7. java.pdf

7/27/2019 7. Java.pdf

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Java

Xiaoqiang Wang

Florida State University

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Why Java?

It’s almost entirely object-oriented

It has a vast library of predefined objects andoperations

It’s more platform independent

this makes it great for Web programming

It’s more secure

It isn’t C++

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Applets, Servlets and

Applications

An applet is designed to be embedded in a

Web page, and run by a browser Applets run in a sandbox with numerous

restrictions; for example, they can’t read files

and then use the network

A servlet is designed to be run by a web

server An application is a conventional program

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Building Standalone JAVA

Programs (on UNIX)

Prepare the file f oo. j ava using an editor

Invoke the compiler: j avac f oo. j ava

This creates f oo. cl ass

Run the java interpreter: j ava f oo

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Java Virtual Machine

The .class files generated by the compiler arenot executable binaries

so Java combines compilation and interpretation

Instead, they contain “byte-codes” to beexecuted by the Java Virtual Machine

other languages have done this, e.g. UCSD

Pascal This approach provides platform

independence, and greater security

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HelloWorld (standalone)

publ i c cl ass Hel l oWor l d {publ i c st at i c voi d mai n( St r i ng[ ] ar gs) {

Syst em. out . pr i nt l n( "Hel l o Wor l d! " ) ;}

}

Note that String is built in

println is a member function for theSystem.out class

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Comments are almost like C++

/* This kind of comment can span multiple lines */

// This kind is to the end of the line

/**

* This kind of comment is a special

* ‘javadoc’

style comment

*/

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Primitive data types are like C

Main data types are int, double, boolean,

char

Also have byte, short, long, float

boolean

has values true

and false

Declarations look like C, for example,

double x, y;

int

count = 0;

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Non-Primitive types in Java

Classes

Everything is a class, defined within a package

Arrays

byte[] b = new byte[100]; Strings

string st = “scientific computing” is initialized like a

primitive type, but it is not a primitive type!

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Array Operations

Subscripts always start at 0 as in C

Subscript checking is done automatically

Certain operations are defined on arrays of

objects, as for other classes

e.g. myArray.length == 5

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Expressions are like C

Assignment statements mostly look like those in C; youcan use =, +=, *= etc.

Arithmetic uses the familiar + - * / %

Java also has ++ and --

Java has boolean operators && || !

Java has comparisons < <= == != >= >

Java does not have pointers or pointer arithmetic

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Control statements are like C

if (x < y) smaller = x;

if (x < y){ smaller=x;sum += x;} else { smaller = y; sum += y; }

while (x < y) { y = y -

x; }

do { y = y - x; } while (x < y)

for (int i = 0; i < max; i++)

sum += i; BUT: conditions must be boolean !

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Control statements II

Java also introduces the try statement,about which more later

switch (n + 1) { case 0: m = n - 1; break; case 1: m = n + 1; case 3: m = m * n; break; default: m = -n; break; }

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Java isn't C!

In C, almost everything is in functions In Java, almost everything is in classes

There is often only one class per file There must be only one public class per file

The file name must be the same as the

name of that public class, but with a .java extension

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Java program layout

A typical Java file looks like:

import java.awt.*; import java.util.*;

public class SomethingOrOther { // object definitions go here . . . }

This must be in a file named SomethingOrOther.java

!

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References

All variable names in java that are not

primitive types are actually references

int[] a = new int[100];int[] b = a;

// b and a are references to the 100 bytes in memory

b[3] = 10;

System.out.println(“a[3]= “ + a[3]); // prints 10

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Argument Passing

methods in java pass arguments by value

the dummy argument in the method is a copyof the argument passed

if a primitive type is passed, it is copied if a non-primitive type is passed, its reference

is passed. More precisely, the variable is a

reference.

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Argument Passingclass Refer {

...

public void refer(int[] a) {

a[3] = 4;

int[] b = new int[10];

// redefines the referencea = b;

a[3] = 12.;

// the caller argument is

// unchanged!!

