Inner class, Static class, Multithreading, JavaBeans

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Why Inner classes?

New feature available in Java and D language.

Different from Nested class and Composition.Provides better code hiding. Logically related classes can be made as Inner classes. Classes implemented through Multiple Inheritance in C++ can be implemented using Inner classes in Java.

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Inner Class

Inner classes are classes defined within other classes. The class that includes the inner class is called the outer class

Inner classes can be hidden from other classes in the samepackage.

They can be nested to any depth

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Inner Class

An inner class definition is a member of the outer class in the

same way that the instance variables and methods of the outerclass are members

An inner class is local to the outer class definition

The name of an inner class may be reused for something elseoutside the outer class definition

If the inner class is private, then the inner class cannot be accessed by name outside the definition of the outer class.

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Structurepublic class Outer {

private class Inner{ // inner class instance variables // inner class methods } // end of inner class definition

// outer class instance variables// outer class methods }9/3/2012 5

TYPESThere are four categories of inner classes in Java:

1.Inner classes (non-static)2.Static inner classes 3.Local classes (defined inside a block of Java code) 4.Anonymous classes (defined inside a block of Java code)

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Non-Static Inner Classes

Non-static inner classes are defined without the keyword static.

An instance of a non-static inner class can only exist with aninstance of its enclosing class.

Methods of a non-static inner class can directly refer to any member of any enclosing class (including private members).

Accessibility : public,package/default,protected or private.

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Non-Static Inner ClassesAccessing the members of the inner class:

Need to instantiate an object instance of an inner class first.An Inner class object can access the implementation of the object that created it including private fields

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Inner Class CreationInner classes can be created as follows:

outerObj = new (arguments); . innerObj = outerObj.new

(arguments);

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ExampleInnerExample variables; methods();

InnerClass variables; methods() ;InnerClass i= new innerClass(); InnerExample2 OuterClass outer = new OuterClass(); outer.new InnerClass().welcome(); OuterClass InnerClass welcome();9/3/2012 10

Example//A named inner class is used. // This is used to show that it can access non-local variables in the enclosing object. public class InnerExample { static String msg= "Welcome"; public static void main(String[] arg) { class InnerClass { public void doWork() { System.out.println(msg); } } InnerClass i = new InnerClass(); i.doWork(); }

}

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// Access inner class from outside public class InnerExample2 { public static void main(String[] args) { OuterClass outer = new OuterClass(); outer.new InnerClass().welcome(); } } class OuterClass { public class InnerClass { public void welcome() { System.out.println("Welcome from InnerClass()"); } } }

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Access

Inner and outer class has access to each others private

members

Within the definition of a method of an inner class: It is legal to reference a private instance variable of the outer class It is legal to invoke a private method of the outer class The inner class has a hidden reference to the outer class

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Cont..

Within the definition of a method of the outer class

It is legal to reference a private instance variable of the innerclass on an object of the inner class It is legal to invoke a (nonstatic) method of the inner class as long as an object of the inner class is used as a calling object

Within the definition of the inner or outer classes, the modifiers

public and private are equivalent

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The .class File for an Inner Class

Compiling any class in Java produces a .class file named

ClassName.class

Compiling a class with one (or more) inner classes causes both (or more) classes to be compiled, and produces two (or more)

.class files Such as ClassName.class and ClassName$InnerClassName.class

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Nesting Inner Classes

It is legal to nest inner classes within inner classes

The rules are the same as before, but the names get longer Given class A, which has public inner class B, which has public inner class C, then the following is valid: A aObject = new A(); A.B bObject = aObject.new B();

A.B.C cObject = bObject.new C();

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Advantages

They can make the outer class more self-contained since they

are defined inside a class.Both of their methods have access to each other's private methods and instance variables.

Using an inner class as a helping class is one of the most usefulapplications of inner classes If used as a helping class, an inner class should be marked private

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Static Class

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An Intro to Static Class

In order to understand the use of the static keyword in class

declaration, we need to understand the class declaration itself.We can declare two kinds of classes: top-level classes and inner classes.

The static keyword does not do to a class declaration what itdoes to a variable or a method declaration.

Inner Class

We define an inner class within a top-level class An inner class is associated with an instance of its enclosing class and has direct access to that object's methods and fields. Also, because an inner class is associated with an instance, it cannot define any static members itself.

