an object that cannot change, not that of a stringvariable, which is just a reference to a string object). All String data members are private, and nostring method modifies the string’s value.

String MethodsAlthough a string represents an array, standard array syntax cannot be used to inquire into

it. These are detailed in the below Table.

String ComparisonString comparison methods listed in below Table.


String SearchingThe String class also provides methods that search a string for the occurrence of a single

character or substring. These return the index of the matching substring or character if found, or -1 if not found.

int indexOf (char ch) int indexOf (char ch, int begin) int lastIndexOf (char ch) int lastIndexOf (char ch, int fromIndex) int indexOf (String str) int indexOf (String str, int begin) int lastIndexOf (String str) int lastIndexOf (String str, int fromIndex)

The following example shows the usage of these functions:if (s1.indexOf (’:’) >= 0){

…}String suffix =

s1.substring (s1.lastIndexOf (’.’));int spaceCount = 0;int index = s1.indexOf (’ ’);while (index >= 0){

++spaceCount;index = s1.indexOf (’ ’, index + 1);



}int index = s1.indexOf (―that‖);

String ConcatenationThe String class provides a method for concatenating two strings:

String concat (String otherString)The + and += String operators are more commonly used: The Java compiler recognizes the + and+= operators as String operators. For each + expression, the compiler generates calls to methodsthat carry out the concatentation. For each += expression, the compiler generates calls tomethods that carry out the concatenation and assignment.

Converting Objects To StringsThe String + operator accepts a non-string operand, provided the other operand is a

string. The action of the + operator on non-string operands is to convert the non-string to a string,then to do the concatenation. Operands of native types are converted to string by formatting theirvalues. Operands of class types are converted to a string by the method toString() that is definedfor all classes. Any object or value can be converted to a string by explicitly using one of thestatic valueOf() methods defined in class String:

String str = String.valueOf (obj);If the argument to valueOf() is of class type, then valueOf() calls that object’s toString() method.Any class can define its own toString() method, or just rely on the default. The output producedby toString() is suitable for debugging and diagnostics. It is not meant to be an elaborate textrepresentation of the object, nor is it meant to be parsed. These conversion rules also apply to theright-hand side of the String += operator.

Converting Strings To NumbersMethods from the various wrapper classes, such as Integer and Double, can be used to

convert numerical strings to numbers.The wrapper classes contain static methods such as parseInt() which convert a string to its

own internal data type.

12. PackagesJava allows you to group classes in a collection called a package. A class can use all

classes from its own package and all public classes from other packages.You can import a specific class or the whole package. You place import statements at the

top of your source files (but below any package statements). For example, you can import allclasses in the java.util package with the statement:

import java.util.*;

Adding a class into a packageTo place classes inside a package, you must put the name of the package at the top of

your source file, before the code that defines the classes in the package. For example,package com.horstmann.corejava;public class Employee


{...

}

Package scope Public features can be used by any class. Private features can only be used by the class that defines them. If you don't specify either public or private, then the feature (that is, the class, method, or

variable) can be accessed by all methods in the same package.

13. Documentation CommentsThe Java SDK contains a very useful tool, called javadoc, that generates HTML

documentation from your source files. If you add comments that start with the special delimiter/** to your source code, you too can produce professional-looking documentation easily. This isa very nice scheme because it lets you keep your code and documentation in one place. If youput your documentation into a separate file, then you probably know that the code and commentstend to diverge over time. But since the documentation comments are in the same file as thesource code, it is an easy matter to update both and run javadoc again.

How to Insert CommentsThe javadoc utility extracts information for the following items:

• Packages• Public classes and interfaces• Public and protected methods• Public and protected fieldsYou can (and should) supply a comment for each of these features. Each comment is

placed immediately above the feature it describes. A comment starts with a /** and ends with a*/. Each /** . . . */ documentation comment contains free-form text followed by tags. A tag startswith an @, such as @author or @param. The first sentence of the free-form text should be asummary statement. The javadoc utility automatically generates summary pages that extractthese sentences. In the free-form text, you can use HTML modifiers such as ... foremphasis, <code>...</code> for a monospaced ―typewriter‖ font, ... for strongemphasis, and even <img ...> to include an image. You should, however, stay away fromheading <h1> or rules <hr> since they can interfere with the formatting of the document.

Class CommentsThe class comment must be placed after any import statements, directly before the class

definition. Here is an example of a class comment:/**A <code>Card</code> object represents a playing card, such as "Queen of Hearts". Acard has a suit (Diamond, Heart, Spade or Club) and a value (1 = Ace, 2 . . . 10, 11 =Jack, 12 = Queen, 13 = King).*/public class Card


{...

}Method Comments

Each method comment must immediately precede the method that it describes. Inaddition to the general-purpose tags, you can use the following tags:

@param variable descriptionThis tag adds an entry to the ―parameters‖ section of the current method. The description canspan multiple lines and can use HTML tags. All @param tags for one method must be kepttogether.

@return descriptionThis tag adds a ―returns‖ section to the current method. The description can span multiple linesand can use HTML tags.

@throws class description

Field CommentsYou only need to document public fields—generally that means static constants. For

example,/**The "Hearts" card suit*/public static final int HEARTS = 1;

General CommentsThe following tags can be used in class documentation comments.

@author nameThis tag makes an ―author‖ entry. You can have multiple @author tags, one for each author.

@version text

How to Extract CommentsHere, docDirectory is the name of the directory where you want the HTML files to go. Followthese steps:

1. Change to the directory that contains the source files you want to document. If you havenested packages to document, such as com.horstmann.corejava, you must be in thedirectory that contains the subdirectory com. (This is the directory that contains theoverview.html file, if you supplied one.)

2. Run the command javadoc -d docDirectory nameOfPackage for a single package. Or runjavadoc -d docDirectory nameOfPackage1 nameOfPackage2... to document multiplepackages. If your files are in the default package, then run javadoc -d docDirectory *.javainstead. If you omit the -d docDirectory option, then the HTML files are extracted to thecurrent directory. That can get messy, and we don't recommend it. The javadoc programcan be fine-tuned by numerous command-line options. For example, you can use the -author and -version options to include the @author and @version tags in thedocumentation. (By default, they are omitted.)



14. References1. Cay S. Horstmann and Gary Cornell, ―Core Java: Volume I – Fundamentals‖, Eighth

Edition, Sun Microsystems Press, 2008.2. Herbert Schildt ―JAVA2:The complete reference‖ Fifth Edition, McGraw-Hill/Osborne.3. http://java.sun.com

http://java.sun.com/


UNIT II OBJECT-ORIENTED PROGRAMMING – INHERITANCE

1. Inheritance 1.1. Member Access And Inheritance 1.2. Super Keyword

2. Class Hierarchy 3. Polymorphism

3.1. Overridden Methods 3.2. Overloaded Methods

4. Dynamic Binding 4.1. Dynamic Binding 4.2. Static Binding

5. Final Keyword 6. Abstract Classes

6.1. Abstract Methods 7. The Object Class 8. Reflection 9. Interfaces 10. Object Cloning 11. Inner Classes 12. Proxies

Inheritance

Inheritance is a process of making a new class that derives from an existing class. Theexisting class is called the superclass, base class, or parent class. The new class is called thesubclass, derived class, or child class.

Therefore, a subclass is a specialized version of a superclass. It inherits all of theinstance variables and methods defined by the superclass and add its own, unique elements.

Subclasses of a class can define their own unique behaviors and yet share some of thesame functionality of the parent class.

To inherit a class, you simply incorporate the definition of one class into another by usingthe extends keyword. The general form of a class declaration that inherits a superclass is shownhere:

class subclass-name extends superclass-name{

// body of class}You can only specify one superclass for any subclass that you create. Java does not

support the inheritance of multiple superclasses into a single subclass.Protected features in Java are visible to all subclasses as well as all other classes in the

same package.



Member Access and InheritanceA class member that has been declared as private will remain private to its class. It is not

accessible by any code outside its class, including subclasses.

Super keywordSuper is a special keyword that directs the compiler to invoke the superclass method.

super has two general forms. to invoke a superclass constructor. to invoke a superclass members(variables &methods). invoke a superclass

constructor

Invoke superclass constructor: A subclass can call a constructor method defined by its superclass by use of the following

form of super:super(parameter-list);

Here, parameter-list specifies any parameters needed by the constructor in the superclass. super( ) must always be the first statement executed inside a subclass constructor. The compiler implicitly calls the base class’s no-parameter constructor or default

constructor. If the superclass has parameterized constructor and the subclass constructor does not call

superclass constructor explicitly, then the Java compiler reports an error.

Invoke superclass members: Super always refers to the superclass of the subclass in which it is used. This usage has

the following general form:super.member;

Here, member can be either a method or an instance variable. This second form of superis most applicable to situations in which member names of a subclass hide members bythe same name in the superclass.

If a parent class contains a finalize() method, it must be called explicitly by the derivedclass’s finalize() method.

super.finalize();

When Constructors are CalledConstructors are called in order of derivation, from superclass to subclass. Because a

superclass has no knowledge of any subclass, any initialization it needs to perform is separatefrom and possibly prerequisite to any initialization performed by the subclass. Therefore, it mustbe executed first.

CLASS HIERARCHY

The collection of all classes extending from a common superclass is called an inheritancehierarchy; the path from a particular class to its ancestors in the inheritance hierarchy is itsinheritance chain.

Simple class hierarchies consist of only a superclass and a subclass. But you can buildhierarchies that contain as many layers of inheritance as you like.



For example, create three classes called A, B, and C, C can be a subclass of B, which is asubclass of A. When this type of situation occurs, each subclass inherits all of the traits found inall of its superclasses. In this case, C inherits all aspects of B and A.

POLYMORPHISMPolymorphism means the ability of methods to behave differently based on the kinds of

input.Types of polymorphism

Method Overloading Method overriding

Overloaded methodsOverloaded methods are methods with the same name but different method signature

(either a different number of parameters or different types in the parameter list).

public class Test{public static void main(String arg[]){A a=new A();a.display();B b=new B();b.display("Hello");}

}class A{void display(){

System.out.println("Hai");}

}class B extends A{void display(String s){

System.out.println(s);}

}

OutputHaiHello

Overridden methodsThe process of a subclass redefining a method contained in the superclass (with the same

parameter types) is called overriding the method.Overridden methods allow Java to support run time polymorphism. Whenever a method

is called for a subclass object, the compiler calls the overriding version instead of the superclassversion. The version of the method defined by the superclass will be hidden.



Call to an overridden method is resolved at run time, rather than compile time. Superclassreference variable can refer to a subclass object. Java uses this fact to resolve calls to overriddenmethods at run time. When an overridden method is called through a superclass reference, Javadetermines which version of that method to execute based upon the type of the object beingreferred to at the time the call occurs. This determination is made at run time.

class Dispatch {public static void main(String args[]) {

A a = new A(); // object of type AB b = new B(); // object of type BC c = new C(); // object of type CA r; // obtain a reference of type A

r = a; // r refers to an A objectr.callme(); // calls A's version of callmer = b; // r refers to a B objectr.callme(); // calls B's version of callmer = c; // r refers to a C objectr.callme(); // calls C's version of callme

}}

When you override a method, the subclass method must be at least as visible as thesuperclass method.

In particular, if the superclass method is public, then the subclass method must also bedeclared as public. It is a common error to accidentally omit the public specifier for the subclassmethod. Then the compiler complains that you try to supply a weaker access privilege.

public class Test1{public static void main(String arg[]){A a=new A();a.display();B b=new B();b.display();}

}class A{protected void display(){


}class B extends A{public void display(){

System.out.println("Hello");}

}



OutputHaiHello

Methods labeled final, private, or static are not subject to dynamic lookup because theymay not be overridden. private methods are simply not inherited because they would never becallable anyway. static methods apply to a particular class’s static data and thus make no sense ina derivation. final methods are those designated as not-overridable for reasons of complexity orsafety.

public class Test1{public static void main(String arg[]){A a1=new A();a1.display();A a2=new B();a2.display();}

}class A{void display(){


}class B extends A{void display(){

System.out.println("Hello");}

}

OutputHaiHello

DYNAMIC BINDINGSelecting the appropriate method at runtime is called dynamic binding. Dynamic Binding

refers to the case where compiler is not able to resolve the call and the binding is done at runtime

only. All the instance methods in Java follow dynamic binding.

Dynamic binding has a very important property:It makes programs extensible without recompiling the existing code.

Suppose a new class is added, and there is the possibility that the variable refers to anobject of that class. The code contains the method invoking statement of that class need not berecompiled. The method is called automatically if the object happens to refer to the class.



Dynamic Binding or Late BindingIt is important to understand what happens when a method call is applied to an object. Here

are the details:

1. The compiler looks at the declared type of the object and the method name. The compilerknows all possible candidates for the method to be called.

2. Next, the compiler determines the types of the parameters that are supplied in the methodcall. If among all the methods called fun there is a unique method whose parameter typesare a best match for the supplied parameters, then that method is chosen to be called. Thisprocess is called overloading resolution.

3. If the method is private, static, final, or a constructor, then the compiler knows exactlywhich method to call.

4. When the program runs and uses dynamic binding to call a method, then the virtualmachine must call the version of the method that is appropriate for the actual type of theobject. The virtual machine precomputes a method table for each class that lists allmethod signatures and the actual methods to be called. When a method is actually called,the virtual machine simply makes a table lookup. This is used to reduce the timeconsumed by searching process.

STATIC BINDING OR EARLY BINDING

If the compiler can resolve the binding at the compile time only then such a binding iscalled Static Binding or Early Binding. Resolve the call and binding at compile time.

If the method is private, static, final, or a constructor, then the compiler knows exactlywhich method to call. This is called static binding.

All the member variables in Java follow static binding. All the static method calls are resolved at compile time itself. All private methods are resolved at compile time itself.

FINAL KEYWORDThe keyword final has three uses.

Used to create the equivalent of a named constant. Used to Prevent Overriding Used to Prevent Inheritance

Named constantA variable can be declared as final. Doing so prevents its contents from being modified.

This means that you must initialize a final variable when it is declared. For example:final int constant = 1;

Variables declared as final do not occupy memory on a per-instance basis. Thus, a final variableis essentially a constant. The keyword final can also be applied to methods, but its meaning issubstantially different than when it is applied to variables. Attempts to change it will generateeither a compile-time error or an exception.



Using final to Prevent OverridingMethods declared as final cannot be overridden.The syntax is simple, just put the

keyword final after the access specifier and before the return type like this:class A{

final void meth() {System.out.println("This is a final method.");

}}class B extends A{

void meth() { // ERROR! Can't override.System.out.println("Illegal!");

}}

Normally, Java resolves calls to methods dynamically, at run time. This is called latebinding. However, since final methods cannot be overridden, a call to one can be resolved atcompile time. This is called early binding.

Using final to Prevent InheritanceDeclaring a class as final implicitly declares all of its methods as final. So it prevents a

class from being inherited. To do this, precede the class declaration with final.It is illegal to declare a class as both abstract and final since an abstract class is

incomplete by itself and relies upon its subclasses to provide complete implementations.Here is an example of a final class:

final class A{

// ...}// The following class is illegal.class B extends A{ // ERROR! Can't subclass A

// ...}

ABSTRACT CLASSES

An abstract class is a type of class that is not allowed to be instantiated. The only reasonit exists is to be extended. Abstract classes contain methods and variables common to all thesubclasses, but the abstract class itself is of a type that will not be used directly. Even a singleabstract method requires that a class be marked abstract.



To declare a class abstract, you simply use the abstract keyword in front of the classkeyword at the beginning of the class declaration.

abstract class classname{

public abstract type methodname();// no implementation required

……..}

There can be no objects of an abstract class. That is, an abstract class cannot be directlyinstantiated with the new operator. Such objects would be useless, because an abstract class isnot fully defined.

Also, you cannot declare abstract constructors, or abstract static methods. Any subclassof an abstract class must either implement all of the abstract methods in the superclass, or beitself declared abstract.

Here is a simple example of a class with an abstract method, followed by a class whichimplements that method:// A Simple demonstration of abstract.abstract class A{

abstract void callme();// concrete methods are still allowed in abstract classesvoid callmetoo() {

System.out.println("This is a concrete method.");}

}class B extends A{

void callme() {System.out.println("B's implementation of callme.");

}}class AbstractDemo{

public static void main(String args[]) {B b = new B();b.callme();b.callmetoo();

}}

That is, sometimes you will want to create a superclass that only defines a generalizedform that will be shared by all of its subclasses, leaving it to each subclass to fill in the details.Such a class determines the nature of the methods that the subclasses must implement. You canrequire that certain methods be overridden by subclasses by specifying the abstract type



modifier. These methods are sometimes referred to as subclasser responsibility because theyhave no implementation specified in the superclass.

Thus, a subclass must override them—it cannot simply use the version defined in thesuperclass.

Abstract methods

An abstract method is a method declaration that contains no functional code. The reason forusing an abstract method is to ensure that subclasses of this class will include an implementationof this method. Any concrete class (that is, a class that is not abstract, and therefore capable ofbeing instantiated) must implement all abstract methods it has inherited.

Methods declared with the keyword abstract define a skeleton for a method but do notimplement it. This requires a derived class to provide the code for this class.

A class with one or more abstract methods must itself be declared abstract and cannot beinstantiated. It can only be used for derivation.

To declare an abstract method, use this general form:abstract type name(parameter-list);

UAGEBut while an abstract class can define both abstract and nonabstract methods, an interface canhave only abstract methods. Interface as being similar to an abstract class with no instance fields.

InterfacesAn interface is a collection of method definitions (without implementations) and constant values.

Defining an InterfaceThis is the general form of an interface:

An interface must be declared with the keyword interface.

access interface name {return-type method-name(parameter-list);type final-varname = value;}

It is also possible to declare that an interface is protected so that it can only beimplemented by classes in a particular package. However this is very unusual.

Rules for interface constants. They must always be public static final

Once the value has been assigned, the value can never be modified. The assignment happens inthe interface itself (where the constant is declared), so the implementing class can access it anduse it, but as a read-only value.To make a class implement an interface, have to carry out two steps:



Declare that your class intends to implement the given interface. Supply definitions for all methods in the interface.

To declare that a class implements an interface, use the implements keyword:access class classname implements interfacename{

//definitions for all methods in the interface}

Implementation classes must adhere to the same rules for method implementation as a classextending an abstract class. In order to be a legal implementation class, a nonabstractimplementation class must do the following:

When you implement an interface method, method must be declared as public. (Theaccess level can be more accessible than that of the overridden method.)

Type signature of the implementing method must match exactly the type signaturespecified in the interface definition. (The argument list and return type must exactlymatch that of the overridden method.)

A class can implement one or more interfaces. Partial Implementations: If a class includes an interface but does not fully implement the

methods defined by that interface, then that class must be declared as abstract.