}

}

...

Refer ref = new Refer();int[] d = new int[20];

ref.refer(d);

...System.out.println("d[3]= "

+ d[3]);

// prints the value 4

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Scoping

As in C/C++, scope is determined by the placement of curly

braces {}.

A variable defined within a scope is available only to the end

of that scope.

{ int x = 12;

/* only x available */

{ int q = 96;

/* both x and q available */

}

/* only x available */

/* q “out of scope” */

}

{ int x = 12;

{ int x = 96; /* illegal */

}

}

This is ok in C/C++ but not in Java.

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An array is an object

Person mary = new Person ( );

int myArray[ ] = new int[5];

int myArray[ ] = {1, 4, 9, 16, 25};

String languages [ ] = {"Prolog","Java"};

Since arrays are objects they are allocated

dynamically

Arrays, like all objects, are subject to garbage

collection when no more references remain

so fewer memory leaks Java doesn’t have pointers!

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Scope of Objects

Java objects don’t have the same lifetimes

as primitives.

When you create a Java object using new,

it hangs around past the end of the scope. Here, the scope of name s is delimited by

the {}s but the String object hangs around

until GC’d{

String s = new String("a string");} /* end of scope */

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Name conventions

Java is case-sensitive; maxval, maxVal, andMaxVal are three different names

Class names begin with a capital letter

All other names begin with a lowercase letter Subsequent words are capitalized:

theBigOne Underscores are not used in names

These are very strong conventions!

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JAVA Classes

The class is the fundamental concept in JAVA(and other OOPLs)

A class describes some data object(s), and theoperations (or methods) that can be applied tothose objects

Every object and method in Java belongs to aclass

Classes have data (fields) and code (methods)and classes (member classes or inner classes)

Static methods and fields belong to the classitself

Others belong to instances

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Examplepublic class Circle {// A class fieldpublic static final double PI= 3.14159; // A usefulconstant

// A class method: just compute a value based on the argumentspublic static double radiansToDegrees(double

rads) {

return rads

* 180 / PI;

}

// An instance fieldpublic double r; // The radius of the circle

// Two methods which operate on the instance fields of anobjectpublic double area() { // Compute the area of thecirclereturn PI * r * r;

}public double circumference() { // Compute the circumference

of the circlereturn 2 * PI * r;

}

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Constructors

Classes should define one or more methods to create or construct instances of the class

Their name is the same as the class name

note deviation from convention that methods begin with lower case

Constructors are differentiated by the number and types of their arguments

An example of overloading

If you don’t define a constructor, a default one will be created.

Constructors automatically invoke the zero argumentconstructor of their superclass when they begin (note that thisyields a recursive process!)

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Constructor examplepubl i c cl ass Ci r cl e {

publ i c st at i c f i nal doubl e PI = 3. 14159; / / A const ant

publ i c doubl e r ; / / i nst ance f i el d hol ds ci r cl e’ s

r adi us

/ / The const r uct or met hod: i ni t i al i ze t he r adi us f i el d

publ i c Ci r cl e( doubl e r ) { t hi s. r = r ; }

/ / Const r uct or t o use i f no ar gument s

publ i c Ci r cl e( ) { r = 1. 0; }

/ / bet t er : publ i c Ci r cl e( ) { t hi s( 1. 0) ; }

/ / The i nst ance met hods: comput e val ues based on r adi us

publ i c doubl e ci r cumf er ence( ) { r et ur n 2 * PI * r ; }

publ i c doubl e ar ea( ) { r et ur n PI * r *r ; }

}

this.r refers to the r

field of the class

This() refers to a

constructor for the class

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Extending a class

Class hierarchies reflect subclass-superclass relations among

classes.

One arranges classes in hierarchies:

A class inherits instance variables and instance methods fromall of its superclasses. Tree -> BinaryTree -> BST

You can specify only ONE superclass for any class.