To instantiate an inner class, you must first instantiate the outer class. Then, create the inner object within the outer object with this syntax:

OuterClass.InnerClass innerObject = outerObject.new

InnerClass();

An inner class can be one of the following four types:

1.

Local

2.

Member

3.

Nested

Local Class

Local classes are the same as local variables, in the sense that

they're created and used inside a block.

Once you declare a class within a block, it can be instantiated as many times as you wish within that block.

Like local variables, local classes aren't allowed to be declaredpublic, protected, private, or static.

Member Class

Member classes are defined within the body of a class.

You can use member classes anywhere within the body of thecontaining class.

You declare member classes when you want to use variables

and methods of the containing class without explicitdelegation.

The member class is the only class that you can declare static. When you declare a member class, you can instantiate that member class only within the context of an object of the outer class in which this member class is declared. If you want to remove this restriction, you declare the member class a static class.

When you declare a member class with a static modifier, it becomes a nested top-level class and can be used as a normal

top-level class

Nested Top-level Class

A nested top-level class is a member classes with

a static modifier.

A nested top-level class is just like any other top-level class except that it is declared within another class or interface.

Nested top-level classes are typically used as a convenient wayto group related classes without creating a new package.

Nested classes are divided into two categories: static and nonstatic. Nested classes that are declared static are simply called

static nested classes. Non-static nested classes are called innerclasses.

Static nested classes are accessed using the enclosing class name:

Outer Class. Static Nested Class For example, to create an object for the static nested class, use this syntax:

Outer Class.StaticNestedClass nested Object = new Outer

Class.StaticNestedClass();

Static Nested Classes

As with class methods and variables, a static nested class is associated with its outer class.

And like static class methods, a static nested class cannot refer directly to instance variables or methods defined in its enclosing class it can use them only through an object reference.

Note: A static nested class interacts with the instance members of its outer class (and other classes) just like any other top-level class. In effect, a static nested class is behaviorally a top-level class that has been nested in another top-level class for packaging convenience.

It is a way of logically grouping classes that are only used inone place.It increases encapsulation.Nested classes can lead to more readable and maintainable code.

Multithreading

Multithreading

A thread is part of process in execution Two or more threads executed concurrently is multithreading

Advantage of Multithreading

Maximum CPU Utilization

Thread in JAVA

By default every JAVA program has a main thread.

How to access main threadThread t = Thread.currentThread();

Methods in Thread ClassMethod getName() getPriority() isAlive() join() run() sleep() start() Meaning Obtain threads name Obtain threads priority Determine if a thread is alive Wait for a thread to terminate Entry point of the thread Suspend for few times start a thread by calling run().

Life Cycle of ThreadThread t = new Thread(this)

t.Start()

Wait() Notify()

t.sleep(1000)

Creating a ThreadTwo ways

Runnable interface

Extending thread class

Thread Using Runnable Interfaceclass NewThread implements Runnable { Thread t; NewThread() { t = 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--) {

Thread Using Runnable InterfaceSystem.out.println("Child Thread: " + i); Thread.sleep(500); } } catch (InterruptedException e) {

System.out.println("Child interrupted.");} System.out.println("Exiting child thread.");

}}9/3/2012 37

class ThreadDemo { public static void main(String args[]) { new NewThread(); // create a new thread try { for(int i = 5; i > 0; i--) { System.out.println("Main Thread: " + i);

Thread.sleep(1000);} } catch (InterruptedException e) { System.out.println("Main thread interrupted."); } System.out.println("Main thread exiting.");}

Output

Child thread: Thread[Demo Thread,5,main] Main Thread: 5 Child Thread: 5 Child Thread: 4 Main Thread: 4 Child Thread: 3 Child Thread: 2 Main Thread: 3 Child Thread: 1 Exiting child thread. Main Thread: 2 Main Thread: 1 Main thread exiting.