Inner Classes

An inner class is a class that is defined inside another class.

Inner classes let you make one class a member of another class. Just as classes havemember variables and methods, a class can also have member classes.

Regular Inner Class

You define an inner class within the curly braces of the outer class, as follows:

class MyOuter {class MyInner { }

}And if you compile it, %javac MyOuter.java , you’ll end up with two class files:

MyOuter.class

MyOuter$MyInner.class

The inner class is still, in the end, a separate class, so a class file is generated. But theinner class file isn’t accessible to you in the usual way. The only way you can access the innerclass is through a live instance of the outer class.



Instantiating an Inner Class

To instantiate an instance of an inner class, you must have an instance of the outer class.An inner class instance can never stand alone without a direct relationship with a specificinstance of the outer class.

Instantiating an Inner Class from Within Code in the Outer Class

From inside the outer class instance code, use the inner class name in the normal way:

class MyOuter {private int x = 7;MyInner mi = new MyInner();class MyInner {

public void seeOuter() {System.out.println("Outer x is " + x);

}}public static void main(String arg[]){

MyOuter mo=new MyOuter();mo.mi.seeOuter();

}}

Output:Outer x is 7

Method-Local Inner Classes

A method-local inner class is defined within a method of the enclosing class.

class MyOuter {void inner(){final int c=9;class MyInner {

int x=5;public void display() {

System.out.println("Inner x is " + x);System.out.println("Inner c is " + c);

}}MyInner mi = new MyInner();mi.display();

}public static void main(String arg[]){

MyOuter mo = new MyOuter();mo.inner();

}}



Output: x is 5c is 9

Anonymous Inner Classes Anonymous inner classes have no name, and their type must be either a subclass of the

named type or an implementer of the named interface.

An anonymous inner class is always created as part of a statement, so the syntax will endthe class definition with a curly brace, followed by a closing parenthesis to end themethod call, followed by a semicolon to end the statement: });

An anonymous inner class can extend one subclass, or implement one interface. It cannotboth extend a class and implement an interface, nor can it implement more than oneinterface.

public class Test{public static void main(String arg[]){

B b=new B();b.ob.display();

}}class A{

void display(){System.out.println("Hai");}

}class B {

A ob=new A(){void display(){System.out.println("Hello");}

};}Output: Hello

And if you compile it, %javac Test.java , you’ll end up with two class files:A.classB.classB$1.classTest.class

Static Nested Classes Static nested classes are inner classes marked with the static modifier.

Technically, a static nested class is not an inner class, but instead is considered a top-level nested class.



Because the nested class is static, it does not share any special relationship with aninstance of the outer class. In fact, you don’t need an instance of the outer class toinstantiate a static nested class.

Instantiating a static nested class requires using both the outer and nested class names asfollows:

BigOuter.Nested n = new BigOuter.Nested(); A static nested class cannot access nonstatic members of the outer class, since it does not

have an implicit reference to any outer instance (in other words, the nested class instancedoes not get an outer this reference).

public class Test{public static void main(String arg[]){

A.B b=new A.B();b.display();

}}class A {static class B {

int m=5;void display(){

System.out.println("m="+m);}

}}

Output: m=5

The Object Class - The Cosmic Superclass

The Object class sits at the top of the class hierarchy tree in the Java developmentenvironment. The Object class is the ultimate ancestor

Every class in Java extends Object class.The Object class defines the basic state and behavior that all objects must have, such as

the ability to compare with another object, to convert to a string, to wait on a condition variable,to notify other objects that a condition variable has changed, and to return the object's class.

Object defines the following methods, which means that they are available in everyobject.

Reflection Reflection is the ability of the software to analyze itself at runtime.

Reflection is provided by the java.lang.reflect package and elements in class.

This mechanism is helpful to tool builders, not application programmers.The reflection mechanism is extremely used to



Analyze the capabilities of classes at run time Inspect objects at run time Implement generic array manipulation code

Analyze the capabilities of classes at run time - Examine the structure of class.

Java.lang.Class

Field[] getFields()

Method[] getMethods()

Constructor[] getConstructors()

Return an array of public fields, methodsand constructors that the class supports. Thisincludes all public members of super class.

Field[] getDeclaredFields()

Method[] getDeclaredMethods()

Constructor[] getDeclaredConstructors()

Return an array of fields, methods andconstructors that are declared in the class.This includes private, protected and public

members of class but not members of superclass.

Java.lang.reflect

Field Method Constructor

Methods (Field, Method & Constructor): String getName() – return the name of the item. int getModifiers()- return an integer, that describes the modifier.

Field

Provide information about fields. The getFields method returns an array containing Field objects for the public fields. The getDeclaredField method returns an array of Field objects for all fields. The methods

return an array of length 0 if there are no such fields.

import java.lang.reflect.*;


class Test{public int var1;private int var2;protected int var3;int var4;

}public class FieldDemo{public static void main(String args[])throws Exception{

Class c=Class.forName("Test");Field f[]=c.getFields();Field fdec[]=c.getDeclaredFields();System.out.println("public Fields:");for(int i=0;i<f.length;i++)

System.out.println(f[i]);System.out.println("All Fields:");for(int i=0;i<fdec.length;i++)

System.out.println(fdec[i]);}

}

Output:public Fields:public int Test.var1All Fields:public int Test.var1private int Test.var2protected int Test.var3int Test.var4

Method Provides information about method

The getMethods() method return an array containing Method objects that give you all thepublic methods.

The getDeclaredMethods () return all methods of the class or interface. This includesthose inherited from classes or interfaces above it in the inheritance chain.

import java.lang.reflect.*;

class Test{public void method1(){}protected void method2(){}private void method3(){}void method4(){}

}

public class MethodDemo{

public static void main(String args[])throws Exception{

Class c=Class.forName("Test");

Method m[]=c.getMethods();

Method mdec[]=c.getDeclaredMethods();

System.out.println("public Methods of class Test & its Super class:");

for(int i=0;i<m.length;i++)

System.out.println(m[i].getName());

System.out.println("All Methods:");

for(int i=0;i<mdec.length;i++)

System.out.println(mdec[i].getName());

}

}

Output:

public Methods of class Test & its Super

class:

method1

hashCode

getClass

wait

equals

notify

notifyAll

toString

All Methods:

method1

method2

method3

method4


Constructor

Provide information about constructors getConstructors () method return an array containing Constructor objects that give you all

the public constructors getDeclaredConstructors () method return all constructors of the class represented by the

Class object.

Using Reflection to Analyze Objects at Run Time

Look at the contents of the data fields. It is easy to look at the contents of a specific field

of an object whose name and type are known when you write a program. But reflection lets youlook at fields of objects that were not known at compile time.

f.set(obj, value) sets the field represented by f of the object obj to the new value. f.get(obj) returns an object whose value is the current value of the field of obj.

import java.lang.reflect.*;class A{

public int var1,var2;A(int i, int j){

var1=i;var2=j;

}}public class ConstructorDemo {

public static void main(String args[]) throws Exception{A obj=new A(10,20);System.out.println("Before \n var1= "+obj.var1);Field f1 = obj.getClass().getField("var1");

int v1 = f1.getInt(obj) + 1;f1.setInt(obj, v1);

System.out.println("After \n var1= "+v1);System.out.println("Before \n var2= "+obj.var2);Field f2 = obj.getClass().getField("var2");f2.set(obj,21);System.out.println("After \n var2= "+f2.get(obj));

}}

Output:

Before var1= 10Aftervar1= 11

Before var2= 20After



var2= 21

Using Reflection to implement generic array manipulation code

The Array class in the java.lang.reflect package allows you to create arrays dynamically.First the given array can be converted to an Object[] array. newInstance() method of Array class,constructs a new array.

Object newarray= Array.newInstance(ComponentType, newlength)newInstance() method needs two parameters

Component Type of new arrayTo get component type

1. Get the class object using getClass() method.2. Confirm that it is really an array using isArray().3. Use getComponentType method of class Class, to find the right type for the array.

Length of new array

Length is obtained by getLength() method. It returns the length of any array(method isstatic method, Array.getLengh(array name)).

import java.lang.reflect.*;public class TestArrayRef {

static Object arrayGrow(Object a){Class cl = a.getClass();if (!cl.isArray()) return null;Class componentType = cl.getComponentType();int length = Array.getLength(a);int newLength = length + 10;Object newArray = Array.newInstance(componentType,newLength);System.arraycopy(a, 0, newArray, 0, length);return newArray;

}public static void main(String args[]) throws Exception{

int arr[]=new int[10];System.out.println(arr.length);arr = (int[])arrayGrow(arr);System.out.println(arr.length);

}}

Output:1020

Object cloning



A clone of an object is a new object that has the same state as the original but a different

identity. You can modify the clone without affecting the original.

When you make a copy of a variable, the original and the copy are references to the same object.

This means a change to either variable also affects the other.

If you want to make a clone of any class then the class must

implement the Cloneable interface, and

Redefine the clone method with the public access modifier.

import java.util.*;

class Test implements Cloneable{int a=10;public void display(){

System.out.println("a="+a);}public Object clone(){

try{Test cloned = (Test)super.clone();return cloned;

}catch(CloneNotSupportedException e){

return null;}

}}

public class CloneDemo{public static void main(String arg[]){

Test original=new Test();original.a=20;Test copy=(Test)original.clone();copy.a=80;original.display();copy.display();

}}

Output:a=20a=80

ProxyProxy used to create new classes at runtime that implement a given set of interfaces. The

proxy class can create brand-new classes at runtime.



The proxy class can create brand-new classes at runtime. Such a proxy class implementsthe interfaces that you specify. In particular, the proxy class has the following methods:

• All methods required by the specified interfaces;• All methods defined in the Object class (toString, equals, and so on).

To create a proxy object, you use the newProxyInstance method of the Proxy class. The method hasthree parameters:

1. A class loader. As part of the Java security model, it is possible to use differentclass loaders for system classes, classes that are downloaded from the Internet,and so on.

2. An array of Class objects, one for each interface to be implemented.3. An invocation handler.

Proxies can be used for many purposes, such as:• Routing method calls to remote servers;• Associating user interface events with actions in a running program;• Tracing method calls for debugging purposes.


Unit III Event-Driven Programming

Graphics programming

Frame

Frame is a top-level window that has a title bar, menu bar, borders, and resizing corners. By

default, a frame has a size of 0 × 0 pixels and it is not visible.

Frames are examples of containers. It can contain other user interface components such as

buttons and text fields.

Class hierarchy for FrameJava.awt

Component

Container

Window

Frame

Component & Window class Methodsjava.awt.Component

void setVisible(boolean b) - shows or hides the component depending on whether b is

true or false.

void setSize(int width, int height) - resizes the component to the specified width and

height.

void setBounds(int x, int y, int width, int height) - moves and resizes this component. The

location of the top-left corner is given by x and y, and the new size is given by the width

and height parameters.

void setBackground(java.awt.Color) – set Background color to the window.

void setForeground(java.awt.Color)- set Foreground color to the window.

void repaint() - causes a repaint of the component ―as soon as possible.‖



java.awt.Window void setTitle(String s) - sets the text in the title bar for the frame to the string s.

Frame’s constructors:Frame( ) - creates a standard window that does not contain a title.Frame(String title) - creates a window with the title specified by titleMethods of Frame class:

void setSize(int newWidth, int newHeight) - method is used to set the dimensions of thewindow. The dimensions are specified in terms of pixels.

void setVisible(boolean visibleFlag)- The component is visible if the argument to thismethod is true. Otherwise, it is hidden.

void setTitle(String newTitle)- change the title in a frame window.

Creating a Frame WindowMethod 1: In main() method

Create an instance of a Frame class.Frame f=new Frame(―frame name‖);

Set the frame sizef.setSize(500,500);

Make the frame visiblef.setVisible(true);Example:The following java program creates a frame with the dimension as 600 x 400 and makes itvisible in the screen. Import java.awt package because Frame class is available in that package.import java.awt.*;

public class Demo{

public static void main(String arg[]){

Frame f=new Frame("Demo");

f.setSize(600,400);

f.setVisible(true);

}

}

Out Put:



Method :2 Create a subclass of Frame. In the Subclass constructor

Change the frame title by calling superclass [Frame] constructor using super(String)method call.

Set the size of the window explicitly by calling the setSize( ) method. Make the frame visible by calling setVisible() method.

In main() method Create an instance of subclass.

Example:The following java program creates a frame with the dimension as 600 x 400 and makes itvisible in the screen. Import java.awt package because Frame class is available in that package.

import java.awt.*;

public class Demo extends Frame{

Demo(String s){

super(s);

setSize(600,400);

setVisible(true);

}

Out Put:

}


Demo ob=new Demo("Demo");

}

Working with 2D shapes Graphics: java.awt.GraphicsThe Graphics class is part of the java.awt package. The Graphics class defines a number ofdrawing functions. Each shape can be drawn edge-only or filled. Objects are drawn and filled inthe currently selected graphics color, which is black by default. When a graphics object is drawnthat exceeds the dimensions of the window, output is automatically clipped.Java Coordinate System Java’s coordinate system has the origin (0, 0) in the top left corner. Positive x values are to the



right, and positive y values are down. Coordinate units are measured in pixels (picture element).All pixel values are integers; there are no partial or fractional pixels.

X coordinate: Horizontal distance moving right from the left of the screen.Y coordinate: Vertical distance moving from top to bottom of the screen.The Graphics class provides a set of simple built-in graphics primitives for drawing, includinglines, rectangles, polygons, ovals, and arcs.LinesTo draw straight lines, use the drawLine method. drawLine takes four arguments: the x and ycoordinates of the starting point and the x and y coordinates of the ending point.

void drawLine(int startX, int startY, int endX, int endY) - displays a line in the currentdrawing color that begins at startX,startY and ends at endX,endY.

Example: Demo.java

import java.awt.*;


Demo(String s){

super(s);

setSize(100,100);

setVisible(true);

}

public void paint(Graphics g) {

g.drawLine(10,10,60,60);

}


Demo ob=new Demo("Line Demo");

}

}

Output:



RectanglesThe Java graphics primitives provide two kinds of rectangles: Plain rectangles and Roundedrectangles(which are rectangles with rounded corners).

void drawRect(int x, int y, int width, int height) void fillRect(int x, int y, int width, int height) void drawRoundRect(int x, int y, int width, int height, int xDiam, int yDiam) void fillRoundRect(int x, int y, int width, int height, int xDiam, int yDiam)

A rounded rectangle has rounded corners. The upper-left corner of the rectangle is at x, y. Thedimensions of the rectangle are specified by width and height. The diameter of the rounding arcalong the X axis is specified by xDiam. The diameter of the rounding arc along the Y axis isspecified by yDiam.

Example: Demo.java

import java.awt.*; Demo ob=new Demo("Line Demo");


Demo(String s){

super(s);

setSize(500,500);

setVisible(true);

}


g.drawRect(100,100,60,60);

g.fillRect(250,100,60,60);

g.drawRoundRect(100,250,60,60,10,10);

g.fillRoundRect(250,250,60,60,20,20);

}


}

}

Output:



PolygonsPolygons are shapes with an unlimited number of sides. Set of x and y coordinates are needed todraw a polygon, and the drawing method starts at one, draws a line to the second, then a line tothe third, and so on.

void drawPolygon(int x[ ], int y[ ], int numPoints) void fillPolygon(int x[ ], int y[ ], int numPoints)

x[]- An array of integers representing x coordinatesy[]- An array of integers representing y coordinatesnumPoints- An integer for the total number of points

Example: Demo.java

import java.awt.*;



int x1[] = { 39,94,97,112,53,58,26 };

int y1[] = { 133,174,136,170,208,180,206 };

g.drawPolygon(x1,y1,7);

int x2[] = { 139,194,197,212,153,158,126 };

int y2[] = { 133,174,136,170,208,180,206 };

g.fillPolygon(x2,y2,7);

}


Demo ob=new Demo("Polygon Demo");

}

}

Output:


OvalsUse ovals to draw ellipses or circles.

void drawOval(int top, int left, int width, int height) void fillOval(int top, int left, int width, int height)


Example:Demo.java

import java.awt.*;


}

}

Demo(String s){

super(s);

setSize(300,300);

setVisible(true);

}


g.drawOval(20,120,70,70);

g.fillOval(140,120,100,70);

}


Demo ob=new Demo("Oval Demo");

ArcThe arc is drawn from startAngle through the angular distance specified by arkAngle. Angles arespecified in degrees. The arc is drawn counterclockwise if sweepAngle is positive, and clockwiseif arkAngle is negative.

void drawArc(int x, int y, int width, int height, int startAngle, int arcAngle) void fillArc(int x, int y, int width, int height, int startAngle, int arcAngle)

x=0y=0width=100height=100Startangle=90arcAngle=180


Example:Demo.java

import java.awt.*;


Demo(String s){

super(s);

setSize(300,300);

setVisible(true);


}

}

Output:

}


g.drawArc(20,120,90,90,90,180);

g.fillArc(120,120,90,90,90,180);

g.drawArc(170,120,90,90,90,-180);

}


Demo ob=new Demo("Arc Demo");

Drawing Text Draw text on the screen using the method drawstring().

void drawstring(String text, int x, int y)- draws a string in the current font and color.

import java.awt.*;


Demo(String s){

super(s);

setSize(200,200);

setVisible(true);

}


g.drawString("welcome", 75, 100);

}


Demo ob=new Demo("Text Demo");

}

}



Using color, fonts, and images Color: java.awt.ColorColor class creates a solid RGB color with the specified red, green, blue value in the range (0-255). Java’s abstract color model uses 24-bit color. The values of each color must be in the range(0- 255).Constructor:Color(int red, int green, int blue) - create new color object for any combination of red, green,and blue.Java.awt.Color class used to create new color object for any combination of red, green, and blue,and it predefines a few color constants. They are stored in class variables,

Color.white (255,255,255)

Color.black (0,0,0)

Color.lightGray (192,192,192)

Color.gray (128,128,128)

Color.darkGray (64,64,64)

Color.red (255,0,0)

Color.blue (0,0,255)

Color.yellow (255,255,0)

Color.magenta (255,0,255)

Color.cyan( 0,255,255)

Color.pink (255,175,175)

Color.orange (255,200,0)

Color.green(0,255,0)

java.awt.Component Component class defines setBackground() and setForeground() methods for setting backgroundand foreground colors to the window. Since Frame is a subclass of Component class it can usethese methods.

void setBackground(java.awt.Color) – set Background color to the window. void setForeground(java.awt.Color)- set Foreground color to the window.

java.awt.GraphicssetColor() method used to set the current color for the graphics context which is defined inGraphics class.

void setColor(java.awt.Color)


Example: The following program for designing a frame with square boxes, each of which has arandomly chosen color in it.