When a subclass-superclass chain contains multiple instancemethods with the same signature (name, arity, and argument

types), the one closest to the target instance in the subclass-

superclass chain is the one executed.

All others are shadowed/overridden.

Something like multiple inheritance can be done via interfaces

(more on this later)

What’s the superclass of a class defined without an extendsclause?

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Extending a classpubl i c cl ass Pl aneCi r cl e ext ends Ci r cl e {

/ / We aut omat i cal l y i nher i t t he f i el ds and met hods of Ci r cl e,/ / so we onl y have t o put t he new st uf f her e./ / New i nst ance f i el ds t hat st or e t he cent er poi nt of t he ci r cl epubl i c doubl e cx, cy;

/ / A new const r uct or met hod t o i ni t i al i ze t he new f i el ds/ / I t uses a speci al synt ax t o i nvoke t he Ci r cl e( ) const r uct orpubl i c Pl aneCi r cl e( doubl e r , doubl e x, doubl e y) {

super ( r ) ; / / I nvoke t he const r uctor of t he super cl ass, Ci r cl e( )t hi s . cx = x; / / I ni t i al i ze t he i nst ance f i el d cxt hi s . cy = y; / / I ni t i al i ze t he i nst ance f i el d cy

}

/ / The ar ea( ) and ci r cumf er ence( ) met hods ar e i nher i t ed f r om Ci r cl e/ / A new i nst ance met hod t hat checks whet her a poi nt i s i nsi de t he ci r cl e/ / Not e t hat i t uses t he i nher i t ed i nst ance f i el d rpubl i c bool ean i sI nsi de( doubl e x, doubl e y) {

doubl e dx = x - cx, dy = y - cy; / / Di st ance f r om cent erdoubl e di st ance = Mat h. sqr t ( dx*dx + dy*dy) ; / / Pyt hagor ean t heor emr et ur n ( di st ance < r ) ; / / Ret ur ns t r ue or f al se

}}

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Data hiding and encapsulation

Data-hiding or encapsulation is an important partof the OO paradigm.

Classes should carefully control access to their data and methods in order to

Hide the irrelevant implementation-level details so theycan be easily changed

Protect the class against accidental or maliciousdamage.

Keep the externally visible class simple and easy to

document

Java has a simple access control mechanism tohelp with encapsulation

Modifiers: public, protected, private, and package(default)

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Package

Every class name lies in a file with an identical name

Every class is defined within a package

// this file is in directory /path../gordon/grid

package gordon.grid

class Shape {...}

---------------------------------

package gordon

// import all classes in the package gordon.grid

import gordon.grid.*

class SubShape extends Shape {

Shape sh1; // short name allowed since class is imported (.*)

gordon.grid.Shape sh2; // fully qualified name (not necessary)

public void perimeter(...);

}

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Access control

Access to packages

Java offers no control mechanisms for packages.

If you can find and read the package you can access it

Access to classes

All top level classes in package P are accessibleanywhere in P

All public top-level classes in P are accessibleanywhere

Access to class members (in class C in packageP)

Public: accessible anywhere C is accessible

Protected: accessible in P and to any of C’s subclasses

Private: only accessible within class C

Package: only accessible in P (the default)

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The static keyword

Java methods and variables can be declared static (to createfields and methods that belong to the class, rather than to aninstance of the class. )

These exist independent of any object

This means that a Class’s

static methods can be called even if no objects of that classhave been created and

static data is “shared” by all instances (i.e., one rvalue per class instead of one per instance

class StaticTest {static int i = 47;}

StaticTest st1 = new StaticTest();StaticTest st2 = new StaticTest();

// st1.i == st2.I == 47

StaticTest.i++; // or st1.I++ or st2.I++// st1.i == st2.I == 48

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Abstract classes and methods

Abstract vs. concrete classes

Abstract classes can not be instantiatedpublic abstract class shape { }

An abstract method is a method w/o a bodypublic abstract double area();

(Only) Abstract classes can have abstract

methods

In fact, any class with an abstract method is

automatically an abstract class

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Example: abstract classpubl i c abst r act cl ass Shape {

publ i c abst r act doubl e ar ea( ) ; / / Abst r act met hods: not epubl i c abst r act doubl e ci r cumf er ence( ) ; / / semi col on i nst ead of body.