Using Extending Threadclass NewThread extends Thread { NewThread() { // Create a new, second thread super("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 {

Using Extending Threadfor(int i = 5; i > 0; i--) { System.out.println("Child Thread: " + i);

Thread.sleep(500);} } catch (InterruptedException e) { System.out.println("Child interrupted."); }

System.out.println("Exiting child thread.");} }9/3/2012 41

class ExtendThread { public static void main(String args[]) { new NewThread(); // create a new thread

try {for(int i = 5; i > 0; i--) { System.out.println("Main Thread: " + i); Thread.sleep(1000); } } catch (InterruptedException e) {

System.out.println("Main thread interrupted."); }System.out.println("Main thread exiting."); } }

Multiple Threadingclass NewThread implements Runnable { String name;

Thread t;NewThread(String threadname) { name = threadname; t = new Thread(this, name); System.out.println("New thread: " + t);

t.start();} public void run() {

try { for(int i = 5; i > 0; i--) { System.out.println(name + ": " + i); Thread.sleep(1000); } } catch (InterruptedException e) {

System.out.println(name + "Interrupted");} System.out.println(name + " exiting."); } }

class MultiThreadDemo { public static void main(String args[]) { new NewThread("One"); // start threads new NewThread("Two"); new NewThread("Three"); try {

Thread.sleep(10000);} catch (InterruptedException e) { System.out.println("Main thread Interrupted"); } System.out.println("Main thread exiting."); } }

OutputNew thread: Thread[One,5,main] New thread: Thread[Two,5,main] Two: 3 One: 2

New thread: Thread[Three,5,main]One: 5 Two: 5 Three: 5 One: 4 Two: 4 Three: 4 One: 3 Three: 3

Three: 2Two: 2 One: 1 Three: 1 Two: 1 One exiting. Two exiting. Three exiting. Main thread exiting.

isAlive()

Final boolean isAlive()

Join()Final void join()

Thread Priority

Low priority to high priority Numbered from 1 to 10

Final void setPriority(int level) Arguments in constants MIN_PRIORITY NORM_PRIORITY

MAX_PRIORITY

Final int getPriority()

A Java Bean is a reusable software component that can bemanipulated visually in a builder tool.

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Introduction

JavaBeans brings component technology to Java.

JavaBeans lets you write Beans, that you can visuallymanipulate within application builder tools.

Beans are classes written in the Java programming language conforming to a particular convention.

They are used to encapsulate many objects into a single object (the bean), so that they can be passed around as a single bean object instead of as multiple individual objects.

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Introduction

A Beans methods are no different than Java methods, and can be called from other Beans or a scripting environment Beans can be used with both builder tools, or manually manipulated by text tool through programmatic interfaces. Compact and Easy Portable Leverages the Strengths of the Java Platform Flexible Build-Time Component Editors Multithreading Security9/3/2012 52

Convention

A JavaBean is a Java Object that is serializable

It allows applications and frameworks to reliably save,store, and restore the bean's state

has a 0-argument constructor The class must have a public default constructor. This allows easy instantiation within editing and activation frameworks.

and allows access to properties using getter and setter methods

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package sunw.demo.simple;

public void setColor(Color newColor)

Example

import java.awt.*;import java.io.Serializable; public class SimpleBean extends Canvas implements Serializable

{color = newColor;repaint(); }

public void paint(Graphics g) {g.setColor(color); g.fillRect(20, 5, 20, 30); }

{private Color color = Color.green; public SimpleBean(){ setSize(60,40); setBackground(Color.red); } }9/3/2012 54

public Color getColor(){return color; }

In addition to the Beans Development Kit (BDK) version 1.0, you

will need the Java Development Kit (JDK) version 1.1.

http://www.mediafire.com/?4hn81bgt83m29dg

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Builder tool

A builder tool maintains Beans in a palette or toolbox. The user can select a component from the toolbox and place it into a container, choosing its size and position.

None of these require a single line of code to be written by the

application developer.

The application development tool does that. This is accomplished through a set of standard interfaces provided by the component environment that allow the components to publish, or expose, their properties.9/3/2012 56

BeanBox

The BeanBox is an application for testing Beans, and serves as a

simple demonstration for how more sophisticated Beansawarebuilder tools should behave.

The BeanBox is also a nice tool for learning about Beans.

The \BDK1.1\beanbox. directory contains Windows (run.bat)and Unix (run.sh) scripts that start the BeanBox.

When started, the BeanBox displays three windows: The BeanBox, ToolBox, and the Properties sheet.