Demo ob=new Demo("Color Demo");

import java.awt.*;


Demo(String s){

}

}

Output:

super(s);setSize(200,200);setVisible(true);

setBackground(Color.black);

}


int rval, gval, bval;

for (int j = 30; j < 200-30; j += 30)

for (int i = 30; i < 200-30; i+= 30){

rval = (int)Math.floor(Math.random() * 256);

gval = (int)Math.floor(Math.random() * 256);

bval = (int)Math.floor(Math.random() * 256);

g.setColor(new Color(rval,gval,bval));

g.fillRect(i,j,25,25);

}

}


Font: java.awt.FontFont class is used to create Font Objects to set the font for drawing text, labels, textField, buttonsetc..,One Font constructor has this general form:Font(String fontName, int fontStyle, int pointSize)Here, fontName specifies the name of the desired font. The style of the font is specified byfontStyle. It may consist of one or more of thesethree constants: Font.PLAIN, Font.BOLD, andFont.ITALIC. The size, in points, of the font is specified by pointSize. point size may or maynot be the height of the characters.To draw characters in a font, first create an object of the class Font. then specify the font name,the font style, and the point size.


MethodsJava.awt.Graphics

void setFont(Font fontObj)- selects a font for the graphics context. That font will be usedfor subsequent text drawing operations.

Font getFont( )- get the current font by calling getFont( ).

Java.awt.Component void setFont(Font fontObj)

Example: The following program that draws several lines of text in different fonts.

import java.awt.*;


Demo(String s){


}


Font f = new Font("TimesRoman", Font.PLAIN, 18);

Font fb = new Font("TimesRoman", Font.BOLD, 18);

Font fi = new Font("TimesRoman", Font.ITALIC, 18);

Font fbi = new Font("TimesRoman", Font.BOLD + Font.ITALIC, 18);

g.setFont(f);

g.drawString("This is a plain font", 10, 50);

g.setFont(fb);

g.drawString("This is a bold font", 10, 75);

g.setFont(fi);

g.drawString("This is an italic font", 10, 100);

g.setFont(fbi);

g.drawString("This is a bold italic font", 10, 125);


Demo ob=new Demo("Font Demo");

}

}

Output:

}

Image: java.awt.ImageImage class provides support for displaying and manipulation of graphical images. Image issimply a rectangular graphical object.


Java provides the functions for reading images that are stored in local files and display them ongraphics object.

Step1: Loading an imagejava.awt.Toolkit

Toolkit getDefaultToolkit()- returns the default toolkit. Image getImage(String filename) - returns an image that will read its pixel data from a

file.Toolkit object can only read GIF and JPEG files.Step2: displaying an imagejava.awt.Graphics boolean drawImage(Image img, int x, int y, ImageObserver observer)- draws a scaled image. boolean drawImage(Image img, int x, int y, int width, int height, ImageObserver observer) -

draws a scaled image. The system scales the image to fit into a region with the given widthand height. Note: This call may return before the image is drawn.

Example: the following program draws a tiled graphics image from the top-left corner to bottomright corner of the window.

import java.awt.*;


Demo(String s){


}


Toolkit tk = Toolkit.getDefaultToolkit();

Image img= tk.getImage("E:/Sample/ball.jpg");

for (int i = 20; i <300-20; i=i+20)

for (int j = 40; j <300-20; j=j+20)

g.drawImage(img,i,j,20,20,null);

}


Demo ob=new Demo("Image Demo");

}

}

Output:


AWT Components All components are subclass of Component class Components allow the user to interact with application. A layout manager

arranges components within a container (Frame/Applet/Panel).

Adding and Removing Controls add(Component compObj)- add components to the conatainer. Once it is added, it will

automatically be visible whenever its parent window is displayed. Here, compObj is aninstance of the control that you want to add.

void remove(Component obj)- remove a control from a window

Component Constructor Methods

Label Label( )Label(String str)Label(String str, int how)

void setText(String str)String getText( )

Button Button( )Button(String str)

void setLabel(String str)String getLabel( )

List List( )List(int numRows)List(int numRows, boolean multipleSelect)

void add(String name)void add(String name, intindex)String getSelectedItem( )int getSelectedIndex( )String[ ] getSelectedItems( )

Choice Choice( ) void add(String name)String getSelectedItem( )int getSelectedIndex( )

Checkbox Checkbox( )Checkbox(String str)Checkbox(String str, boolean on)Checkbox(String str, boolean on, CheckboxGroup cbGroup)Checkbox(String str, CheckboxGroup cbGroup, boolean on)

boolean getState( )void setState(boolean on)String getLabel( )void setLabel(String str)

TextField TextField( )TextField(int numChars)TextField(String str)TextField(String str, int numChars)

String getText( )void setText(String str)void setEditable(booleancanEdit)

TextArea TextArea( )TextArea(int numLines, int numChars)TextArea(String str)TextArea(String str, int numLines, int numChars)

void append(String str)void insert(String str, intindex)


Scrollbars Scrollbar( )

Scrollbar(int style)

Scrollbar(int style, int initialValue, int thumbSize, int min, int max)

int getMinimum( )

int getMaximum( )

void setValues(int

initialValue, int thumbSize,

int min, int max)

Label Labels are components that hold text. Labels don’t react to user input. It is used to identify components.

Constructors Label(String str) - constructs a label with left-aligned text. Label(String str, int how) - constructs a label with the alignment specified by how.

Methods void setText(String str)- set the text in the label String getText( )- return the text of label

Example: The following example creates three labels and adds them to a frame..The labels areorganized in the frame by the flow layout manager.

import java.awt.*; Demo ob=new Demo("Label Demo");


Label lb1 = new Label("One");

Label lb2 = new Label("Two");

Label lb3 = new Label("Three");

FlowLayout flow= new FlowLayout();

Demo(String s){

super(s);

setSize(200,200);

setLayout(flow);

add(lb1);add(lb2);add(lb3);

setVisible(true);

}


}

}

Output:


ButtonA push button is a component that contains a label and that generates an event when it is pressed.Push buttons are objects of type Button.Constructors

Button( )- creates an empty button Button(String str)- creates a button that contains str as a label.

Methods void setLabel(String str) -set the label in the button String getLabel( ) -return the label of button

Example: The following example creates three buttons and adds them to a frame. The buttonsare organized in the frame by the flow layout manager.


import java.awt.*; Demo ob=new Demo("Button Demo");



}}

Output:

Button b=new Button();

Button b1=new Button();

Button b2=new Button("Button 2");

Demo(String s){

super(s);setSize(200,200);

setLayout(flow);

b1.setLabel("Button 1");

add(b);add(b1);add(b2);

setVisible(true);

}

ListThe List class provides a compact, multiple-choice, scrolling selection list. List object can beconstructed to display any number of choices in the visible window. It allows the user to selectmultiple items.Constructors

List( )- allows to select one item at any one time List(int numRows)- the value of numRows specifies the number of entries in the list that



will be visible always List(int numRows, boolean multipleSelect)- if multipleSelect is true, then the user may

select two or more items at a timeMethod

void add(String name)- Here, name is the name of the item added to the list. The firstform adds items to the end of the list.

void add(String name, int index) -adds the item at the index specified by index String getSelectedItem( )- return the selected item String[ ] getSelectedItems( )- return the selected items.

Example: The following example creates a list and adds it to a frame.

import java.awt.*;



List l1=new List(2);

List l2=new List(3);

List l3=new List(4,true);

Label lb1 = new Label("Dept");



Demo(String s){

super(s);

setSize(200,300);

setLayout(flow);

l1.add("CSE");l1.add("ECE");l1.add("EEE");l1.add("MECH");



add(lb1);add(l1);add(lb2);add(l2);add(lb3);add(l3);

setVisible(true);

}


Demo ob=new

Demo("List Demo");

}

}



CheckBoxA check box is a control that is used to turn an option on or off. It consists of a small box that caneither contain a check mark or not. There is a label associated with each check box that describeswhat option the box represents.Constructors

Checkbox( )- check box whose label is initially blank Checkbox(String str)- check box whose label is specified by str. Checkbox(String str, boolean on) - allows you to set the initial state of the check box. If

on is true, the check box is initially checked Checkbox(String str, boolean on, CheckboxGroup cbGroup)- group is specified by

cbGroup Checkbox(String str, CheckboxGroup cbGroup, boolean on)

Methods boolean getState( ) void setState(boolean on) String getLabel( ) void setLabel(String str)

CheckboxGroup

Create a set of mutually exclusive check boxes in which one and only one check box in the groupcan be checked at any one time. These check boxes are often called radio buttons. The defaultconstructor is defined, which creates an empty group.Example: The following example creates a checkbox group (Gender) and checkboxes(Languages Known).

import java.awt.*;



Label l1=new Label("Gender");

Label l2=new Label("Languages Known");

CheckboxGroup cg=new CheckboxGroup();

Checkbox c1=new Checkbox("Male",cg,true);

Checkbox c2=new Checkbox("Female",cg,false);


Checkbox c3=new Checkbox("VisualBasic");

Checkbox c4=new Checkbox("C++");

Checkbox c5=new Checkbox("Java");

Checkbox c6=new Checkbox("C");


}

}

Output:

Demo(String s){

super(s);

setSize(200,200);

setLayout(flow);

add(l1);add(c1);add(c2);

add(l2);add(c3);add(c4);add(c5);add(c6);

setVisible(true);

}


Demo ob=new Demo("Checkbox Demo");

ChoiceThe Choice class is used to create a pop-up list of items from which the user may choose. Itallows the user to select single item at any time. Choice only defines the default constructor,which creates an empty list.To add a item to the list, call add( ). It has this general form:

void add(String name) To determine which item is currently selected, you may call either getSelectedItem( ) or

getSelectedIndex( ).

import java.awt.*;



Label lb=new Label("City");

Choice c=new Choice();

Demo(String s){

super(s);

setSize(200,200);setLayout(flow);

c.add("Chennai");c.add("Coimbatore");

c.add("KanyaKumari");c.add("Madurai");

c.add("Tirunelveli");

add(lb);add(c);

setVisible(true);

}


Demo ob=new Demo("Checkbox Demo");

}

}



TextFieldText fields allow the user to enter strings and to edit the text using the arrow keys, cut and pastekeys, and mouse selections.

TextField( )- creates a default text field TextField(int numChars)- creates a text field that is numChars characters wide TextField(String str)- initializes the text field with the string contained in str TextField(String str, int numChars)

Methods String getText( ) void setText(String str)

TextAreaSimple multiline editor allow the user to enter strings.TextArea and TextField are subclass of TextComponent. Therefore, it supports the getText( ),setText( ), getSelectedText( ), select( ), isEditable( ), and setEditable( ) methods described inTextField class.

import java.awt.*;



Label lb1=new Label("Name");

Label lb2=new Label("No");

Label lb3=new Label("Message");

TextField t1=new TextField(20);


TextArea ta=new TextArea(2,10);

Demo(String s){

super(s);

setSize(250,200);

setLayout(flow);

add(lb1);add(t1);

add(lb2);add(t2);

add(lb3);add(ta);



setVisible(true);

}


Demo ob=new Demo("TextComponents Demo");

}

}

output:


Basics of Event Handling

Event handling concept is quite simple: a source generates an event and sends it to one or morelisteners. Listener simply waits until it receives an event. Once received, the listener processesthe event and then returns.

EventsAn event is an object that describes a state change in a source. Some of the activities that causeevents to be generated are pressing a button, entering a character via the keyboard, selecting anitem in a list, and clicking the mouse.Event SourcesA source is an object that generates an event. Sources may generate more than one type of event.A source must register listeners in order for the listeners to receive notifications about a specifictype of event.

Event ListenersListener is an object that is notified when an event occurs. It has two major requirements.

It must have been registered with one or more sources to receive notifications aboutspecific types of events.

It must implement methods to receive and process these notifications.The package java.awt.event defines several types of events that are generated by various userinterface elements.


Event Source Description

Button Generates action events when the button is pressed.

Checkbox Generates item events when the check box is selected or deselected.

Choice Generates item events when the choice is changed.

List Generates action events when an item is double-clicked; generates item

events when an item is selected or deselected.

Mouse Generates Mouse events when Mouse input occurs.

Keyboard Generates Key events when keyboard input occurs.


How event handling in the AWT works A listener object is an instance of a class that implements a special interface called

(naturally enough) a listener interface. An event source is an object that can register listener objects and send them event objects. The event source sends out event objects to all registered listeners when that event

occurs. The listener objects will then use the information in the event object to determine their

reaction to the event.You register the listener object with the source object by using lines of code that follow themodel:eventSourceObject.addEventListener(eventListenerObject);

AWT Event Hierarchy java.awt.event


Source Event Class Class Methods Listener Interface Interface Methods

Button ActionEvent String

getActionCommand( )

ActionListener actionPerformed(ActionEvent ae)

List,

Choice,

Checkbox

ItemEvent Object getItem( )

ItemSelectable

getItemSelectable( )

ItemListener itemStateChanged(ItemEvent ie)

Keyboard KeyEvent char getKeyChar( )

int getKeyCode( )

KeyListener keyPressed(KeyEvent ke)

keyReleased(KeyEvent ke)

keyTyped(KeyEvent ke)

Mouse MouseEvent int getX( )

int getY( )

MouseListener mouseClicked(MouseEvent me)

mouseEntered(MouseEvent me)

mouseExited(MouseEvent me)

mousePressed(MouseEvent me)

mouseReleased(MouseEvent me)

MouseMotionListener mouseDragged(MouseEvent me)

mouseMoved(MouseEvent me)

Handling Keyboard Events To handle keyboard events, class must implement the KeyListener interface.Register key listener to receive notifications about KeyEvents.addKeyListener(this);

Source: KeyBoardEvent Class: java.awt.event.KeyEventListener Interface: java.awt.event.KeyListener

Example: The following program demonstrates keyboard input. When program receiveskeystrokes, identifies the key and perform the corresponding actions specified by the program.

import java.awt.*;

import java.awt.event.*;

public class Demo extends Frame implements

KeyListener{

int x=20,y=20;

Demo(String s){

super(s);

setSize(300,300);

setVisible(true);

addKeyListener(this);

}

public void keyPressed(KeyEvent ke){

int kc=ke.getKeyCode();

if(kc==37 && x<300)

x=x-10;

if(kc==39 && x>0)

x=x+10;

if(kc==38 && y>0)

y=y-10;

if(kc==40 && y<300)

y=y+10;

repaint();

}

public void keyTyped(KeyEvent ke){

}

public void keyReleased(KeyEvent ke){

}

public void paint(Graphics g){

g.drawRect(x,y,30,30);

}


Demo ob=new Demo("Key event Demo");

}

}

Output:


Handling Mouse Events To handle Mouse events, class must implement the MouseListener & MouseMotionLiatenerinterface. Register mouse listener & mouse motion listener to receive notifications aboutMouseEvents.addMouseListener(this);addMouseMotionListener(this);Source: MouseEvent Class: java.awt.event.MouseEventListener Interface: java.awt.event.MouseListener

java.awt.event.MouseMotionListenerExample: The following program demonstrates Mouse event handling. When user drag themouse it draws a line along the motion path.

import java.awt.*;



MouseListener,MouseMotionListener{

int x1,y1,x2,y2;

Demo(String s){

super(s);

setSize(300,300);

setVisible(true);

addMouseListener(this);

addMouseMotionListener(this);

}

public void mouseDragged(MouseEvent me){

x1=x2;y1=y2;

x2=me.getX();

y2=me.getY();

Graphics g=this.getGraphics();

g.drawLine(x1,y1,x2,y2);

}

public void mouseMoved(MouseEvent me){}

public void mouseClicked(MouseEvent me){}

public void mouseEntered(MouseEvent me){}

public void mouseExited(MouseEvent me){

}

public void mousePressed(MouseEvent me){

x1=x2=me.getX();

x1=y2=me.getY();

}

public void mouseReleased(MouseEvent me){}


public void paint(Graphics g){}


Demo ob=new Demo("Mouse event Demo");

}

}


Action Event To handle Action events, class must implement the ActionListener interface. Source objectsneeds to register action listener to receive events.SourceObject.addActionListener(this);Source: ButtonEvent Class: java.awt.event.ActionEventListener Interface: java.awt.event.ActionListener

Example: The following program demonstrates Action event handling. Application window has3 Text Fields and two buttons. Text fields are used to get user input and show the result to theuser. When the user presses Button, it will generate an action event. Listener receives the eventand performs the operation specified in the actionPerformed Method.

import java.awt.*;


import javax.swing.*;


ActionListener{

FlowLayout flow=new FlowLayout();

Label lb1 = new Label("Number1");

Label lb2 = new Label("Number2");

Label lb3 = new Label("Result");




Button b1=new Button("Add");

Button b2=new Button("Multiply");

Demo(String s){

super(s);

setLayout(flow);

add(lb1);add(t1);

add(lb2);add(t2);

add(b1);add(b2);

add(lb3);add(t3);

b1.addActionListener(this);

b2.addActionListener(this);

setSize(280,280);

setVisible(true);

}

public void actionPerformed(ActionEvent ae){

String s=ae.getActionCommand();

int n1,n2;

n1=Integer.parseInt(t1.getText());

n2=Integer.parseInt(t2.getText());

if(s.equals("Add")){

t3.setText(String.valueOf(n1+n2));

}

if(s.equals("Multiply")){

t3.setText(String.valueOf(n1*n2));

}

}


Demo my=new Demo("Action event

demo");

}

}


Item Event To handle Item events, class must implement the ItemListener interface. Source objects needs toregister action listener to receive events.SourceObject.addActionListener(this);Source: List, Choice, CheckboxEvent Class: java.awt.event.ItemEventListener Interface: java.awt.event.ItemListener

Example: The following program demonstrates Item event handling. Application window hasText Field, Checkbox, and Choice. Text fields are used to show the result to the user. When theuser select item from any component, then the selected item will be displayed in text field. Alistener receives the event and performs the operation specified in the itemStateChanged()Method.

import java.awt.*;



ItemListener{

String msg="";


Label lb1 = new Label("City");

Label lb2 = new Label("Qualification");

Label lb3 = new Label("Languages Known");

Choice c=new Choice();

CheckboxGroup cg=new CheckboxGroup();

Checkbox c1=new Checkbox("UG",cg,true);

Checkbox c2=new Checkbox("PG",cg,false);

Checkbox c3=new Checkbox("Visual Basic");

Checkbox c4=new Checkbox("C++");

Checkbox c5=new Checkbox("Java");

Checkbox c6=new Checkbox("dotnet");

TextField t=new TextField(20);

Demo(String s){

super(s);

setLayout(flow);

c.add("Chennai");c.add("Coimbatore");

c.add("KanyaKumari");c.add("Madurai");

c.add("Tirunelveli");

add(lb1);add(c);

add(lb2);add(c1);add(c2);

add(lb3);add(c3);add(c4);add(c5);add(c6);

add(t);

setSize(200,300);

setVisible(true);

c.addItemListener(this);

c1.addItemListener(this);








}

public void itemStateChanged(ItemEvent ie){

String msg=(String)ie.getItem();

t.setText(msg);

}


Demo ob=new Demo("List event Demo");

}

}

Output:

Layout Management Layout manager automatically arranges several components within a window. Each containerobject has a layout manager associated with it.