}

cl ass Ci r cl e ext ends Shape {publ i c st at i c f i nal doubl e PI = 3. 14159265358979323846;pr ot ect ed doubl e r ; / / I nst ance dat apubl i c Ci r cl e( doubl e r ) { t hi s. r = r ; } / / Const r uctorpubl i c doubl e get Radi us( ) { r et ur n r ; } / / Accessorpubl i c doubl e ar ea( ) { r et ur n PI *r *r ; } / / I mpl ement at i ons of publ i c doubl e ci r cumf er ence( ) { r et ur n 2*PI *r ; } / / abst r act met hods.

}

cl ass Rect angl e ext ends Shape {pr ot ect ed doubl e w, h; / / I nst ance dat apubl i c Rect angl e( doubl e w, doubl e h) { / / Const r uct or

t hi s. w = w; t hi s. h = h;}publ i c doubl e get Wi dt h( ) { r et ur n w; } / / Accessor met hodpubl i c doubl e get Hei ght ( ) { r et ur n h; } / / Anot her accessorpubl i c doubl e ar ea( ) { r et ur n w*h; } / / I mpl ement at i ons of publ i c doubl e ci r cumf er ence( ) { r et ur n 2*( w + h) ; } / / abst r act met hods.

}

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Syntax Notes

No global variables

class variables and methods may be applied toany instance of an object

methods may have local (private?) variables

No pointers

but complex data objects are “referenced”

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Single Inheritance, but

A class may extend only one class, but it may

implement many others A subclass inherits the variables and

methods of its superclass(es), but mayoverride them

Overrides the methods defined in the

class(es) it implements

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Classes and Interfaces The methods of an abst r act class are

implemented elsewhere A f i nal class cannot be extended

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Interfaces

Java does not allow “multiple inheritance” because it introduces

problems as well as benefits. Fortunately,

Java allows you to impose requirements on a class from multipleclass-like interfaces.

An interface is like an abstract class in that it can hold abstractmethod definitions that force other classes to implement ordinarymethods.

But it is also different:

An interface does NOT have instance variables (but itcan have constants)

All methods in an interface are abstract (they each havea name, parameters, and a return type, but noimplementation)

All methods in an interface are automatically public.

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Classes vs. Interfaces

A class definition that implements an interface mustdefine all the methods specified in that interface. Inthis respect, an interface is like an abstract class.

An interface differs from an abstract class, however,in several respects:

An interface only imposes definition requirements;

interfaces do not supply definitions.

A class extends exactly one superclass; a class canimplement an unlimited number of interfaces.

Thus, the purpose of the interface is strictly toimpose requirements via its abstract methods; thereare no method implementations:

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Interfaces

Interfaces provide no mechanism for enforcingmethod specifications, other than method

signatures

you are free to deposit descriptive comments inan interface, however.

Interfaces are excellent places for descriptivecomments for two reasons:

Interfaces, unlike class definitions, are free of

clutter from implementing code.

Programmers look to interfaces for method andclass documentation.

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Interfaces

The interface mechanism is an enormously

important aid to good programming practice.

Interfaces allow you to shift to the Java compiler arequirement-managing responsibility

Interfaces encourage you to document your classes by acting, by convention, asdocumentation centers.