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Creating Your Bean

Save the program and compile it using javac programname.java

Create a file called manifest.tmp usingedit manifest.tmp

The file manifest.tmp should contain Name: programname.class Java-Bean: True

Create the jar file using

jar cfmv jarfilename.jar manifest.tmp programname.class

Copy the jar file in \BDK1.1\jars folder9/3/2012 58

Loading bean

When the BeanBox is started, it automatically loads all the Beans it finds within the JAR files contained in the \BDK1.1\jars

directory.

You can also load Beans Using the File|LoadJar menu. The ToolBox contains the Beans available for use by the BeanBox.

To add a Bean to the BeanBox

Click on the bean name in the ToolBox. The cursor will change to

a crosshair.

Click within the BeanBox. The Bean will appear and will be selected.9/3/2012 59

JavaBean Concepts- Properties

Properties are attributes of a Bean that are referenced by

name.

These properties are usually read and written by calling methods on the Bean specifically created for that purpose.

A programmer would set or get the value of this propertythrough method calls, while an application developer using a visual development tool would manipulate the value of this property using a visual property editor.

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Edit Properties

The Properties sheet displays each propertys name and itscurrent value.

You can edit the property value by clicking on a property within

the Properties sheet.

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JavaBean Concepts- Events

Beans use events to communicate with other Beans. A Bean that wants to receive events (a listener Bean) registers its interest with the Bean that fires the event (a source Bean).

Builder tools can examine a Bean and determine which events that Bean can fire (send) and which it can handle (receive).

To see which events a Bean can send, choose the Edit|Events BeanBox menu item.

A list of events, grouped by interface, will be displayed. Under each interface group is a list of event methods. These are all inherited from Canvas.9/3/2012 62

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Hooking Up Events in the BeanBox

In this example you will use two OurButton bean instances to

stop and start an instance of the animated Juggler bean.

You will label the buttons "start" and "stop"; Make the start button, when pressed, invoke the Juggler beans start method.

Make the stop button, when pressed, invoke the Juggler beans stop method.

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JavaBean Concepts- Persistence

A Bean persists by having its properties, fields, and state saved and restored to and from storage.

The mechanism that makes persistence possible is calledserialization.

When a Bean instance is serialized, it is converted into a data stream and written to storage.

Serialization done by implementing the java.io.Serializable

interface

Select the File|Save SerializeComponent menu item.

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Bean Introspection Reports

Introspection is the process of discovering a Beans designtime features

You can generate a Bean introspection report by choosing the the Edit|Report menu item.

The report lists Bean events, properties, and methods, and their characteristics.

By default Bean reports are sent to the java interpreters

standard output.

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Using the BeanBox to Generate Applets

Choose File|Make Applet to bring up the MakeApplet dialog.

Use the default JAR file and applet name for this example. Pressthe OK button.

The generated files were placed in the beanbox/tmp/myApplet directory.

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Merits and Demerits

Write once, run anywhere. The configuration settings of a Bean can be saved in a persistent storage and can be restored at a later time.

A Bean may register to receive events from other objects and

can generate events that are sent to it.

A class with a null constructor is subject to being instantiated in an invalid state. The developer might not realize that. The

compiler cant detect such a problem.

This can be used in web based application also.9/3/2012 68

Interface AppletInitializer

Description Methods in this interface are used to initialize Beans that are also applets. This interface allows the designer to specify information about the events, methods and properties of a Bean. This interface allows the designer to provide a graphical user interface through which a bean may be configured. Methods in this interface determine if a bean is executing in design mode. A method in this interface is invoked when an exception has occurred. A method in this interface is invoked when a bound property is changed. Objects that implement this interface allow the designer to change and display property values. A method in this interface is invoked when a Constrained property is changed. Methods in this interface allow a bean to execute in environments where GUI is not available.9/3/2012 69

BeanInfos

Customizer

DesignMode ExceptionListener PropertyChangeListener PropertyEditor VetoableChangeListener

Visibility

References

Using the Beans Development Kit1.0, September1997, A

Tutorial, Alden DeSoto.

http://inside.mines.edu/~crader/cs443/Chapters/Chap06.html http://web.mit.edu/1.124/LectureNotes/JavaBeans.html http://java.sun.com/developer/onlineTraining/Beans/JBeansAPI /Magercises/BeansDevelopmentKit/index.html

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Thank yu!

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