Panel,Applet - Flow Layout Frame - Border Layout

Whenever a container is resized, the layout manager is used to position each of the componentswithin it. General syntax for setting layout to container

Void setLayout(LayouManager obj)Layout Managers are

FlowLayout BorderLayout Grid Layout GridbagLayout BoxLayout

Arrange component without using layout Manager:You can position components manually using setBounds() method defined by

Component class.1. Disable the default manager of your container.

setLayout(null);2. Givethe location and size of the component which is to be added in the container.



setBounds(int x, int y, int width, int height);eg:Button b=new Button(―click me‖);b.setBounds(10,10,50,20);FlowLayoutFlowLayout arranges the components in rows from left-to-right and top-to-bottom order basedon the order in which they were added to the container.FlowLayout arranges components in rows, and the alignment specifies the alignment of the rows.For example, if you create a FlowLayout that’s left aligned, the components in each row willappear next to the left edge of the container.The FlowLayout constructors allow you to specify the horizontal and vertical gaps that shouldappear between components, and if you use a constructor that doesn’t accept these values, theyboth default to 5.Constructor:

FlowLayout() - create default layout, which centers component and leaves 5 pixelsspaces between each component.

FlowLayout(int how)-specify how ech line is aligned.

The flow layout manager lines the components horizontally until there is no more room and thenstarts a new row of components. When the user resizes the container, the layout managerautomatically reflows the components to fill the available space. If you reduce the width of theframe further, then portions of the wider components begin to disappear. Similarly, if you reducethe frame’s vertical size so that there’s not enough vertical space to display all rows, some of thecomponents will become partially or completely inaccessible

import java.awt.*;



Button b1=new Button("one");

Button b2=new Button("Two");

Button b3=new Button("Three");

Demo(String s){

super(s);

setLayout(flow);

add(b1);add(b2);add(b3);

setSize(200,200);

setVisible(true);

}


Demo ob=new Demo("FlowLayout Demo");

}

}



GridLayoutThe GridLayout layout manager divides the available space into a grid of cells, evenly allocatingthe space among all the cells in the grid and placing one component in each cell.Cells are always same size. When you resize the window, the cells grow and shrink, but all thecells have identical sizes.Constructor

GridLayout(int rows, int cols)- construct a grid with specified rows and cols.

GridLayout(int rows, int cols, int hspace, int vspace) - to specify the amount of horizontaland vertical space that should appear between adjacent components.

When you create a GridLayout, you can specify a value of 0 for either the row count or thecolumn count, but not both. If you set the number of rows to 0, GridLayout creates as many rowsas it needs to display all the components using the specified number of columns.

import java.awt.*;


GridLayout grid=new GridLayout(2,2);

Demo ob=new Demo("GridLayout Demo");

}

}




Demo(String s){

super(s);

setLayout(grid);

add(b1);add(b2);add(b3);

setSize(200,200);

setVisible(true);

}


BorderLayout

BorderLayout divides the container into five areas, and you can add a component to each area.



The five regions correspond to the top, left, bottom, and right sides of the container, along withone in the center. Each of the five areas is associated with a constant value defined inBorderLayout: NORTH, SOUTH, EAST, WEST, and CENTER for the top, bottom, right, left,and center regions, respectively.Constructor

BorderLayout() BorderLayout(int hspace, int vspace) – leave space between components.

Border layout grows all components to fill the available space.You can add components byspecifying a constraint - BorderLayout.CENTER|NORTH|SOUTH|EAST|WESTvoid add(Component obj, constraint)

import java.awt.*;


BorderLayout grid=new BorderLayout();




Button b4=new Button("four");

Button b5=new Button("five");

Demo(String s){

super(s);

setLayout(grid);

add(b1,BorderLayout.NORTH);

add(b2,BorderLayout.SOUTH);

add(b3,BorderLayout.CENTER);

add(b4,BorderLayout.EAST);

add(b5,BorderLayout.WEST);

setSize(200,200);

GridBag Layout - Gridlayout without limitations

setVisible(true);

}


Demo ob=new Demo("BorderLayout Demo");

}

}


In Grid bag layout, the rows and columns have variable sizes. It is possible to merge twoadjacent cells and make a space for placing larger components.To describe the layout to grid bag manager, you must follow the procedure

1. Create an object of type GridBagLayout. No need to specify rows and column.2. Set this GridBagLayout object to the container.3. Create an object of type GridBagConstraints. This object will specify how the

components are laid out within the grid bag.4. For each components, fill in the GridBagConstraints object.Finally add the component

with the constraint by using the calladd(Component, constraint);

GridBagConstraints: Gridx – specify the column position of the component to be added Gridy - specify the row position of the component to be added Gridwidth- specify how many columns occupied by the component Gridheight - specify how many rows occupied by the component

import java.awt.*;


GridBagLayout gb=new GridBagLayout();

GridBagConstraints gc1= new GridBagConstraints();






Demo(String s){

super(s);

setLayout(gb);

gc1.gridx=0;

gc1.gridy=0;

gc1.gridwidth=2;

gc1.gridheight=1;

gc2.gridx=0;

gc2.gridy=1;

gc2.gridwidth=1;

gc2.gridheight=1;

gc3.gridx=1;

BoxLayout – javax.swing

gc3.gridy=1;

gc3.gridwidth=1;

gc3.gridheight=1;

add(b1,gc1);add(b2,gc2);add(b3,gc3);

setSize(200,200);

setVisible(true);

}


Demo ob=new Demo("GridBagLayout Demo");

}}


Arrange components in single row or single column. Components are arranged either verticallyfrom top to bottom or horizontally from left to right

BoxLayout(Container Obj, BoxLayout.X_AXIS | Y_AXIS);

Adapter classes An adapter class provides an empty implementation of all methods in an event listener interface.It is useful when you want to receive and process only some of the events that are handled by anevent listener interface.Define a new class that extends the adapter class and overrides the desired methods only.Extends adapter class to create Listener and override the desired methods. If you implement theinterface, you have to define all of methods in it. This adapter class defines null methods for allevents, so you can only redefine the methods that you needed.Some Adapter classes are

KeyAdapter MouseAdapter MouseMotionAdapter

Example: If the user only interest in mouse pressed event,then simply extends MouseAdapterclass and redefine the mousePressed event. The following program listening MouseEventwhenever the user press the mouse it will print ―mouse pressed‖ text on command prompt.

import java.awt.*;


public class Demo extends MouseAdapter{


Frame f=new Frame("Mouse Adapter Demo");

f.setSize(200,200);

f.setVisible(true);

Demo ob=new Demo();

f.addMouseListener(ob);

}

public void mousePressed(MouseEvent me){

System.out.println("Mouse pressed..");

}

}



Model-View-Controller design pattern Swing uses the model-view-controller architecture (MVC) as the fundamental designbehind each of its components. Essentially, MVC breaks GUI components into threeelements. Each of these elements plays a crucial role in how the component behaves.

Model - (includes state data for each component)The model encompasses the state data for each component. There are different models fordifferent types of components. Model has no user interface. Model data always existsindependent of the component's visual representation.For example, model of a Choice list might contain the information about list of items andcurrently selected item. This information remains the same no matter how the componentis painted on the screen.

View - (to display component on screen)The view refers to how you see the component on the screen. It determines exactly whereand how to draw the choice list by the information offered by the model.

Controller- (handles user Input)The controller decides the behavior of each component with respect to the events. Eventscome in many forms (a mouse click, a keyboard event).The controller decides how eachcomponent will react to the event—if it reacts at all.

© A.V.C college of engineering


MVC Interaction In MVC, each of the three elements—the model, the view, and the controller—requiresthe services of another element to keep itself continually updated.


Introduction to Swing

Swing Features Pluggable look-and feels -- as above Lightweight components

o Do not depend on native peers to render themselves.o Simplified graphics to paint on screeno Similar behaviour across all platformso Portable look and feelo Only a few top level containers not lightweight.

New components -- tress tables, sliders progress bars, frames, text components. Tooltips -- textual popup to give additional help arbitrary keyboard event binding Debugging support

Difference between AWT and Swing


S.No AWT(Abstract Window Toolkit) Swing

1 AWT components are heavy weight,

components are platform dependent.

Swing components are light weight,

components are platform independent.

2 AWT components support Delegate

Event Model.

Swing components support MVC (model,

view, control architecture)

3 AWT components provide static look

and feel.

Swing components provide dynamic look

and feel

4 It does not provide Tooltip text for

components

It provide Tooltip text for components

javax.swing


Swing Components

The JLabel Class


Component Constructor Methods

JLabel JLabel()

JLabel(String text)

JLabel(String text, int horizontalAlignment)

void setText(String str)

String getText( )

JButton JButton( )

JButton(String str)

void setLabel(String str)

String getLabel( )

JList JList()

JList(String[] )

Object getSelectedValue( )

int getSelectedIndex( )

String[] getSelectedItems( )

JRadioButton JRadioButton()

JRadioButton(String text)

JRadioButton(String text, boolean selected)

JRadioButton(String text, Icon icon, boolean

selected)

JComboBox JComboBox()

JComboBox(Object items[])

void add(String name)

String getSelectedItem( )

int getSelectedIndex( )

JCheckbox JCheckbox( )

JCheckbox(String str)

JCheckbox(String str, boolean on)

JCheckBox(String text, Icon icon)

boolean getState( )

void setState(boolean on)

String getLabel( )

void setLabel(String str)

JTextField JTextField(int numChars)

JTextField(String str, int numChars)

String getText( )

void setText(String str)

void setEditable(boolean

canEdit)

JTextArea JTextArea(int numLines, int numChars)

JTextArea(String str, int numLines, int numChars)

void append(String str)

void insert(String str, int index)

JPasswordField JPasswordField(String text, int columns) void setEchoChar(char echo)


Swing allows you to create labels that can contain text, images, or both. Unlike

java.awt.Label objects, JLabel objects may consist of both text and graphics (icons).

Text Input

In Java, two components are used to get text input:

JTextField

JTextArea.

The difference between them is that a text field can accept only one line of text and a text

area can accept multiple lines of text. The classes are called JTextField for single-line

input and JTextArea for multiple lines of text.

The JPasswordField Class

Password fields are a special kind of text field. To avoid nosy bystanders being able to

glance at a password, the characters that the user entered are not actually displayed.

Instead, each typed character is represented by an echo character, typically an asterisk

(*). The Swing set supplies a JPasswordField class that implements such a text field.

The JButton Class

They are typically used much like java.awt.Buttons. JButtons fire ActionEvents when

they are clicked.

The JCheckBox Class

It is used to allow the user to turn a given feature on or off, or to make multiple selections

from a set of choices. A JCheckBox is usually rendered by showing a small box into

which a "check" is placed when selected. The user could check either, both, or none of

the two check boxes.

The JRadioButton Class

JRadioButtons, allowing users to make a single selection from a set of options.



The JList Class A list is a graphical component from which the user can select choices. Lists typicallydisplay several items at a time, allowing the user to make either a single selection ormultiple selections.AWT limited the contents of its List component to strings. The SwingJList component lifts this restriction. List elements can now be strings, images.The JComboBox Class A combo box component is actually a combination of a Swing list and a text field. Unlikelists, a combo box only allows the user one selection at a time, which is usually copiedinto an editable component at the top, such as a text field. The user can be permitted,however, to manually enter in a selection as well.JComboBox is the Swing version of a combo box component. It is very similar to theAWT Choice component.The JPanel Class JPanel is an extension of JComponent (which, remember, extends java.awt.Container)used for grouping together other components. It gets most of its implementation from itssuperclasses.Typically, using JPanel amounts to instantiating it, setting a layout manager (this can beset in the constructor and defaults to a FlowLayout), and adding components to it usingthe add() methods inherited from Container.



Unit IV

Generic Programming:

Generics are a facility of generic programming that was added to the Java programminglanguage.Generic programming is a style of computer programming in which algorithms arewritten in terms of to-be-specified-later types that are then instantiated when needed forspecific types provided as parameters.

As per Java Language Specification:

A type variable is an unqualified identifier. Type variables are introduced bygeneric class declarations, generic interface declarations, generic methoddeclarations, and by generic constructor declarations.

A class is generic if it declares one or more type variables. These type variablesare known as the type parameters of the class. It defines one or more typevariables that act as parameters. A generic class declaration defines a set ofparameterized types, one for each possible invocation of the type parametersection. All of these parameterized types share the same class at runtime.

An interface is generic if it declares one or more type variables. These typevariables are known as the type parameters of the interface. It defines one or moretype variables that act as parameters. A generic interface declaration defines a setof types, one for each possible invocation of the type parameter section. Allparameterized types share the same interface at runtime.

A method is generic if it declares one or more type variables. These type variablesare known as the formal type parameters of the method. The form of the formaltype parameter list is identical to a type parameter list of a class or interface.

A constructor can be declared as generic, independently of whether the class theconstructor is declared in is itself generic. A constructor is generic if it declaresone or more type variables. These type variables are known as the formal typeparameters of the constructor. The form of the formal type parameter list isidentical to a type parameter list of a generic class or interface.

Motivation for Generics:Overloaded methods are often used to perform similar operations on different types ofdata. To motivate generic methods, let’s begin with an example that contains threeoverloaded printArray methods These methods print the string representations of theelements of an Integer array, a Double array and a Character array, respectively. Notethat we could have used arrays of primitive types int, double and char in this example.We chose to use arrays of typeInteger, Double and Character to set up our genericmethod example, because only reference types can be used with generic methods andclasses.


http://www.worldlingo.com/ma/enwiki/en/Java_(programming_language)




public class OverloadedMethods{// method printArray to print Integer arraypublic static void printArray( Integer[] inputArray ){

// display array elementsfor ( Integer element : inputArray )

System.out.printf( "%s ", element );

System.out.println();} // end method printArray

// method printArray to print Double arraypublic static void printArray( Double[] inputArray ){

// display array elementsfor ( Double element : inputArray )



// method printArray to print Character arraypublic static void printArray( Character[] inputArray ){

// display array elementsfor ( Character element : inputArray )



public static void main( String args[] ){

// create arrays of Integer, Double and CharacterInteger[] integerArray = { 1, 2, 3, 4, 5, 6 };Double[] doubleArray = { 1.1, 2.2, 3.3, 4.4, 5.5, 6.6, 7.7 };

Character[] characterArray = { 'H', 'E', 'L', 'L', 'O' };

System.out.println( "Array integerArray contains:" );printArray( integerArray ); // pass an Integer arraySystem.out.println( "\nArray doubleArray contains:" );printArray( doubleArray ); // pass a Double arraySystem.out.println( "\nArray characterArray contains:" );printArray( characterArray ); // pass a Character array


} // end main} // end class OverloadedMethods

Array integerArray contains:123456

Array doubleArray contains:1.1 2.2 3.3 4.4 5.5 6.6 7.7

Array characterArray contains:HELLO

Generic class definitions

Here is an example of a generic class:

public class Pair<T, S>{public Pair(T f, S s){first = f;second = s;}

public T getFirst(){return first;}

public S getSecond(){return second;}

public String toString(){return "(" + first.toString() + ", " + second.toString() + ")";}

private T first;private S second;}

This generic class can be used in the following way:Pair<String, String> grade440 = new Pair<String, String>("mike", "A");Pair<String, Integer> marks440 = new Pair<String, Integer>("mike",100);System.out.println("grade:" + grade440.toString());System.out.println("marks:" + marks440.toString());

Type erasure

Generics are checked at compile-time for type correctness. The generic type informationis then removed via a process called type erasure. For example, List<Integer> will be

converted to the raw type (non-generic type) List, which can contain arbitrary objects.However, due to the compile-time check, the resulting code is guaranteed to be typecorrect, as long as the code generated no unchecked compiler warnings.

As a result, there is no way to tell at runtime which type parameter is used on an object.For example, when you examine an ArrayList at runtime, there is no general way to tellwhether it was an ArrayList<Integer> or an ArrayList<Float>. The exception to thisis by using Reflection on existing list elements. However, if the list is empty or if itselements are subtypes of the parameterized type, even Reflection will not divulge theparameterized type.

The following code demonstrates that the Class objects appear the same.ArrayList<Integer> li = new ArrayList<Integer>();ArrayList<Float> lf = new ArrayList<Float>();if (li.getClass() == lf.getClass()) // evaluates to trueSystem.out.println("Equal");

Java generics differ from C++ templates. Java generics generate only one compiledversion of a generic class or function regardless of the number of types used.Furthermore, the Java compiler does not need to know which parameterized type is usedbecause the type information is validated at compile-time and erased from the compiledcode. Consequently, one cannot instantiate a Java class of a parameterized type becauseinstantiation requires a call to a constructor, which is not possible when the type isunknown at both compile-time and runtime.T instantiateElementType(List<T> arg){return new T(); //causes a compile error}

Because there is only one copy of a generic class, static variables are shared among allthe instances of the class, regardless of their type parameter. As a result, the typeparameter cannot be used in the declaration of static variables or in static methods. Staticvariables and static methods are "outside" of the scope of the class's parameterized types.

Generic methods:

Generic Methods

Genericity is not limited to classes and interfaces, you can define generic methods. Staticmethods, nonstatic methods, and constructors can all be parameterized in almost the sameway as for classes and interfaces, but the syntax is a bit different. Generic methods arealso invoked in the same way as non-generic methods.

Before we see an example of a generics method, consider the following segment of codethat prints out all the elements in a collection:public void printCollection(Collection c) {

Iterator i = c.iterator();for(int k = 0;k<c.size();k++) {System.out.println(i.next());

}

}

Using generics, this can be re-written as follows. Note that the Collection<?> is thecollection of an unknown type.void printCollection(Collection<?> c) {

for(Object o:c) {System.out.println(o);

}

}

Example:

public class GenericMethodTest{

// generic method printArraypublic static < E > void printArray( E[] inputArray ){

// display array elementsfor ( E element : inputArray )



public static void main( String args[] ){

// create arrays of Integer, Double and CharacterInteger[] intArray = { 1, 2, 3, 4, 5 };Double[] doubleArray = { 1.1, 2.2, 3.3, 4.4, 5.5, 6.6, 7.7 };Character[] charArray = { 'H', 'E', 'L', 'L', 'O' };

System.out.println( "Array integerArray contains:" );printArray( integerArray ); // pass an Integer arraySystem.out.println( "\nArray doubleArray contains:" );printArray( doubleArray ); // pass a Double arraySystem.out.println( "\nArray characterArray contains:" );printArray( characterArray ); // pass a Character array

} // end main} // end class GenericMethodTest

Array integerArray contains:1 2 3 4 5 6

Array doubleArray contains:1.1 2.2 3.3 4.4 5.5 6.6 7.7

Array characterArray contains:H E L L O

Bounded Type Parameters:There may be times when you'll want to restrict the kinds of types that are allowed to bepassed to a type parameter. For example, a method that operates on numbers might only

want to accept instances of Number or its subclasses. This is what bounded typeparameters are for.