Syntax:publ i c cl ass someClassName i mpl ement s I1, I2

{ … }

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Interfaces Example

java.lang defines a Comparable interface as:publ i c i nt er f ace Compar abl e {i nt

compar eTo( Obj ect ot her ) ; } // noimplementation

If you wantclass to provide some function, implementan appropriate interface

publ i c cl ass Movi e3 ext ends At t r act i on i mpl ement sComparabl e {

publ i c i nt compar eTo ( Obj ect ot her Movi e){ Movi e3 ot her = ( Movi e3) ot her Movi e;

i f ( r at i ng( ) < ot her . r at i ng( ) ) r et ur n - 1;el se i f ( r at i ng( ) > ot her . r at i ng( ) ) r et ur n 1;

el se r et ur n 0; } }

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Exceptions

If an error does occur, that error is said to beexceptional behavior that throws an exception.

Whenever an expression has the potential tothrow an exception, you can embed thatexpression in a try – catch statement, in which you

specify explicitly what Java is to do when anexception actually is thrown.

Exceptions are objects in their own right

They can be generated, caught and handled under program control

Examples: IOException, ArithmeticException, etc.

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try/catch/finally

Associates a set of statements with one or

more exceptions and some handling codet ry {

Thr ead. sl eep( 200) ;

}cat ch( I nt er r upt edExcept i on e) {

Syst em. out . pr i nt l n( e) ;

}f i nal l y {

Syst em. out . pr i nt l n( “Wakeup”) ;

}

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Exceptions

Java will “throw an exception” when unusual

conditions arise during execution of programs, e.g.,

E.g., Attempt to divide an integer by zero

To handle the exception, use the following:

t ry {statement with potential to throw exception}cat ch ( exception-class-name parameter )

{exception-handling-code }

To catch I/O exceptions, use:

FileNotFoundException or IOException class.

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Exceptions

Suppose, for example, that you want to open a file

for reading using a FileInputStream instance.

You can acknowledge that the attempt may throw

an exception by embedding the reading expressions

in a block following the try keyword.

Java stops executing statements in the try block as

soon as an exception is thrown:

try {

... <-- An attempt to attach a stream to a file occurs here

}

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Exceptions

You specify what to do in the event that the

exception is an instance of the IOExceptionclass by writing the keyword catch, followed

by a parameter typed by IOException,

surrounded by parentheses, followed by

another block:

catch (IOException e) {...

}

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Exceptions

To shut a program down, use System.exit(0);

To have a block of statements executed after a try (whether or

not an exception was thrown) use:f i nal l y { clean-up statements }

You can create (and throw) your own exceptions, e.g.,

public class StrangeNewException extends Exception { }throw (new StrangeNewException () )

catch ( StrangeNewException e) { … }

Alternative method to handle exceptions:publ i c st at i c voi d f ( params) t hr ows Except i on- cl ass{ … }

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variables & objects

what happens when you run this?

String a = “foo”;

System.out.println (a);

it prints

foo

what is “foo”?

a string literal that evaluates to a String object

what is a?

a variable whose value is an object reference

what is String a = “foo”?

a declaration and an assignment in one

“foo”(String)

a

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method calls

what happens when you run this?String a = “foo”;

String b = a.toUpperCase ();


it prints

foo

what is toUpperCase?

a method of class String

type is String -> String

declared as public String toUpperCase ()

what is a.toUpperCase ()?a method call on the object a

does it change a?

no, it creates a new string

“foo”

(String)

a

“foo”

(String)

b

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null references

what happens when you run this?String a = null;


it prints

null

what happens when you run this?String a = null;

String b = a.toUpperCase ();System.out.println (b);

it throws a NullPointerException

why?

because a method call must have a subject

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sharing & equality

what happens when you run this?String a = “foo”;

String b = “foo”;System.out.println (b);

it printsfoo

is that the same as this?String a = “foo”;

String b = a;

System.out.println (b);

yes, because String is immutable.

There is no way to distinguish these cases and, in fact, Java

virtual machine may produce upper or lower state in this case.

