Implementing Interfaces and Event Handling in Java, Slides of Java Programming

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Java-Programming

Day-

Outline

• Interfaces

 What? Why?

 Creating Interfaces

 Implementing interfaces

• Few Interfaces

• Event Handling

 Java’s Event Model

 Event Listener

 Event Handling Techniques

 java.awt.event package

 Keyboad and Mouse Events

• Anonymous classes

Interfaces..cont..

 One of the key features in software development

 Various programs communicating with each other require standard

set of methods to be present

 A software saying I have Printable interface implemented mean it

an communicate with printer

 Example

 EditAble interface

 Methods

 Cut  Copy  Paste

 Constant

 Clipboard capacity

Creating Interfaces

• Declaration

 An interface is simply declared using keyword “ interface ”

 interface selfService { -- - - - body of interface---}

 Optionals

 public, private, protected

 Extends

Examples

 interface selfService { - - - -body of interface - - - - - }

 public interface selfService { - - - - body of interface - - - }

 public interface selfPayment extends selfService {- - -body- -

Where do interface reside

• Interfaces follow almost same rules as that of classes

 Public interfaces should be saved in a separate file with same file name

 Interfaces other than public can be saved in any source file

 There could be only one public interface in a source file others should

be non-public

 A source file can contain an interface and a class.

Interface-Example

 The BasicColors interface

public interface BasicColors{

String RED=“Red”;

String GREEN=“Green”;

String BLUE=“Blue” ;

public void changeToRed(); //method with no body

pubic void changeToGreen();

public void changeToBlue();

 Save this interface to a file BasicColors.java

 Compile it [you will get BasicColor.class]

Using interfaces..

• Few important concepts about interfaces

 To use an interface the interface definition follow the rules of classes

 Import an interface if it is in some other package

 If a package is imported the interface are also imported like

classes

 What if some methods in an interface are not required?

 Provide an empty implementation

 Example

public void ChangeToBlue(){}

 Interface cannot contain variables

 Interface constants are static this mean these can be used without

implementation

 Example

 BasicColors.RED

 All method in an interface are by default public

 Each interface is abstract by default

Using interfaces as abstract type

• Interfaces are good for using as abstract data type

• Often uses as parameter to a constructor or a method

 Example

1. public class BasicColorReader {
2. public class BasicColorReader(){ }
3. public void BasicColorReader(BasicColor obj){
4. String colorInformation=obj.getColorInformation();
5. System.out.println(“Color information = “+colorInformation);
6. } }

 An object of interface mean object of a class that implements that

constructor

Interfaces cont…

• List

 Pacakge: java.util

 Methods

 add(Object obj)

 remove(Object obj)

 Many other methods

 Usage

 Implemented by classes that like to server as a list abstract data

type

• ActionListener

 Package: java.awt.event

 Methods: public void actionPerformed(ActionEvent e)

 Used for listening button click events

Event Handling

• Events! Where do they come from

 When a change in a program occur an even it generted

 When ever a change occurs appropriate even object is created and passed either

to operating system or to some other event handler

 Java intercepts events generated , translate them and route them to proper

event handler

 Example

 When a button of an applet is pressed  A button press event object is generated  Applet translates the event and try to find some handler in the applet  If a handler in the program is found the event is handed over to it  If no handler is found the default handler is used

• Handling Events

 Handling Event: detecting an Event and doing something useful

 A class/Object must listen for some event

 To listen an event it must register for listening itself for that event

 Registration require implementation of proper event listener interface

 Java provide a rich set of interfaces for events

 A component must specify its event listener for its events to be processed

Creating our First Event Hander

• Handling a button click

 Package required  Classes and Listeners for awt component events are defined in java.awt.event package

1. Creating an Applet public class myApplet extends Applet{ }
2. What interface should we use to listen a button click  The ActionListener interface
3. Completing class declaration public class myApplet extends Applet implements ActionListener{ }
4. Adding a button to our applet  Button b=new Button(“Click It”);  add(b);
5. Specifying event listener for our button  b.addActoinListener(this);
6. Implementing ActionListener’s method  public void actionPerformed(ActionEvent event){- - -code- - - }

Execution of the code

• Java registers MyApplet for Button Clicks

 A component can use this class for listening its event

 Whenever a buttonclick event is received by MyApplet it will execute its

actionPerformed() method

• Button b1 assigns its event listening to MyApplet

 When ever this button b1 is clicked the event will handed over to

MyApplet by Java

• When a button is pressed

 actionPerformed() method in MyApplet executed

Event handling using other classes

import java.awt.*;

import java.awt.event.*;

import java.applet.*;

public class myApplet extends Applet implements ActionListener{

private Button button1 = new Button();

MyListener listener=new MyListener();

public void init () {

button1.setText(“Click It");

button1.addActionListener(listener);

add(button1);

}

}

import java.awt.; import java.awt.event.; import java.applet.*; public class MyListener implements ActionListener {

//implementing method of ActionListener

public void actionPerformed(ActionEvent event){

System.out.println("The Button is pressed");

}

Handling events for more than one button

import java.awt.*;

import java.awt.event.*;

import java.applet.*;

public class myApplet extends Applet implements ActionListener{

private Button button1 = new Button();

private Button button2 = new Button();

public void init () {

button1.setText(“Click It");

button1.addActionListener(this);

button2.addActionListener(this);

add(button2);

add(button1); }

//implementing method of ActionListener

public void actionPerformed(ActionEvent event){

System.out.println("The Button is pressed");

} }