To declare a bounded type parameter, list the type parameter's name, followed by theextends keyword, followed by its upper bound, which in this example is Number. Notethat, in this context, extends is used in a general sense to mean either "extends" (as inclasses) or "implements" (as in interfaces).public class Box<T> {

private T t;

public void add(T t) {this.t = t;

}

public T get() {return t;

}

public void inspect(U u){System.out.println("T: " + t.getClass().getName());System.out.println("U: " + u.getClass().getName());

}

public static void main(String[] args) {Box<Integer> integerBox = new Box<Integer>();integerBox.add(new Integer(10));integerBox.inspect("some text"); // error: this is still

String!}

}

By modifying our generic method to include this bounded type parameter, compilationwill now fail, since our invocation of inspect still includes a String:Box.java:21: inspect(U) in Box<java.lang.Integer> cannot

be applied to (java.lang.String)integerBox.inspect("10");

^1 error

To specify additional interfaces that must be implemented, use the & character, as in:



Exceptions:

Exceptions are such anomalous conditions (or typically an event) whichchanges the normal flow of execution of a program. Exceptions are used forsignaling erroneous (exceptional) conditions which occur during the run timeprocessing. Exceptions may occur in any programming language.


Occurrence of any kind of exception in java applications may result in anabrupt termination of the JVM or simply the JVM crashes which leaves theuser unaware of the causes of such anomalous conditions. However Javaprovides mechanisms to handle such situations through its superb exceptionhandling mechanism. The Java programming language uses Exceptionclasses to handle such erroneous conditions and exceptional events.

Exception Hierarchy:There are three types of Exceptions:

1. Checked Exceptions - These are the exceptions which occur duringthe compile time of the program. The compiler checks at the compiletime that whether the program contains handlers for checked exceptionsor not. These exceptions do not extend RuntimeException class andmust be handled to avoid a compile-time error by the programmer.These exceptions extend the java.lang.Exception class These exceptionalconditions should be anticipated and recovered by an application.Furthermore Checked exceptions are required to be caught. Rememberthat all the exceptions are checked exceptions unless and until thoseindicated by Error, RuntimeException or their subclasses.For example if you call the readLine() method on a BufferedReaderobject then the IOException may occur or if you want to build a programthat could read a file with a specific name then you would be promptedto input a file name by the application. Then it passes the name to theconstructor for java.io.FileReader and opens the file. However if you donot provide the name of any existing file then the constructorthrowsjava.io.FileNotFoundException which abrupt the application tosucceed. Hence this exception will be caught by a well-written

application and will also prompt to correct the file name.

Here is the list of checked exceptions.



NoSuchFieldException

InstantiationException

IllegalAccessException

ClassNotFoundException

NoSuchMethodException

CloneNotSupportedException

InterruptedException

2. Unchecked Exceptions - Unchecked exceptions are the exceptionswhich occur during the runtime of the program. Unchecked exceptionsare internal to the application and extendthe java.lang.RuntimeException that is inheritedfrom java.lang.Exceptionclass. These exceptions cannot be anticipated andrecovered like programming bugs, such as logic errors or improper useof an API. These type of exceptions are also calledRuntimeexceptions that are usually caused by data errors, like arithmeticoverflow, divide by zero etc.

Lets take the same file name example as described earlier. In thatexample the file name is passed to the constructor for FileReader.However, the constructor will throwNullPointerException if a logic errorcauses a null to be passed to the constructor. Well in this case theexception could be caught by the application but it would rather try toeliminate the bug due to which the exception has occurred. You musthave encountered the most common exception in your program i.e.the ArithmeticException. I am sure you must be familiar with the reasonof its occurrence that is when something tries to divide by zero. Similarlywhen an instance data member or method of a reference variable is tobe accessed that hasn't yet referenced an objectthrows NullPointerException.

Here is the list of unchecked exceptions.

IndexOutOfBoundsException

ArrayIndexOutOfBoundsException

ClassCastException

ArithmeticException

NullPointerException

IllegalStateException

SecurityException



3. Error - The errors in java are external to the application. These are theexceptional conditions that could not be usually anticipated by theapplication and also could not be recovered from. Error exceptionsbelong to Error and its subclasses are not subject to the catch orSpecify requirement. Suppose a file is successfully opened by anapplication for input but due to some system malfunction could not beable to read that file then the java.io.IOError would be thrown. This errorwill cause the program to terminate but if an application wants then theerror might be caught. An Error indicates serious problems that areasonable application should not try to catch. Most such errors areabnormal conditions.

Hence we conclude that Errors and runtime exceptions are togethercalled as unchecked exceptions.

Throwing Exceptions

If a method needs to be able to throw an exception, it has todeclare the exception(s) thrown in the method signature, andthen include a throw-statement in the method. Here is anexample:

public void divide(int numberToDivide, int numberToDivideBy)throws BadNumberException{

if(numberToDivideBy == 0){throw new BadNumberException("Cannot divide by 0");

}return numberToDivide / numberToDivideBy;

}

When an exception is thrown the method stops execution rightafter the "throw" statement. Any statements following the "throw"statement are not executed. In the example above the "returnnumberToDivide / numberToDivideBy;" statement is notexecuted if a BadNumberException is thrown. The programresumes execution when the exception is caught somewhere bya "catch" block. Catching exceptions is explained later.

You can throw any type of exception from your code, as long asyour method signature declares it. You can also make up yourown exceptions. Exceptions are regular Java classes that



extends java.lang.Exception, or any of the other built-inexception classes. If a method declares that it throws anexception A, then it is also legal to throw subclasses of A.

Catching Exceptions

If a method calls another method that throws checkedexceptions, the calling method is forced to either pass theexception on, or catch it. Catching the exception is done using atry-catch block. Here is an example:

public void callDivide(){try {

int result = divide(2,1);System.out.println(result);

} catch (BadNumberException e) {//do something clever with the exceptionSystem.out.println(e.getMessage());

}System.out.println("Division attempt done");

}

The BadNumberException parameter e inside the catch-clausepoints to the exception thrown from the divide method, if anexception is thrown.

If no exeception is thrown by any of the methods called orstatements executed inside the try-block, the catch-block issimply ignored. It will not be executed.

If an exception is thrown inside the try-block, for instance fromthe divide method, the program flow of the calling method,callDivide, is interrupted just like the program flow inside divide.The program flow resumes at a catch-block in the call stack thatcan catch the thrown exception. In the example above the"System.out.println(result);" statement will not get executed if anexception is thrown fromt the divide method. Instead programexecution will resume inside the "catch (BadNumberExceptione) { }" block.

If an exception is thrown inside the catch-block and thatexception is not caught, the catch-block is interrupted just likethe try-block would have been.



When the catch block is finished the program continues with anystatements following the catch block. In the example above the"System.out.println("Division attempt done");" statement willalways get executed.

Propagating Exceptions

You don't have to catch exceptions thrown from other methods.If you cannot do anything about the exception where the methodthrowing it is called, you can just let the method propagate theexception up the call stack to the method that called thismethod. If you do so the method calling the method that throwsthe exception must also declare to throw the exception. Here ishow the callDivide() method would look in that case.

public void callDivide() throws BadNumberException{int result = divide(2,1);System.out.println(result);

}

Notice how the try-catch block is gone, and the callDividemethod now declares that it can throw a BadNumberException.The program execution is still interrupted if an exception isthrown from the divide method. Thus the"System.out.println(result);" method will not get executed if anexception is thrown from the divide method. But now theprogram execution is not resumed inside the callDivide method.The exception is propagated to the method that calls callDivide.Program execution doesn't resume until a catch-blocksomewhere in the call stack catches the exception. All methodsin the call stack between the method throwing the exception andthe method catching it have their execution stopped at the pointin the code where the exception is thrown or propagated.

Example: Catching IOException's

If an exception is thrown during a sequence of statements insidea try-catch block, the sequence of statements is interrupted andthe flow of control will skip directly to the catch-block. This codecan be interrupted by exceptions in several places:

public void openFile(){try {

// constructor may throw FileNotFoundExceptionFileReader reader = new FileReader("someFile");



int i=0;while(i != -1){

//reader.read() may throw IOExceptioni = reader.read();System.out.println((char) i );

}reader.close();System.out.println("--- File End ---");

} catch (FileNotFoundException e) {//do something clever with the exception

} catch (IOException e) {//do something clever with the exception

}}

If the reader.read() method call throws an IOException, thefollowing System.out.println((char) i ); is not executed. Neither isthe last reader.close() or the System.out.println("--- File End ---"); statements. Instead the program skips directly to thecatch(IOException e){...}catchclause.IfthenewFileReader("someFile"); constructor call throws an exception,none of the code inside the try-block is executed.

Example: Propagating IOException's

This code is a version of the previous method that throws theexceptions instead of catching them:

public void openFile() throws IOException {FileReader reader = new FileReader("someFile");int i=0;while(i != -1){

i = reader.read();System.out.println((char) i );

}reader.close();System.out.println("--- File End ---");

}

If an exception is thrown from the reader.read() method thenprogram execution is halted, and the exception is passed up thecall stack to the method that called openFile(). If the callingmethod has a try-catch block, the exception will be caught there.If the calling method also just throws the method on, the callingmethod is also interrupted at the openFile() method call, and the



exception passed on up the call stack. The exception ispropagated up the call stack like this until some method catchesthe exception, or the Java Virtual Machine does.

Finally

You can attach a finally-clause to a try-catch block. The codeinside the finally clause will always be executed, even if anexception is thrown from within the try or catch block. If yourcode has a return statement inside the try or catch block, thecode inside the finally-block will get executed before returningfrom the method. Here is how a finally clause looks:

public void openFile(){FileReader reader = null;try {

reader = new FileReader("someFile");int i=0;while(i != -1){


}} catch (IOException e) {

//do something clever with the exception} finally {

if(reader != null){try {

reader.close();} catch (IOException e) {

//do something clever with the exception}

}System.out.println("--- File End ---");

}}

No matter whether an exception is thrown or not inside the try orcatch block the code inside the finally-block is executed. Theexample above shows how the file reader is always closed,regardless of the program flow inside the try or catch block.

Note: If an exception is thrown inside a finally block, and it is notcaught, then that finally block is interrupted just like the try-blockand catch-block is. That is why the previous example had the



reader.close() method call in the finally block wrapped in a try-catch block:

} finally {if(reader != null){

try {reader.close();

} catch (IOException e) {//do something clever with the exception

}}System.out.println("--- File End ---");

}

That way the System.out.println("--- File End ---"); method callwill always be executed. If no unchecked exceptions are thrownthat is. More about checked and unchecked in a later chapter.

You don't need both a catch and a finally block. You can haveone of them or both of them with a try-block, but not none ofthem. This code doesn't catch the exception but lets itpropagate up the call stack. Due to the finally block the code stillcloses the filer reader even if an exception is thrown.

public void openFile() throws IOException {FileReader reader = null;try {

reader = new FileReader("someFile");int i=0;while(i != -1){


}} finally {

if(reader != null){try {

reader.close();} catch (IOException e) {

//do something clever with the exception}

}System.out.println("--- File End ---");

}}



Notice how the catch block is gone.

Logging:

The JDK contains the "Java Logging API". Via a logger you can save text to acentral place to report on errors, provide additional information about your program, etc.This logging API allows to configure how messages are written by which class withwhich priority

Overview of Control Flow

Applications make logging calls on Logger objects. Loggers areorganized in a hierarchical namespace and child Loggers mayinherit some logging properties from their parents in thenamespace.

Applications make logging calls on Logger objects. TheseLogger objects allocate LogRecord objects which are passedto Handler objects for publication. Both Loggers and Handlersmay use logging Levels and (optionally) Filters to decide if theyare interested in a particular LogRecord. When it is necessary topublish a LogRecord externally, a Handler can (optionally) usea Formatter to localize and format the message beforepublishing it to an I/O stream.

Each Logger keeps track of a set of output Handlers. By defaultall Loggers also send their output to their parent Logger. ButLoggers may also be configured to ignore Handlers higher upthe tree.

Some Handlers may direct output to other Handlers. Forexample, the MemoryHandler maintains an internal ring buffer

of LogRecords and on trigger events it publishes its LogRecordsthrough a target Handler. In such cases, any formatting is doneby the last Handler in the chain.



The APIs are structured so that calls on the Logger APIs can becheap when logging is disabled. If logging is disabled for a givenlog level, then the Logger can make a cheap comparison testand return. If logging is enabled for a given log level, the Loggeris still careful to minimize costs before passing the LogRecordinto the Handlers. In particular, localization and formatting(which are relatively expensive) are deferred until the Handlerrequests them. For example, a MemoryHandler can maintain acircular buffer of LogRecords without having to pay formattingcosts.

Log Levels

Each log message has an associated log Level. The Level givesa rough guide to the importance and urgency of a log message.Log level objects encapsulate an integer value, with highervalues indicating higher priorities.

The Level class defines seven standard log levels, ranging fromFINEST (the lowest priority, with the lowest value) to SEVERE(the highest priority, with the highest value).

Loggers

As stated earlier, client code sends log requests to Loggerobjects. Each logger keeps track of a log level that it isinterested in, and discards log requests that are below this level.

Loggers are normally named entities, using dot-separatednames such as "java.awt". The namespace is hierarchical and ismanaged by the LogManager. The namespace should typicallybe aligned with the Java packaging namespace, but is notrequired to follow it slavishly. For example, a Logger called

"java.awt" might handle logging requests for classes in thejava.awt package, but it might also handle logging for classes in



sun.awt that support the client-visible abstractions defined in thejava.awt package.

In addition to named Loggers, it is also possible to createanonymous Loggers that don't appear in the sharednamespace. See section 1.14.

Loggers keep track of their parent loggers in the loggingnamespace. A logger's parent is its nearest extant ancestor inthe logging namespace. The root Logger (named "") has noparent. Anonymous loggers are all given the root logger as theirparent. Loggers may inherit various attributes from their parentsin the logger namespace. In particular, a logger may inherit:

Logging level. If a Logger's level is set to be null then the Logger will use aneffective Level that will be obtained by walking up the parent tree and using thefirst non-null Level.

Handlers. By default a Logger will log any output messages to its parent'shandlers, and so on recursively up the tree.

Resource bundle names. If a logger has a null resource bundle name, then it willinherit any resource bundle name defined for its parent, and so on recursively upthe tree.

Logging Methods

The Logger class provides a large set of convenience methodsfor generating log messages. For convenience, there aremethods for each logging level, named after the logging levelname. Thus rather than calling"logger.log(Constants.WARNING,..." a developer can simply callthe convenience method "logger.warning(..."

There are two different styles of logging methods, to meet theneeds of different communities of users.

First, there are methods that take an explicit source class nameand source method name. These methods are intended fordevelopers who want to be able to quickly locate the source ofany given logging message. An example of this style is:

void warning(String sourceClass, String sourceMethod, String msg);

Second, there are a set of methods that do not take explicitsource class or source method names. These are intended for


http://download.oracle.com/javase/1.4.2/docs/guide/util/logging/overview.html#1.14


developers who want easy-to-use logging and do not requiredetailed source information.

void warning(String msg);

For this second set of methods, the Logging framework willmake a "best effort" to determine which class and method calledinto the logging framework and will add this information into theLogRecord. However, it is important to realize that thisautomatically inferred information may only be approximate. Thelatest generation of virtual machines perform extensiveoptimizations when JITing and may entirely remove stackframes, making it impossible to reliably locate the calling classand method.

Handlers

J2SE provides the following Handlers:

StreamHandler: A simple handler for writing formatted records to anOutputStream.

ConsoleHandler: A simple handler for writing formatted records to System.err FileHandler: A handler that writes formatted log records either to a single file, or

to a set of rotating log files. SocketHandler: A handler that writes formatted log records to remote TCP ports. MemoryHandler: A handler that buffers log records in memory.

It is fairly straightforward to develop new Handlers. Developersrequiring specific functionality can either develop a Handler fromscratch or subclass one of the provided Handlers.

Formatters

J2SE also includes two standard Formatters:

SimpleFormatter: Writes brief "human-readable" summaries of log records. XMLFormatter: Writes detailed XML-structured information.

As with Handlers, it is fairly straightforward to develop newFormatters.

The LogManager

There is a global LogManager object that keeps track of global logging information. Thisincludes:

A hierarchical namespace of named Loggers.



A set of logging control properties read from the configuration file. There is a single LogManager object that can be retrieved using the static

LogManager.getLogManager method. This is created during LogManagerinitialization, based on a system property. This property allows containerapplications (such as EJB containers) to substitute their own subclass ofLogManager in place of the default class.

Create a logger

The package java.util.logging provides the logging capabilitiesvia the class Logger.

To create a logger in your Java coding.

import java.util.logging.Logger;

private final static Logger LOGGER = Logger.getLogger(MyClass.class .getName());

Level

The log levels define the severity of a message. The class Levelis used to define which messages should be written to the log.

Die Log Levels in descending order are:

SEVERE (highest) WARNING INFO CONFIG FINE FINER FINEST

In addition to that you have also the levels OFF and ALL to turnthe logging of or to log everything.

For example the following will set the logger to level info whichmeans all messages with severe, warning and info will belogged.

LOGGER.setLevel(Level.INFO);



An Example

To help motivate the use of the logging API, we will use asimple program, based on the idea of a "Hello World" program.

public class HelloWorld {public static void main(String[] args) {

HelloWorld hello = new HelloWorld("Hello world!");hello.sayHello();

}

private String theMessage;

public HelloWorld(String message) {theMessage = message;

}

public void sayHello() {System.err.println(theMessage);

}}

We will expand upon this simple program in order todemonstrate and motivate the logging API. The first thing we willdo is actually generate a logging message. This isstraightforward; we need to create a Logger object, whichrepresents theHelloWorld class, like this:

package logging.example2;import java.util.logging.Logger;/*** A simple Java Hello World program, in the tradition of* Kernighan and Ritchie.*/public class HelloWorld {private static Logger theLogger =

Logger.getLogger(HelloWorld.class.getName());

public static void main(String[] args) {HelloWorld hello = new HelloWorld("Hello world!");hello.sayHello();

}





}

public void sayHello() {System.err.println(theMessage);

}}

This generates a Logger, which we can use to generate loggingmessages. Since you typically use one logger per class, we canmake it a static field of the HelloWorld class. We use the fullyqualified name of the HelloWorld class as the name of theLogger.We'll look at the reasons for this in more detail later. Now, let'sactually use the Logger to try to generate a logging message:

import java.util.logging.Logger;public class HelloWorld {

private static Logger theLogger =Logger.getLogger(HelloWorld.class.getName());

public static void main(String[] args) {HelloWorld hello = new HelloWorld("Hello world!");hello.sayHello();

}



}

public void sayHello() {theLogger.finest("Hello logging!");System.err.println(theMessage);

}}

The logging API has a configuration file and by default, itprovides a handler that will send logging messages, in the formof aLogRecord to the console (System.err). So we would expect tosee the string "Hello logging!"; appear before our message of "Helloworld!". But if you run this program you only see "Hello world!":



% java logging.example3.HelloWorld

Hello world

What's going on here? The answer lies in the notion of logginglevels.