“foo”

(String)

a

“foo”(String)

b

“foo”

(String)

a

b

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mutable containerswhat happens when you run this?Vector v = new Vector ();


v.addElement (a);

System.out.println (v.lastElement ());

it prints

foo

what happens when you run this?

Vector v = new Vector ();String a = “foo”;

String b = “foo”;

v.addElement (a);

System.out.println (v.lastElement ());

v.addElement (b);System.out.println (v.lastElement ());

it prints

foo

foo

(Vector)

v

“foo”

(String)

a

“foo”

(String)

b

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aliasingwhat about this?

Vector v = new Vector ();

Vector q = v;

String a = “foo”;v.addElement (a);

System.out.println (q.lastElement ());

it prints

foowhy?

because v and q are aliased: they arenames for the same object

what if we now do this?if (v == q) System.out.println (“same object”);

if (v.equals (q)) System.out.println (“same value”);

it printssame object

same value

(Vector)

v

“foo”

(String)

a

q

Aliasing occurs when several different

identifiers refer to the same object. The termis very general and is used in many contexts.

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aliasing & immutables

what does this do?String a = “foo”;

String b = a;a.toUpperCase ();


it printsfoo

why?

because strings are immutable

The objects created by thetoUpperCase method is eventuallyGCed (garbage collected.)

“foo”

(String)

a

b

“foo”

(String)

a

“FOO”

(String)

b

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polymorphism

what does this do?


Vector e = new Vector ()

v.addElement (e);

e.addElement (“foo”);

System.out.println (((Vector) v.lastElement ()).lastElement ());

it printsfoo

what kind of method is addElement?

declared as public void addElement (Object o)

(Vector)

v

(Vector)

e

(String)

“foo”

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On polymorphism

First identified by Christopher Strachey (1967) anddeveloped by Hindley and Milner, allowing types such as alist of anything.

E.g. in Haskell we can define a function which operates on alist of objects of any type a (a is a type variable).

l engt h : : [ a] - > I nt

Polymorphic typing allows strong type checking as well asgeneric functions. ML in 1976 was the first language withpolymorphic typing.

Ad-hoc polymorphism (aka overloading) is the ability to use

the same syntax for objects of different types, e.g. "+" for addition of reals and integers.

In OOP, the term is used to describe variables which mayrefer at run-time to objects of different classes.

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reference loops

can i even add v to itself?


v.addElement (v);

System.out.println (v.lastElement ())

yes, try it!

and this?

v.addElement (5);

no, 5 is a primitive value, not an object

(Vector)

v

“foo”

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a pair of methods

some types

what are the types of addElement, lastElement?

addElement (Object), return voidlastElement (), return Object

a puzzle

how are x and e related after this?

v.addElement (e);

x = v.lastElement ();

they denote the same object

can the compiler infer that?

no! not even that x and e have the same class

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downcasts

what does this do?



v.addElement (a);String b = v.lastElement ();


compiler rejects it: v.lastElement doesn’t return a String!

what does this do?Vector v = new Vector ();


v.addElement (a);

String b = (String) v.lastElement ();


it prints

foo

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upcasting and downcasting Suppose we have object O of class C1 with

superclass C2 In Java, upcasting is automatic but

downcasting must be explicit.

Upcasting: treating O as a C2

Downcasting: treating O as a C1

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variable & object classes

what does this do?Vector v = new Vector ();


v.addElement (a);

Object o = v.lastElement ();

System.out.println (o.getClass ());

it prints java.lang.String

what’s going on here?

getClass returns an object representing a class

o.getClass () is the class o has at runtime

System.out.println prints a string representation, ie, thename

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Some key concepts

variables & objects

variables hold object references (or primitive values like 5)

null is a special object reference

sharing, equality & mutability

distinct objects can have the same value

state is held in value of instance variables

an object can be mutable (state may change) or immutable

two variables can point to the same object; changing oneaffects the other

methods

a method has a ‘subject’ or ‘target’ object

may be polymorphic, ie. work on several types of object

compile-time & runtime types

an object has a type at runtime: the class of its constructor

a variable has a declared, compile-time type or class

runtime class is subclass of compile-time class

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The Java API

java.applet

Applet class

java.awt

Windows, buttons,

mouse, etc.