AssertionsAn assertion has a Boolean expression that, if evaluated as false,indicates a bug in the code. This mechanism provides a way todetect when a program starts falling into an inconsistent state.Assertions are excellent for documenting assumptions andinvariants about a class. Here is a simple example of assertion:

BankAccount acct = null;

// ...// Get a BankAccount object// ...

// Check to ensure we have oneassert acct != null;

This asserts that acct is not null. If acct is null, an AssertionError isthrown. Any line that executes after the assert statement cansafely assume that acct is not null.

Using assertions helps developers write code that is morecorrect, more readable, and easier to maintain. Thus, assertionsimprove the odds that the behavior of a class matches theexpectations of its clients.

Note that assertions can be compiled out. In languages such asC/C++, this means using the preprocessor. In C/C++, you canuse assertions through the assert macro, which has the followingdefinition in ANSI C:

void assert(int expression)

The program will be aborted if the expression evaluates to false, andit has no effect if the expression evaluates to true. When testing anddebugging is completed, assertions do not have to be removedfrom the program. However, note that the program will be largerin size and therefore slower to load. When assertions are nolonger needed, the line

#define NDEBUG



is inserted at the beginning of the program. This causes theC/C++ preprocessor to ignore all assertions, instead of deletingthem manually.

In other words, this is a requirement for performance reasons.You should write assertions into software in a form that can beoptionally compiled. Thus, assertions should be executed withthe code only when you are debugging your program -- that is,when assertions will really help flush out errors. You can think ofassertions as a uniform mechanism that replaces the use of adhoc conditional tests.

Implementing Assertions in Java TechnologyJ2SE 1.3 and earlier versions have no built-in support forassertions. They can, however, be provided as an ad hocsolution. Here is an example of how you would roll your ownassertion class.

Here we have an assert method that checks whether a Booleanexpression is true or false. If the expression evaluates to true,then there is no effect. But if it evaluates to false, the assert methodprints the stack trace and the program aborts. In this sampleimplementation, a second argument for a string is used so thatthe cause of error can be printed.

Note that in the assert method, I am checking whether the valueof NDEBUG is on (true) or off (false). If NDEBUG sets to true, then theassertion is to be executed. Otherwise, it would have no effect.The user of this class is able to set assertions on or off bytoggling the value of NDEBUG. Code Sample 1 shows myimplementation.

public class Assertion {

public static boolean NDEBUG = true;

private static void printStack(String why) {Throwable t = new Throwable(why);t.printStackTrace();System.exit(1);

}

public static void assert(boolean expression, String why) {if (NDEBUG && !expression) {

printStack(why);}



}}Note: In order for Code Sample 1 to compile, use -source 1.3 because assert is akeyword as of J2SE 1.4. Otherwise, you will get the following error message:

C:\CLASSES>javac Assertion.javaAssertion.java:11: as of release 1.4, 'assert' is a keyword, and may not be usedas an identifier

(try -source 1.3 or lower to use 'assert' as an identifier)public static void assert(boolean expression, String why) {

^1 error

Code Sample 2 demonstrates how to use the Assertion class. Inthis example, an integer representing the user's age is read. Ifthe age is greater than or equal to 18, the assertion evaluatesto true, and it will have no effect on the program execution. But ifthe age is less than 18, the assertion evaluates to false. Theprogram then aborts, displays the message You are too young to vote,and shows the stack trace.

It is important to note that in this example assertions are used to validate user input andthat no invariant is being tested or verified. This is merely to demonstrate the use ofassertions.import java.util.Scanner;import java.io.IOException;

public class AssertionTest1 {public static void main(String argv[]) throws IOException {

Scanner reader = new Scanner(System.in);System.out.print("Enter your age: ");int age = reader.nextInt();//Assertion.NDEBUG=false;Assertion.assert(age>=18, "You are too young to vote");// use ageSystem.out.println("You are eligible to vote");

}}

import java.io.IOException;

public class AssertionTest2 {

public static void main(String argv[]) throws IOException {System.out.print("Enter your marital status: ");int c = System.in.read();



//Assertion.NDEBUG=false;switch ((char) c) {

case 's':case 'S': System.out.println("Single"); break;case 'm':case 'M': System.out.println("Married"); break;case 'd':case 'D': System.out.println("Divorced"); break;default: Assertion.assert(!true, "Invalid Option"); break;

}

}}

In the three examples, if you do not want assertions to beexecuted as part of the code, uncomment the line

Assert.NDEBUG = false;

Using AssertionsUse the assert statement to insert assertions at particular points inthe code. The assert statement can have one of two forms:

assert booleanExpression;assert booleanExpression : errorMessage;

The errorMessage is an optional string message that would beshown when an assertion fails.

As an example, I will modify Code Sample 3 to usethe assert statement instead of my Assertion class, as shown inCode Sample 4.

import java.io.*;

public class AssertionTest3 {

public static void main(String argv[]) throws IOException {System.out.print("Enter your marital status: ");int c = System.in.read();switch ((char) c) {

case 's':case 'S': System.out.println("Single"); break;case 'm':case 'M': System.out.println("Married"); break;case 'd':



case 'D': System.out.println("Divorced"); break;default: assert !true : "Invalid Option"; break;

}

}}

prompt> javac -source 1.4 AssertionTest3.java

If you try to compile your assertion-enabled classes without using the -source1.4 option, you will get a compiler error saying that assert is a new keyword as ofrelease 1.4.

If you now run the program using the command

prompt> java AssertionTest3

and you enter a valid character, it will work fine. However, if youenter an invalid character, nothing will happen. This is because,by default, assertions are disabled at runtime. To enableassertions, use the switch -enableassertion (or -ea) as follows:

prompt> java -ea AssertionTest3prompt> java -enableassertion AssertionTest3

Following is a sample run:

C:\CLASSES>java -ea AssertionTest3Enter your marital status: wException in thread "main" java.lang.AssertionError: Invalid Option

at AssertionTest3.main(AssertionTest3.java:15)When an assertion fails, it means that the application hasentered an incorrect state. Possible behaviors may includesuspending the program or allowing it to continue to run. A goodbehavior, however, might be to terminate the application,because it may start functioning inappropriately after a failure. Inthis case, when an assertion fails, an AssertionError is thrown.

Note: By default, assertions are disabled, so you must not assume that the Booleanexpression contained in an assertion will be evaluated. Therefore, your expressions mustbe free of side effects.

The switch -disableassertion (or -da) can be used to disableassertions. This, however, is most useful when you wish todisable assertions on classes from specific packages. Forexample, to run the program MyClass with assertions disabled inclass Hello, you can use the following command:



prompt> java -da:com.javacourses.tests.Hello MyClass

And to disable assertions in a specific package and anysubpackages it may have, you can use the following command:

prompt> java -da:com.javacourses.tests... MyClass

Note that the three-dot ellipsis (...) is part of the syntax.

Switches can be combined. For example, to run a program withassertions enabled in the com.javacourses.tests package (and anysubpackages) and disabled in the class com.javacourses.ui.phone,you can use the following command:

prompt> java -ea:com.javacourses.tests... -da:com.javacourses.ui.phone MyClass

Note that when switches are combined and applied topackages, they are applied to all classes, including systemclasses (which do not have class loaders). But if you use themwith no arguments (-ea or -da), they do not apply to systemclasses. In other words, if you use the command

prompt> java -ea MyClass

then assertions are enabled in all classes except systemclasses. If you wish to turn assertions on or off in systemclasses, use the switches -enablesystemassertions (or -esa) and -disablesystemassertions (or -dsa).

Using Assertions for Design by ContractThe assertion facility can help you in supporting an informaldesign-by-contract style of programming. We will now seeexamples of using assertions for preconditions, postconditions,and class invariants. The examples are snippets of code froman integer stack, which provides operations such as push to addan item on the stack and pop to retrieve an item from the stack.

Preconditions

In order to retrieve an item from the stack, the stack must not beempty. The condition that the stack must not be empty is aprecondition. This precondition can be programmed usingassertions as follows:

public int pop() {// preconditionassert !isEmpty() : "Stack is empty";return stack[--num];


}Note: Because assertions might be disabled in some cases, precondition checking can stillbe performed by checks inside methods that result in exceptions suchasIllegalArgumentException or NullPointerException.

Postconditions

In order to push an item on the stack, the stack must not be full.This is a precondition. To add an item on the stack, we assignthe element to be added to the next index in the stack asfollows:

stack[num++] = element;

However, if you make a mistake and you write this statement as

stack[num] = element

then you have a bug. In this case, we need to ensure thatinvoking the push operation is working correctly. So thepostcondition here is to ensure that the new index in the stack isthe old index plus one. Also, we need to make sure that theelement has been added on the stack. The following snippet ofcode shows the push operation with a precondition and apostcondition.

public void push(int element) {// preconditionassert num<capacity : "stack is full";int oldNum = num;stack[num] = element;// postconditionassert num == oldNum+1 && stack[num-1] == element : "problem with counter";

}Note that if a method has multiple return statements, thenpostconditions should be evaluated before each of these returnstatements.

Class Invariants

A class invariant is a predicate that must be true before andafter any method completes. In the stack example, the invariantwould be that the number of elements in the stack is greaterthan or equal to zero and the number of elements should notexceed the maximum capacity of the class. These conditions,for example, can be coded as follows:

private boolean inv() {return (num >= 0 && num < capacity);

}To check that the stack should satisfy this predicate at all times,each public method and constructor should contain theassertion

assert inv();

right before each return.

UNIT VMulti Threading:

Java provides built-in support for multithreaded programming. A multithreadedprogram contains two or more parts that can run concurrently. Each part of such aprogram is called a thread, and each thread defines a separate path of execution.

A multithreading is a specialized form of multitasking. Multitasking threadsrequire less overhead than multitasking processes.

A process consists of the memory space allocated by the operating system thatcan contain one or more threads. A thread cannot exist on its own; it must be a part of aprocess. A process remains running until all of the non-daemon threads are doneexecuting.

Multithreading enables you to write very efficient programs that make maximumuse of the CPU, because idle time can be kept to a minimum.Life Cycle of a Thread:

A thread goes through various stages in its life cycle. For example, a thread isborn, started, runs, and then dies. Following diagram shows complete life cycle of athread.


Above mentioned stages are explained here: New: A new thread begins its life cycle in the new state. It remains in this state

until the program starts the thread. It is also referred to as a born thread. Runnable: After a newly born thread is started, the thread becomes runnable. A

thread in this state is considered to be executing its task. Waiting: Sometimes a thread transitions to the waiting state while the thread waits

for another thread to perform a task.A thread transitions back to the runnable stateonly when another thread signals the waiting thread to continue executing.

Timed waiting: A runnable thread can enter the timed waiting state for a specifiedinterval of time. A thread in this state transitions back to the runnable state whenthat time interval expires or when the event it is waiting for occurs.

Terminated: A runnable thread enters the terminated state when it completes itstask or otherwise terminates.

Creating a Thread:Java defines two ways in which this can be accomplished:

You can implement the Runnable interface. You can extend the Thread class, itself.

Create Thread by Implementing Runnable:The easiest way to create a thread is to create a class that implements the Runnableinterface.To implement Runnable, a class need only implement a single method called run( ),which is declared like this:


public void run( )You will define the code that constitutes the new thread inside run() method. It isimportant to understand that run() can call other methods, use other classes, and declarevariables, just like the main thread can.

After you create a class that implements Runnable, you will instantiate an object of typeThread from within that class. Thread defines several constructors. The one that we willuse is shown here:

Thread(Runnable threadOb, String threadName);

Here threadOb is an instance of a class that implements the Runnable interface and thename of the new thread is specified by threadName.

After the new thread is created, it will not start running until you call its start( ) method,which is declared within Thread. The start( ) method is shown here:

void start( );

Example:

Here is an example that creates a new thread and starts it running:

class NewThread implements Runnable {Thread t;NewThread() {

// Create a new, second threadt = new Thread(this, "Demo Thread");System.out.println("Child thread: " + t);t.start(); // Start the thread

}

public void run() {try {

for(int i = 5; i > 0; i--) {System.out.println("Child Thread: " + i);// Let the thread sleep for a while.Thread.sleep(500);

}} catch (InterruptedException e) {

System.out.println("Child interrupted.");}System.out.println("Exiting child thread.");

}}

class ThreadDemo {public static void main(String args[]) {

new NewThread(); // create a new threadtry {



for(int i = 5; i > 0; i--) {System.out.println("Main Thread: " + i);Thread.sleep(1000);


System.out.println("Main thread interrupted.");}System.out.println("Main thread exiting.");

}

}

This would produce following result:

Child thread: Thread[Demo Thread,5,main]Main Thread: 5Child Thread: 5Child Thread: 4Main Thread: 4Child Thread: 3Child Thread: 2Main Thread: 3Child Thread: 1Exiting child thread.Main Thread: 2Main Thread: 1Main thread exiting.

Create Thread by Extending Thread:

The second way to create a thread is to create a new class that extends Thread, and thento create an instance of that class.

The extending class must override the run( ) method, which is the entry point for the newthread. It must also call start( ) to begin execution of the new thread.

Example:

Here is the preceding program rewritten to extend Thread:

class NewThread extends Thread {NewThread() {

// Create a new, second threadsuper("Demo Thread");System.out.println("Child thread: " + this);start(); // Start the thread

}

// This is the entry point for the second thread.public void run() {



try {for(int i = 5; i > 0; i--) {

System.out.println("Child Thread: " + i);// Let the thread sleep for a while.

Thread.sleep(500);}

} catch (InterruptedException e) {System.out.println("Child interrupted.");

}System.out.println("Exiting child thread.");

}}

class ExtendThread {public static void main(String args[]) {

new NewThread(); // create a new threadtry {

for(int i = 5; i > 0; i--) {System.out.println("Main Thread: " + i);Thread.sleep(1000);


System.out.println("Main thread interrupted.");}System.out.println("Main thread exiting.");

}

}


Child thread: Thread[Demo Thread,5,main]Main Thread: 5Child Thread: 5Child Thread: 4Main Thread: 4Child Thread: 3Child Thread: 2Main Thread: 3Child Thread: 1Exiting child thread.Main Thread: 2Main Thread: 1Main thread exiting.

Thread Methods:

Following is the list of important medthods available in the Thread class.


SN Methods with Description

1public void start()Starts the thread in a separate path of execution, then invokes the run() method on thisThread object.

2public void run()If this Thread object was instantiated using a separate Runnable target, the run() method isinvoked on that Runnable object.

3public final void setName(String name)Changes the name of the Thread object. There is also a getName() method for retrievingthe name.

4 public final void setPriority(int priority)Sets the priority of this Thread object. The possible values are between 1 and 10.

5 public final void setDaemon(boolean on)A parameter of true denotes this Thread as a daemon thread.

6public final void join(long millisec)The current thread invokes this method on a second thread, causing the current thread toblock until the second thread terminates or the specified number of milliseconds passes.

7 public void interrupt()Interrupts this thread, causing it to continue execution if it was blocked for any reason.

8public final boolean isAlive()Returns true if the thread is alive, which is any time after the thread has been started butbefore it runs to completion.

SN Methods with Description

1public static void yield()Causes the currently running thread to yield to any other threads of the same priority thatare waiting to be scheduled

2public static void sleep(long millisec)Causes the currently running thread to block for at least the specified number ofmilliseconds

3 public static boolean holdsLock(Object x)Returns true if the current thread holds the lock on the given Object.

4public static Thread currentThread()Returns a reference to the currently running thread, which is the thread that invokes thismethod.

5public static void dumpStack()Prints the stack trace for the currently running thread, which is useful when debugging amultithreaded application.


The previous methods are invoked on a particular Thread object. The following methods

on the currently running thread

Example:

in the Thread class are static. Invoking one of the static methods performs the operation

The following ThreadClassDemo program demonstrates some of these methods of theThread class:



public class DisplayMessage implements Runnable{

private String message;public DisplayMessage(String message){

this.message = message;}public void run(){

while(true){

System.out.println(message);}

}}

// File Name : GuessANumber.java// Create a thread to extentd Threadpublic class GuessANumber extends Thread{

private int number;public GuessANumber(int number){

this.number = number;}public void run(){

int counter = 0;int guess = 0;do{

guess = (int) (Math.random() * 100 + 1);System.out.println(this.getName()

+ " guesses " + guess);counter++;

}while(guess != number);System.out.println("** Correct! " + this.getName()

+ " in " + counter + " guesses.**");}

}

// File Name : ThreadClassDemo.javapublic class ThreadClassDemo{

public static void main(String [] args)



{Runnable hello = new DisplayMessage("Hello");Thread thread1 = new Thread(hello);thread1.setDaemon(true);thread1.setName("hello");System.out.println("Starting hello thread...");thread1.start();

Runnable bye = new DisplayMessage("Goodbye");Thread thread2 = new Thread(hello);thread2.setPriority(Thread.MIN_PRIORITY);thread2.setDaemon(true);System.out.println("Starting goodbye thread...");thread2.start();

System.out.println("Starting thread3...");Thread thread3 = new GuessANumber(27);thread3.start();try{

thread3.join();}catch(InterruptedException e){

System.out.println("Thread interrupted.");}System.out.println("Starting thread4...");Thread thread4 = new GuessANumber(75);

thread4.start();System.out.println("main() is ending...");

}

}

This would produce following result. You can try this example again and again and youwould get different result every time.

Starting hello thread...Starting goodbye thread...HelloHelloHelloHelloHelloHelloHelloHello



HelloThread-2 guesses 27Hello** Correct! Thread-2 in 102 guesses.**HelloStarting thread4...HelloHello

......... remaining result produced

Interrupting thrreads:

Interrupting a thread means stopping what it is doing before it has completed its task,effectively aborting its current operation. Whether the thread dies, waits for new tasks, orgoes on to the next step depends on the application.

Although it may seem simple at first, you must take some precautions in order to achievethe desired result. There are some caveats you must be aware of as well.

First of all, forget the Thread.stop method. Although it indeed stops a running thread, themethod is unsafe and was deprecated , which means it may not be available in futureversions of the Java.

Another method that can be confusing for the unadvised is Thread.interrupt. Despite whatits name may imply, the method does not interrupt a running thread (more on this later),as Listing A demonstrates. It creates a thread and tries to stop it usingThread.interrupt.The calls to Thread.sleep() give plenty of time for the thread initialization andtermination. The thread itself does not do anything useful.Listing A:class Example1 extends Thread {

public static void main( String args[] ) throws Exception {Example1 thread = new Example1();

System.out.println( "Starting thread..." );thread.start();Thread.sleep( 3000 );System.out.println( "Interrupting thread..." );thread.interrupt();Thread.sleep( 3000 );System.out.println( "Stopping application..." );System.exit( 0 );}

public void run() {while ( true ) {System.out.println( "Thread is running..." );long time = System.currentTimeMillis();


while ( System.currentTimeMillis()-time < 1000 ) {}

}}

}

If you run the code in Listing A, you should see something like this on your console:Starting thread...Thread is running...Thread is running...Thread is running...Interrupting thread...Thread is running...Thread is running...Thread is running...Stopping application...

Even after Thread.interrupt() is called, the thread continues to run for a while.

Really interrupting a threadThe best, recommended way to interrupt a thread is to use a shared variable to signal thatit must stop what it is doing. The thread must check the variable periodically, especiallyduring lengthy operations, and terminate its task in an orderly manner.Listing B demonstrates this technique.Listing B:class Example2 extends Thread {

volatile boolean stop = false;

public static void main( String args[] ) throws Exception {Example2 thread = new Example2();System.out.println( "Starting thread..." );thread.start();Thread.sleep( 3000 );System.out.println( "Asking thread to stop..." );thread.stop = true;Thread.sleep( 3000 );System.out.println( "Stopping application..." );System.exit( 0 );}

public void run() {while ( !stop ) {System.out.println( "Thread is running..." );long time = System.currentTimeMillis();while ( (System.currentTimeMillis()-time < 1000) && (!stop) ) {}

}System.out.println( "Thread exiting under request..." );


http://articles.techrepublic.com.com/5100-10878_11-5144546.html


}}

Running the code in Listing B will generate output like this (notice how the thread exitsin an orderly fashion):Starting thread...Thread is running...Thread is running...Thread is running...Asking thread to stop...Thread exiting under request...Stopping application...

Although this method requires some coding, it is not difficult to implement and give thethread the opportunity to do any cleanup needed, which is an absolute requirement forany multithreaded application. Just be sure to declare the shared variable as volatile orenclose any access to it into synchronized blocks/methods.

So far, so good! But what happens if the thread is blocked waiting for some event? Ofcourse, if the thread is blocked, it can't check the shared variable and can't stop. There areplenty of situations when that may occur, such ascalling Object.wait(),ServerSocket.accept(), and DatagramSocket.receive(), to name afew.

They all can block the thread forever. Even if a timeout is employed, it may not befeasible or desirable to wait until the timeout expires, so a mechanism to prematurely exitthe blocked state must be used.

Unfortunately there is no such mechanism that works for all cases, but the particulartechnique to use depends on each situation. In the following sections, I'll give solutionsfor the most common cases.

Interrupting a thread with Thread.interrupt()As demonstrated in Listing A, the method Thread.interrupt() does not interrupt a runningthread. What the method actually does is to throw an interrupt if the thread is blocked, sothat it exits the blocked state. More precisely, if the thread is blocked at one of themethods Object.wait, Thread.join, or Thread.sleep, it receives anInterruptedException,thus terminating the blocking method prematurely.

So, if a thread blocks in one of the aforementioned methods, the correct way to stop it isto set the shared variable and then call the interrupt() method on it (notice that it isimportant to set the variable first). If the thread is not blocked, calling interrupt() will nothurt; otherwise, the thread will get an exception (the thread must be prepared to handlethis condition) and escape the blocked state. In either case, eventually the thread will test



the shared variable and stop. Listing C is a simple example that demonstrates thistechnique.Listing C:class Example3 extends Thread {

volatile boolean stop = false;

public static void main( String args[] ) throws Exception {Example3 thread = new Example3();System.out.println( "Starting thread..." );thread.start();Thread.sleep( 3000 );System.out.println( "Asking thread to stop..." );thread.stop = true;thread.interrupt();Thread.sleep( 3000 );System.out.println( "Stopping application..." );System.exit( 0 );}

public void run() {while ( !stop ) {System.out.println( "Thread running..." );try {Thread.sleep( 1000 );} catch ( InterruptedException e ) {System.out.println( "Thread interrupted..." );}

}System.out.println( "Thread exiting under request..." );}

}

As soon as Thread.interrupt() is called in Listing C, the thread gets an exception so that itescapes the blocked state and determines that it should stop. Running this code producesoutput like this:Starting thread...Thread running...Thread running...Thread running...Asking thread to stop...Thread interrupted...Thread exiting under request...Stopping application...

Interrupting an I/O operationBut what happens if the thread is blocked on an I/O operation? I/O can block a thread fora considerable amount of time, particularly if network communication is involved. Forexample, a server may be waiting for a request, or a network application may be waitingfor an answer from a remote host.



If you're using channels, available with the new I/O API introduced in Java 1.4, theblocked thread will get a ClosedByInterruptException exception. If that is the case, thelogic is the same as that used in the third example—only the exception is different.

But you might be using the traditional I/O available since Java 1.0, since the new I/O isso recent and requires more work. In this case, Thread.interrupt() doesn't help, since thethread will not exit the blocked state. Listing D demonstrates that behavior. Althoughthe interrupt() method is called, the thread does not exit the blocked state.Listing D:import java.io.*;

class Example4 extends Thread {

public static void main( String args[] ) throws Exception {Example4 thread = new Example4();System.out.println( "Starting thread..." );thread.start();Thread.sleep( 3000 );System.out.println( "Interrupting thread..." );thread.interrupt();Thread.sleep( 3000 );System.out.println( "Stopping application..." );System.exit( 0 );}

public void run() {ServerSocket socket;try {socket = new ServerSocket(7856);

} catch ( IOException e ) {System.out.println( "Could not create the socket..." );return;

}while ( true ) {System.out.println( "Waiting for connection..." );try {Socket sock = socket.accept();} catch ( IOException e ) {System.out.println( "accept() failed or interrupted..." );}

}}

}

Fortunately, the Java Platform provides a solution for that case by callingthe close()method of the socket the thread is blocked in. In this case, if the thread isblocked in an I/O operation, the thread will get a SocketException exception, much liketheinterrupt() method causes an InterruptedException to be thrown.



The only caveat is that a reference to the socket must be available so thatits close()method can be called. That means the socket object must also be shared. Listing Edemonstrates this case. The logic is the same as in the examples presented so far.

And here's the sample output you can expect from running Listing E:Listing E:import java.net.*;import java.io.*;

class Example5 extends Thread {volatile boolean stop = false;volatile ServerSocket socket;

public static void main( String args[] ) throws Exception {Example5 thread = new Example5();System.out.println( "Starting thread..." );thread.start();Thread.sleep( 3000 );System.out.println( "Asking thread to stop..." );thread.stop = true;thread.socket.close();Thread.sleep( 3000 );System.out.println( "Stopping application..." );System.exit( 0 );}

public void run() {try {socket = new ServerSocket(7856);

} catch ( IOException e ) {System.out.println( "Could not create the socket..." );return;

}while ( !stop ) {System.out.println( "Waiting for connection..." );try {Socket sock = socket.accept();} catch ( IOException e ) {System.out.println( "accept() failed or interrupted..." );}

}System.out.println( "Thread exiting under request..." );}

}

Starting thread...Waiting for connection...Asking thread to stop...accept() failed or interrupted...Thread exiting under request...Stopping application...





Multithreading is a powerful tool, but it presents its own set of challenges. One of these ishow to interrupt a running thread. If properly implemented, these techniques makeinterrupting a thread no more difficult than using the built-in operations already providedby the Java Platform.

Thread Priorities:

Every Java thread has a priority that helps the operating system determine the order inwhich threads are scheduled.

Java priorities are in the range between MIN_PRIORITY (a constant of 1) andMAX_PRIORITY (a constant of 10). By default, every thread is given priorityNORM_PRIORITY (a constant of 5).

Threads with higher priority are more important to a program and should be allocatedprocessor time before lower-priority threads. However, thread priorities cannot guaranteethe order in which threads execute and very much platform dependentant.

Setting a threads priority can be very useful if one thread has more critical tasks toperform than another.The Thread class has a method called setPriority(int level) with which you can alter thepriority a Thread instance has.The priority level range from 1 (least important) to 10 (most important) and if no level isexplicitly set, a Thread instance has the priority level of 5.In the first example below no priorites are set, so both threads have the priority level 5.The TestThread class implements the Runnable interface and in itsrun() method loops from 1 to 10 and output the number along with its Thread id, which ispassed to the constructor.

public class Main {public void setPrioritiesOnThreads() {

Thread thread1 = new Thread(new TestThread(1));Thread thread2 = new Thread(new TestThread(2));

thread1.start();thread2.start();

try {thread1.join();

thread2.join();

} catch (InterruptedException ex) {ex.printStackTrace();


}

System.out.println("Done.");

}

public static void main(String[] args) {new Main().setPrioritiesOnThreads();

}

class TestThread implements Runnable {

int id;

public TestThread(int id) {

this.id = id;}

public void run() {

for (int i = 1; i <= 10; i++) {System.out.println("Thread" + id + ": " + i);

}}

}}

Since both threads have the same priority, the output will be a mix between them andcould look like this:

Thread2: 1Thread1: 1Thread2: 2Thread1: 2Thread2: 3Thread1: 3Thread2: 4Thread1: 4Thread2: 5Thread1: 5Thread2: 6Thread1: 6Thread2: 7Thread2: 8


Thread2: 9Thread2: 10Thread1: 7Thread1: 8Thread1: 9Thread1: 10Done.

The output could look different from on execution to another since we have no control ofhow the CPU will prioritize them.

If we set the priority on the threads we still haven't got exact control of the execution, butat least we can tell the CPU which one we think ismost important. The next example is identical to the one above except for the lines wherethe priority of the threads are set:

public class Main {public void setPrioritiesOnThreads() {

Thread thread1 = new Thread(new TestThread(1));Thread thread2 = new Thread(new TestThread(2));

//Setting priorities on the Thread objectsthread1.setPriority(Thread.MAX_PRIORITY);thread2.setPriority(Thread.MIN_PRIORITY);

thread1.start();thread2.start();

try {

//Wait for the threads to finishthread1.join();thread2.join();

} catch (InterruptedException ex) {ex.printStackTrace();

}

System.out.println("Done.");

}

public static void main(String[] args) {new Main().setPrioritiesOnThreads();

}


class TestThread implements Runnable {

int id;

public TestThread(int id) {

this.id = id;}

public void run() {

for (int i = 1; i <= 10; i++) {System.out.println("Thread" + id + ": " + i);

}}

}}

The output from the code looked like this when executed:

Thread1: 1Thread1: 2Thread1: 3Thread1: 4Thread1: 5Thread1: 6Thread1: 7Thread1: 8Thread1: 9Thread1: 10Thread2: 1Thread2: 2Thread2: 3Thread2: 4Thread2: 5Thread2: 6Thread2: 7Thread2: 8Thread2: 9Thread2: 10Done.

It is however not certain that the first thread will be prioritized to finish before the secondthread starts every time. It is up to the CPU to decide.


Thread Synchronization :

When two or more threads need access to a shared resource, they need some way toensure that the resource will be used by only one thread at a time.

The process by which this synchronization is achieved is called thread synchronization.

The synchronized keyword in Java creates a block of code referred to as a critical section.Every Java object with a critical section of code gets a lock associated with the object. Toenter a critical section, a thread needs to obtain the corresponding object's lock.

This is the general form of the synchronized statement:

synchronized(object) {// statements to be synchronized

}

Here, object is a reference to the object being synchronized. A synchronized blockensures that a call to a method that is a member of object occurs only after the currentthread has successfully entered object's monitor.

Here is an example, using a synchronized block within the run( ) method:

class Callme {void call(String msg) {

System.out.print("[" + msg);try {

Thread.sleep(1000);} catch (InterruptedException e) {

System.out.println("Interrupted");}System.out.println("]");

}}

// File Name : Caller.javaclass Caller implements Runnable {

String msg;Callme target;Thread t;public Caller(Callme targ, String s) {

target = targ;msg = s;t = new Thread(this);t.start();

}

// synchronize calls to call()public void run() {



synchronized(target) { // synchronized blocktarget.call(msg);

}}

}// File Name : Synch.javaclass Synch {

public static void main(String args[]) {Callme target = new Callme();Caller ob1 = new Caller(target, "Hello");Caller ob2 = new Caller(target, "Synchronized");Caller ob3 = new Caller(target, "World");

// wait for threads to endtry {

ob1.t.join();ob2.t.join();ob3.t.join();

} catch(InterruptedException e) {System.out.println("Interrupted");

}}

}


[Hello][World]

[Synchronized]

Interthread Communication:

Consider the classic queuing problem, where one thread is producing some data andanother is consuming it. To make the problem more interesting, suppose that the producerhas to wait until the consumer is finished before it generates more data.

In a polling system, the consumer would waste many CPU cycles while it waited for theproducer to produce. Once the producer was finished, it would start polling, wasting moreCPU cycles waiting for the consumer to finish, and so on. Clearly, this situation isundesirable.

To avoid polling, Java includes an elegant interprocess communication mechanism viathe following methods:

wait( ): This method tells the calling thread to give up the monitor and go to sleepuntil some other thread enters the same monitor and calls notify( ).

notify( ): This method wakes up the first thread that called wait( ) on the sameobject.



notifyAll( ): This method wakes up all the threads that called wait( ) on the sameobject.c The highest priority thread will run first.

These methods are implemented as final methods in Object, so all classes have them. Allthree methods can be called only from within a synchronized context.

These methods are declared within Object. Various forms of wait( ) exist that allow youto specify a period of time to wait.

Example:

The following sample program consists of four classes: Q, the queue that you're trying tosynchronize; Producer, the threaded object that is producing queue entries; Consumer, thethreaded object that is consuming queue entries; and PC, the tiny class that creates thesingle Q, Producer, and Consumer.

The proper way to write this program in Java is to use wait( ) and notify( ) to signal inboth directions, as shown here:

class Q {int n;boolean valueSet = false;synchronized int get() {

if(!valueSet)try {

wait();} catch(InterruptedException e) {

System.out.println("InterruptedException caught");}System.out.println("Got: " + n);valueSet = false;notify();return n;

}

synchronized void put(int n) {if(valueSet)try {

wait();} catch(InterruptedException e) {

System.out.println("InterruptedException caught");}this.n = n;valueSet = true;System.out.println("Put: " + n);notify();

}}



class Producer implements Runnable {Q q;Producer(Q q) {

this.q = q;new Thread(this, "Producer").start();

}

public void run() {int i = 0;while(true) {

q.put(i++);}

}}

class Consumer implements Runnable {Q q;Consumer(Q q) {

this.q = q;new Thread(this, "Consumer").start();

}public void run() {

while(true) {q.get();

}}

}class PCFixed {

public static void main(String args[]) {Q q = new Q();new Producer(q);new Consumer(q);System.out.println("Press Control-C to stop.");

}}

Inside get( ), wait( ) is called. This causes its execution to suspend until the Producernotifies you that some data is ready.

When this happens, execution inside get( ) resumes. After the data has been obtained,get( ) calls notify( ). This tells Producer that it is okay to put more data in the queue.

Inside put( ), wait( ) suspends execution until the Consumer has removed the item fromthe queue. When execution resumes, the next item of data is put in the queue, and notify() is called. This tells the Consumer that it should now remove it.



Here is some output from this program, which shows the clean synchronous behavior:

Put: 1Got: 1Put: 2Got: 2Put: 3Got: 3Put: 4Got: 4Put: 5Got: 5

Thread Deadlock:

A special type of error that you need to avoid that relates specifically to multitasking isdeadlock, which occurs when two threads have a circular dependency on a pair ofsynchronized objects.

For example, suppose one thread enters the monitor on object X and another thread entersthe monitor on object Y. If the thread in X tries to call any synchronized method on Y, itwill block as expected. However, if the thread in Y, in turn, tries to call any synchronizedmethod on X, the thread waits forever, because to access X, it would have to release itsown lock on Y so that the first thread could complete.

Example:

To understand deadlock fully, it is useful to see it in action. The next example creates twoclasses, A and B, with methods foo( ) and bar( ), respectively, which pause briefly beforetrying to call a method in the other class.

The main class, named Deadlock, creates an A and a B instance, and then starts a secondthread to set up the deadlock condition. The foo( ) and bar( ) methods use sleep( ) as away to force the deadlock condition to occur.

class A {synchronized void foo(B b) {

String name = Thread.currentThread().getName();System.out.println(name + " entered A.foo");try {

Thread.sleep(1000);} catch(Exception e) {

System.out.println("A Interrupted");}System.out.println(name + " trying to call B.last()");b.last();



}synchronized void last() {

System.out.println("Inside A.last");}

}class B {

synchronized void bar(A a) {String name = Thread.currentThread().getName();System.out.println(name + " entered B.bar");try {

Thread.sleep(1000);} catch(Exception e) {

System.out.println("B Interrupted");}System.out.println(name + " trying to call A.last()");a.last();

}synchronized void last() {

System.out.println("Inside A.last");}

}class Deadlock implements Runnable {

A a = new A();B b = new B();Deadlock() {

Thread.currentThread().setName("MainThread");Thread t = new Thread(this, "RacingThread");t.start();a.foo(b); // get lock on a in this thread.System.out.println("Back in main thread");


b.bar(a); // get lock on b in other thread.System.out.println("Back in other thread");

}public static void main(String args[]) {

new Deadlock();}

}

Here is some output from this program:

MainThread entered A.fooRacingThread entered B.barMainThread trying to call B.last()

RacingThread trying to call A.last()



Because the program has deadlocked, you need to press CTRL-C to end the program.You can see a full thread and monitor cache dump by pressing CTRL-BREAK on a PC .You will see that RacingThread owns the monitor on b, while it is waiting for the monitoron a. At the same time, MainThread owns a and is waiting to get b. This program willnever complete.

As this example illustrates, if your multithreaded program locks up occasionally,deadlock is one of the first conditions that you should check for.

Thread Control:

While the suspend( ), resume( ), and stop( ) methods defined by Thread class seem to bea perfectly reasonable and convenient approach to managing the execution of threads,they must not be used for new Java programs and obsolete in newer versions of Java.

The following example illustrates how the wait( ) and notify( ) methods that are inheritedfrom Object can be used to control the execution of a thread.