java.awt.image

image processing

java.awt.peer

GUI toolkit

java.io

System.out.print

java.lang

length method for arrays;

exceptions

java.net

sockets

java.util

System.getProperty

See http://java.sun.com/j2se/1.3/docs/api/for the current APIs

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The package java.lang

The class Obj ect

The root class in Java

Example methods: cl one( ) , equal s( ) ,t oSt r i ng( )

Subclasses may override these methods

The class Cl ass

Example methods: get Name( ) ,get Super Cl ass( )

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Observing an object’s class

voi d pr i nt Cl assName ( Obj ect obj ) {Syst em. out . pr i nt l n( "The cl ass of " + obj +

" i s " + obj . get Cl ass( ) . get Name( ) ) ;}

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Strings in Java

Strings are not a primitive data type, but

represented as objects.

Many methods defined in class java.lang:

Several constructors

Lots of methods: concat(), equals(), indexOf(), length()

strings are concatenated with +

Strings are constants (immutable)

You can concatenate two strings to produce a new,longer string, using the + operator, but you cannot add,

delete, insert into, or delete from any particular string.

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StringBuffers in Java

Several methods defined in class

java.lang: Constructors can specify length or initial

value append(), insertf(), length(), toString()

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Java.lang.system Printstreams

System.out.err(“Error message”);

Inputstreams

System.in.read(inputCharacter)

System.in.read(inputBuffer, 0, maxChars)

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The Cloneable Interface A class implements the cl oneabl e interface

by overriding the Obj ect method cl one( ) For example, we could add a cl one( )

method to the FIFO class, if we wanted to be

able to make copies of queues.

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The class java.util Interface to host OS

Some basic functions and data structures

BitSet, Dictionary (the superclass of Hashtable),

Stack, Vector

Random number generation

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System Properties System properties are like UNIX environment

variables, but platform independent The API class java.util has methods for

accessing the system properties

/ / d t i i t i bl

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/ / det er mi ne envi r onment var i abl esi mpor t j ava. ut i l . * ;cl ass envSnoop {

publ i c st at i c voi d mai n ( St r i ng ar gs[ ] ) {Pr oper t i es p;

St r i ng s;p = Syst em. get Pr oper t i es( ) ;p. l i st ( Syst em. out ) ;

s = Syst em. get Pr oper t y( "user . name") ;Syst em. out . pr i nt l n( "user . name="+s) ;s = Syst em. get Pr oper t y( "user . home") ;Syst em. out . pr i nt l n( "user . home="+s) ;

}}

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Java GUI The awt class allows you to create

frames

buttons

menus and menubars

checkboxes

text areas

scrolling lists

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Java.net Defines several useful objects:

URLs

Internet Addresses

Sockets

Datagrams

packets

sockets

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Java I/O

In Java, there is clear distinction between

streaming in formation to and from a program (bytes, char, binary)

formatting data (number of decimal places, ints, etc.)

flexibility comes at the price of more coding

flexibility implies generality: read from sockets, createreaders/writers with more capabilities

In C and Fortran this distinction is not there

can read and format with little verbosity

flexible in terms of formatting

price to pay: cannot extend the routines provided by thelanguage to more general situations

In C++, the iostream file (cin/cout) can be extended at the cost of efficiency

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Efficiency

scientific computing usually deals with numbers

written to and read from files

C++

printf/scanf/fprintf/fscanf are more efficient than cin/cout

Java

flexibility comes at the price of efficiency

use buffered I/O for more efficiency

Fortran 90/95

ideally suited to scientific program (its main reason for

being created)