This example is similar to the program in the previous section. However, the deprecatedmethod calls have been removed. Let us consider the operation of this program.

The NewThread class contains a boolean instance variable named suspendFlag, which isused to control the execution of the thread. It is initialized to false by the constructor.

The run( ) method contains a synchronized statement block that checks suspendFlag. Ifthat variable is true, the wait( ) method is invoked to suspend the execution of the thread.The mysuspend( ) method sets suspendFlag to true. The myresume( ) method setssuspendFlag to false and invokes notify( ) to wake up the thread. Finally, the main( )method has been modified to invoke the mysuspend( ) and myresume( ) methods.

Example:

// Suspending and resuming a thread for Java 2class NewThread implements Runnable {

String name; // name of threadThread t;boolean suspendFlag;NewThread(String threadname) {

name = threadname;t = new Thread(this, name);System.out.println("New thread: " + t);suspendFlag = false;t.start(); // Start the thread

}// This is the entry point for thread.public void run() {

try {for(int i = 15; i > 0; i--) {

System.out.println(name + ": " + i);Thread.sleep(200);



synchronized(this) {while(suspendFlag) {

wait();}

}}

} catch (InterruptedException e) {System.out.println(name + " interrupted.");

}System.out.println(name + " exiting.");

}void mysuspend() {

suspendFlag = true;}synchronized void myresume() {

suspendFlag = false;notify();

}}

class SuspendResume {public static void main(String args[]) {

NewThread ob1 = new NewThread("One");NewThread ob2 = new NewThread("Two");try {

Thread.sleep(1000);ob1.mysuspend();System.out.println("Suspending thread One");Thread.sleep(1000);ob1.myresume();System.out.println("Resuming thread One");ob2.mysuspend();System.out.println("Suspending thread Two");Thread.sleep(1000);ob2.myresume();System.out.println("Resuming thread Two");

} catch (InterruptedException e) {System.out.println("Main thread Interrupted");

}// wait for threads to finishtry {

System.out.println("Waiting for threads to finish.");ob1.t.join();ob2.t.join();

} catch (InterruptedException e) {



System.out.println("Main thread Interrupted");}System.out.println("Main thread exiting.");

}

}

Here is the output produced by the above program:

New thread: Thread[One,5,main]One: 15New thread: Thread[Two,5,main]Two: 15One: 14Two: 14One: 13Two: 13One: 12Two: 12One: 11Two: 11Suspending thread OneTwo: 10Two: 9Two: 8Two: 7Two: 6Resuming thread OneSuspending thread TwoOne: 10One: 9One: 8One: 7One: 6Resuming thread TwoWaiting for threads to finish.Two: 5One: 5Two: 4One: 4Two: 3One: 3Two: 2One: 2Two: 1One: 1Two exiting.



One exiting.Main thread exiting.

Using isAlive() and join():

Typically, the main thread is the last thread to finish in a program. However, there isn’tany guarantee that the main thread won’t finish before a child thread finishes. In theprevious example, we told the main method to sleep until the child threads terminate.However, we estimated the time it takes for the child threads to complete processing. Ifour estimate was too short, a child thread could terminate after the main threadterminates. Therefore, the sleep technique isn’t the best one to use to guarantee that themain thread terminates last.

Programmers use two other techniques to ensure that the main thread is the last thread toterminate. These techniques involve calling the isAlive() method and the join() method.Both of these methods are defined in the Thread class.

The isAlive() method determines whether a thread is still running. If it is, the isAlive()method returns a Boolean true value; otherwise, a Boolean false is returned. You can usethe isAlive() method to examine whether a child thread continues to run. The join()method works differently than the isAlive() method. The join() method waits until thechild thread terminates and ―joins‖ the main thread. In addition, you can use the join()method to specify the amount of time you want to wait for a child thread to terminate.

The following example illustrates how to use the isAlive() method and the join() methodin your program. This example is nearly the same as the previous example. Thedifference lies in the main() method of the Demo class definition.

After the threads are declared using the constructor of the MyThread class, the isAlive()method is called for each thread. The value returned by the isAlive() method is thendisplayed on the screen. Next, the join() method is called for each thread. The join()method causes the main thread to wait for all child threads to complete processing beforethe main thread terminates.

class MyThread implements Runnable {String tName;Thread t;MyThread (String threadName) {

tName = threadName;t = new Thread (this, tName);t.start();


try {System.out.println("Thread: " + tName );Thread.sleep(2000);

} catch (InterruptedException e ) {System.out.println("Exception: Thread "

+ tName + " interrupted");



}System.out.println("Terminating thread: " + tName );

}}class Demo {

public static void main (String args []) {MyThread thread1 = new MyThread ("1");MyThread thread2 = new MyThread ("2");MyThread thread3 = new MyThread ("3");MyThread thread4 = new MyThread ("4");System.out.println("Thread Status: Alive");System.out.println("Thread 1: "

+ thread1.t.isAlive());System.out.println("Thread 2: "



+ thread4.t.isAlive());try {

System.out.println("Threads Joining.");thread1.t.join();thread2.t.join();thread3.t.join();thread4.t.join();

} catch (InterruptedException e) {System.out.println(

"Exception: Thread main interrupted.");}System.out.println("Thread Status: Alive");System.out.println("Thread 1: "




+ thread4.t.isAlive());System.out.println(

"Terminating thread: main thread.");}

}



Here is what is displayed on the screen when this program runs:

Thread Status: AliveThread 1: trueThread 2: trueThread 3: trueThread 4: trueThreads Joining.Thread: 1Thread: 2Thread: 3Thread: 4Terminating thread: 1Terminating thread: 2Terminating thread: 3Terminating thread: 4Thread Status: AliveThread 1: falseThread 2: falseThread 3: falseThread 4: falseTerminating thread: main thread.

Thread Pools:Most of the executor implementations in java.util.concurrent use thread pools, whichconsist of worker threads. This kind of thread exists separately from the Runnable andCallable tasks it executes and is often used to execute multiple tasks.

Using worker threads minimizes the overhead due to thread creation. Thread objects usea significant amount of memory, and in a large-scale application, allocating anddeallocating many thread objects creates a significant memory management overhead.

One common type of thread pool is the fixed thread pool. This type of pool always has aspecified number of threads running; if a thread is somehow terminated while it is still inuse, it is automatically replaced with a new thread. Tasks are submitted to the pool via aninternal queue, which holds extra tasks whenever there are more active tasks than threads.

An important advantage of the fixed thread pool is that applications using it degradegracefully. To understand this, consider a web server application where each HTTPrequest is handled by a separate thread. If the application simply creates a new thread forevery new HTTP request, and the system receives more requests than it can handleimmediately, the application will suddenly stop responding to all requests when theoverhead of all those threads exceed the capacity of the system. With a limit on thenumber of the threads that can be created, the application will not be servicing HTTPrequests as quickly as they come in, but it will be servicing them as quickly as the systemcan sustain.


A simple way to create an executor that uses a fixed thread pool is to invoke thenewFixedThreadPool factory method in java.util.concurrent.Executors This class alsoprovides the following factory methods:

The newCachedThreadPool method creates an executor with anexpandable thread pool. This executor is suitable for applications thatlaunch many short-lived tasks.

The newSingleThreadExecutor method creates an executor that executes asingle task at a time.

Several factory methods are ScheduledExecutorService versions of theabove executors.

If none of the executors provided by the above factory methods meet your needs,constructing instances of java.util.concurrent.ThreadPoolExecutor orjava.util.concurrent.ScheduledThreadPoolExecutor will give you additional options.

Callables:Callables are a tweak to the existing runnable construct. Callables differ only by having ageneric return type - most specialized concurrency APIs already available (such asFoxtrot ) already have the concept of a runnable + return type (such as the foxtrot Task and Job classes). Some of you may already have noticed that the ExecutorService API isbiased to Callable objects - and some may consider that a problem since they have a lotof code that already works with Runnable .

public class ComplexCalculation implements Callable<Long> {

public ComplexCalculation(int value1, int value2, long value3) {// ...}

public Long call() {

// perform complex calculation resulting in longlong result = //...return result; // autoboxing!

}}

java.util.concurrentInterface Callable<V>

Type Parameters:

V - the result type of method call


http://foxtrot.sourceforge.net/docs/api.php

http://foxtrot.sourceforge.net/docs/api.php

http://foxtrot.sourceforge.net/

http://foxtrot.sourceforge.net/

http://download.oracle.com/javase/7/docs/api/java/util/concurrent/ScheduledThreadPoolExecutor.html

http://download.oracle.com/javase/7/docs/api/java/util/concurrent/ThreadPoolExecutor.html

http://download.oracle.com/javase/7/docs/api/java/util/concurrent/Executors.html

public interface Callable<V>

A task that returns a result and may throw an exception. Implementors define a singlemethod with no arguments called call.

The Callable interface is similar to Runnable , in that both are designed for classes whoseinstances are potentially executed by another thread. A Runnable, however, does notreturn a result and cannot throw a checked exception.

Executors:

There's a close connection between the task being done by a new thread, as defined by itsRunnable object, and the thread itself, as defined by a Thread object. This works well forsmall applications, but in large-scale applications, it makes sense to separate threadmanagement and creation from the rest of the application. Objects that encapsulate thesefunctions are known as executors.

The Executors class contains utility methods to convert from other common forms toCallable classes.

V call()Computes a result, or throws an exception if unable to do so.

Example:

import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;

class SimpExec {public static void main(String args[]) {CountDownLatch cdl = new CountDownLatch(5);CountDownLatch cdl2 = new CountDownLatch(5);CountDownLatch cdl3 = new CountDownLatch(5);CountDownLatch cdl4 = new CountDownLatch(5);ExecutorService es = Executors.newFixedThreadPool(2);

es.execute(new MyThread(cdl, "A"));es.execute(new MyThread(cdl2, "B"));es.execute(new MyThread(cdl3, "C"));es.execute(new MyThread(cdl4, "D"));

try {cdl.await();cdl2.await();cdl3.await();

cdl4.await();} catch (InterruptedException exc) {System.out.println(exc);

}

es.shutdown();

}}

class MyThread implements Runnable {String name;

CountDownLatch latch;

MyThread(CountDownLatch c, String n) {latch = c;name = n;new Thread(this);

}

public void run() {for (int i = 0; i < 5; i++) {latch.countDown();

}}

}

Synchronizers:

Java 5 introduces general purpose synchronizer classes, including semaphores, mutexes,barriers, latches, and exchangers, which facilitate coordination between threads. Theseclasses are a apart of the java.util.concurrent package. A brief description of each of thesefollows:

SemaphoresA counting semaphore maintains a set of permits. Each acquire() blocks if

necessary until a permit is available, and then takes it. Each release() adds a permit,potentially releasing a blocking acquirer. However, no actual permit objects are used; theSemaphore just keeps a count of the number available and acts accordingly.


Semaphores are often used to restrict the number of threads than can access some(physical or logical) resource.

Example:

import java.util.concurrent.Semaphore;import java.util.Random;

//Solving the mutual exclusion problem using Semaphore class

class Process2 extends Thread{

private static final Random rand = new Random();

private int id;

private Semaphore sem;

public Process2(int i, Semaphore s){

id = i;sem = s;

}

private void busy(){

try{

sleep(rand.nextInt(500));} catch (InterruptedException e){}

}

private void noncritical(){

System.out.println("Thread " + id + " is NON critical");busy();

}

private void critical(){

System.out.println("Thread " + id + " entering critical section");busy();System.out.println("Thread " + id + " leaving critical section");


}

public void run(){

for (int i = 0; i < 2; ++i){

noncritical();try{

sem.acquire();} catch (InterruptedException e){

// ...}critical();sem.release();

}}

public static void main(String[] args){

final int N = 4;

System.out.println("Busy waiting...");

//Semaphore(int permits, boolean fair)Semaphore sem = new Semaphore(N, true);

Process2[] p = new Process2[N];

for (int i = 0; i < N; i++){

p[i] = new Process2(i, sem);p[i].start();

}}

}

Barrier

A synchronization aid that allows a set of threads to all wait for each other toreach a common barrier point. CyclicBarriers are useful in programs involving a fixedsized group of threads that must occasionally wait for each other. The barrier is calledcyclic because it can be re-used after the waiting threads are released.


For the java.util.concurrent class see CyclicBarrier A barrier is a type of synchronization method. A barrier for a group of threads orprocesses in the source code means any thread/process must stop at this point and cannotproceed until all other threads/processes reach this barrier.[1]

The basic Java synchronization mechanism centers on the keyword synchronized , whichpredicates methods. In java, all objects can be used like monitors (protected types).

That means that given an object A, no more than one thread can execute a synchronizedmethod of A at any time.

Barrier implementation

/** Given object A, no more than one thread can execute a synchronized

method of A at any time.*/

public class Barrier{

public synchronized void block() throws InterruptedException{

wait();}

public synchronized void release() throws InterruptedException{

notify();}

public synchronized void releaseAll() throws InterruptedException{

notifyAll();}

}

And an example with two threads.

class BarrierExample{

static class MyThread1 implements Runnable{

public MyThread1(Barrier barrier){

this.barrier = barrier;}


http://programmingexamples.wikidot.com/cyclicbarrier


public void run(){

try{

Thread.sleep(1000);System.out.println("MyThread1 waiting on barrier");barrier.block();System.out.println("MyThread1 has been released");

} catch (InterruptedException ie){

System.out.println(ie);}

}

private Barrier barrier;

}

static class MyThread2 implements Runnable{

Barrier barrier;

public MyThread2(Barrier barrier){

this.barrier = barrier;}

public void run(){

try{

Thread.sleep(3000);System.out.println("MyThread2 releasing blocked threads\n");barrier.release();System.out.println("MyThread1 releasing blocked threads\n");

} catch (InterruptedException ie){

System.out.println(ie);}

}}

public static void main(String[] args) throws InterruptedException{

Barrier BR = new Barrier();


Thread t1 = new Thread(new BarrierExample.MyThread1(BR));Thread t2 = new Thread(new BarrierExample.MyThread2(BR));t1.start();t2.start();t1.join();t2.join();

}}

ExchangersA synchronization point at which two threads can exchange objects. Each thread

presents some object on entry to the exchange method, and receives the object presentedby the other thread on return.

Example:

import java.util.concurrent.Exchanger;

class ExgrDemo {public static void main(String args[]) {Exchanger<String> exgr = new Exchanger<String>();

new UseString(exgr);

new MakeString(exgr);}

}

class MakeString implements Runnable {Exchanger<String> ex;

String str;

MakeString(Exchanger<String> c) {ex = c;str = new String();

new Thread(this).start();

}

public void run() {char ch = 'A';

for (int i = 0; i < 3; i++) {for (int j = 0; j < 5; j++)str += (char) ch++;

try {str = ex.exchange(str);

} catch (InterruptedException exc) {System.out.println(exc);

}}

}}

class UseString implements Runnable {Exchanger<String> ex;

String str;

UseString(Exchanger<String> c) {ex = c;new Thread(this).start();

}

public void run() {

for (int i = 0; i < 3; i++) {try {

str = ex.exchange(new String());System.out.println("Got: " + str);

} catch (InterruptedException exc) {System.out.println(exc);

}}

}}Latches

A synchronization aid that allows one or more threads to wait until a set ofoperations being performed in other threads completes. A CountDownLatch is initializedwith a given count. The await methods block until the current count reaches zero due to


invocations of the countDown() method, after which all waiting threads are released andany subsequent invocations of await return immediately.

Latching variables specify conditions that once set never change. This provides a way tostart several threads and have them wait until a signal is received from a coordinatingthread. [1]

The following program creates a set of threads, but doesn't let any thread start until all thethreads are created.

import java.util.concurrent.*;

public class LatchTest{

private static final int COUNT = 10;

private static class Worker implements Runnable{

CountDownLatch startLatch;

CountDownLatch stopLatch;

String name;

Worker(CountDownLatch startLatch, CountDownLatch stopLatch, String name){

this.startLatch = startLatch;this.stopLatch = stopLatch;this.name = name;

}

public void run(){

try{

startLatch.await(); // wait until the latch has counted down to// zero

} catch (InterruptedException ex){

ex.printStackTrace();}System.out.println("Running: " + name);stopLatch.countDown();

}}


public static void main(String args[]){

// CountDownLatch(int count)// Constructs a CountDownLatch initialized with the given count.CountDownLatch startSignal = new CountDownLatch(1);CountDownLatch stopSignal = new CountDownLatch(COUNT);for (int i = 0; i < COUNT; i++){

new Thread(new Worker(startSignal, stopSignal, Integer.toString(i))).start();

}System.out.println("Go");startSignal.countDown();try{

stopSignal.await();} catch (InterruptedException ex){

ex.printStackTrace();}System.out.println("Done");

}}

Thread and event driven programming:

The single-thread rule:

Here's the rule:

Once a Swing component has been realized, all code that might affect or depend on thestate of that component should be executed in the event-dispatching thread.

This rule might sound scary, but for many simple programs, you don't have to worryabout threads. Before we go into detail about how to write Swing code, let's define twoterms: realized and event-dispatching thread.

Realized means that the component's paint() method has been or might be called. ASwing component that's a top-level window is realized by having one of these methodsinvoked on it: setVisible(true), show(), or (this might surprise you) pack(). Once awindow is realized, all components that it contains are realized. Another way to realize acomponent is to add it to a container that's already realized. You'll see examples ofrealizing components later.

The event-dispatching thread is the thread that executes drawing and event-handlingcode. For example, the paint() and actionPerformed() methods are automatically executed


in the event-dispatching thread. Another way to execute code in the event-dispatchingthread is to use the SwingUtilities invokeLater() method.

Example:

import java.awt.FlowLayout;import java.lang.reflect.InvocationTargetException;

import javax.swing.JFrame;import javax.swing.JLabel;import javax.swing.JPanel;import javax.swing.SwingUtilities;

public class InvokeAndWaitDemo extends Object {private static void print(String msg) {String name = Thread.currentThread().getName();System.out.println(name + ": " + msg);

}

public static void main(String[] args) {final JLabel label = new JLabel("--------");

JPanel panel = new JPanel(new FlowLayout());

panel.add(label);

JFrame f = new JFrame("InvokeAndWaitDemo");f.setContentPane(panel);f.setSize(300, 100);f.setVisible(true);

try {print("sleeping for 3 seconds");Thread.sleep(3000);

print("creating code block for event thread");Runnable setTextRun = new Runnable() {public void run() {print("about to do setText()");label.setText("New text!");



}};print("about to invokeAndWait()");SwingUtilities.invokeAndWait(setTextRun);print("back from invokeAndWait()");

} catch (InterruptedException ix) {print("interrupted while waiting on invokeAndWait()");

} catch (InvocationTargetException x) {print("exception thrown from run()");

}

}

}

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