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Streams Reader/Writer: abstract classes

cannot be instantiated

meant for character streams (mostly unicode)

unicode: 2 bytes/character (for most languages)

InputStream/OutputStream: regular class

for byte input and output

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Reader Subclasses of Reader (package

java.io.reader)

BufferedReader

LineNumberReader (subclass of BufferedReader)

CharArrayReader

FilterReader

InputStreamReader

PipedReader

StringReader

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Writer Subclasses of Writer (package java.io.writer)

BufferedWriter

CharArrayWriter

FilterWriter

OutputStreamWriter

PipedWriter

PrintWriter

StringWriter

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InputStream Subclasses of InputStream

ByteArrayInputStream

FileInputStream

FilterInputStream

DataInputStream

ObjectInputStream

PipedInputStream

SequenceInputStream

StringBufferInputStream

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OuputStream Subclasses of OuputStream

ByteArrayInputStream

FileOutputStream

FilterOutputStream

DataOutputStream

ObjectOutputStream

PipedOutputStream

SequenceOutputStream

StringBufferOutputStream

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java.io Package Most (if not all) of the Java I/O classes are

contained in the package java.io To use these classes, simply insert at the top

of your code (below any package definitions):

import java.io.*

This will import all the classes (*) in the java.io package

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Unbuffered Reading FileInputStream os = new

FileInputStream(new File(“gordon”));

Disk/file Read asingle byte

Program

Read 1 byte from the file/disk

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Buffered Reading

BufferedInputStream(InputStream in); // constructor

BufferedInputStream(InputStream in, int size);BufferedInputStream os = new BufferedInputStream(new File(“gordon”));

String s = os.readLine(); // s=reference, readLine specific to BufferedInputStream


Program

Read 1 byte from the buffer

Buffer of n bytes

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Buffered Writing

BufferedOutputStream(OutputStream out); // constructor

BufferedOutputStream(OutputStream out, int size);void write(byte[] b, int off, int len) throws IOException; // particular method

BufferedOutputStream os = new BufferedOuputStream(new FileInputStream(“gordon”));


Program

write 1 byte to the buffer

Buffer of n bytes

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Binary Data

Consider an int (or float) (4 bytes): float f;

Computers can write this data to a file in one of twoways:

BIG_ENDIAN (network order: order used byDataOutputStream)

byte 1 (msb), byte 2, byte 3, byte 4 (lsb)

LITTLE_ENDIAN

byte 4 (lsb), byte 3, byte 2, byte 1 (msb);

msb: most significant bit

lsb: least significantn b

http://www.netrino.com/Embedded-Systems/How-To/Big-Endian-Little-Endian







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Writing Binary// public class DataOutputStream extends FilterOutputStream

// implements DataOutput { // class definition

// DataOutputStream(OutputStream out); // constructor

DataOutputStream os = new DataOutputStream(new File(“gordon.bin”));

os.writeBytes(“gordon”);

double d=4.5; os.writeDouble(d);

//int j; os.writeInt(j); // will not compile: j is not defined

long lg=3455; os.writeLong(lg);

byte[] b = new byte[100];

… initialize b …

offset=10; len=20;

os.writeBytes(b, offset, len); // write bytes b[9] through b[29]

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Reading Binary//DataInputStream(InputStream out);

DataInputStream is = new DataInputStream(new File(“gordon.bin”));

double d = os.readDouble(d);

int j = os.readInt();

long lg = os.readLong(lg);

int offset = 10; len = 30;

byte[] b = new byte[100];os.read(b, off, len); // read len bytes starting from b[offset]

// Cannot read multiple longs, ints, etc. at a given time.

// To do this, one must call writeLong(), etc. multiple times

// This is inefficient since “writeLong()” has a cost associated with calling it

// In C++, fread(char* ptr, int size, int nb, FILE* fd);

// can read n bytes calling the routine once. Therefore, it is expected to be

// quite a bit faster than the Java version.

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