Event Handling

Change in the state of an object is known as event i.e. event describes the change in state of source. Events are generated as result of user interaction with the graphical user interface components.

For example, clicking on a button, moving the mouse, entering a character through keyboard, selecting an item from list, scrolling the page are the activities that causes an event to happen.

The events can be broadly classified into two categories:

• Foreground Events -
  Those events which require the direct interaction of user.They are generated as consequences of a person interacting with the graphical components in Graphical User Interface. For example, clicking on a button, moving the mouse, entering a character through keyboard,selecting an item
  from list, scrolling the page etc.


• Background Events -
  Those events that do not require the interaction of end user are known as background events. Operating system interrupts, hardware or software failure, timer expires, an operation completion are the example of background events.

Event Handling is the mechanism that controls the event and decides what should happen if an event occurs. This mechanism has the code which is known as event handler that is executed when an event occurs.

Event handling in Java programming because is integral to the creation of applets and other sorts of GUI-based programs.

The java.awt.event package provides many event classes and Listener interfaces for event handling.

Java Uses the Delegation Event Model to handle the events. This model defines the standard mechanism to generate and handle the events.

The Delegation Event Model has the following key participants:

• Source - The source is an object on which event occurs. Source is responsible for providing information of the occurred event to its handler.
• Listener - It is also known as event handler. Listener is responsible for generating response to an event. Listener waits until it receives an event. Once the event is received , the listener process the event an then returns.

The benefit of this approach is that the user interface logic is completely separated from the logic that generates the event. The user interface element is able to delegate the processing of an event to the separate piece of code.

In this model, Listener needs to be registered with the source object so that the listener can receive the event notification.

For registering the component with the Listener, many classes provide the registration methods, such as:

• Button

public void addActionListener(ActionListener a){}

• MenuItem

public void addActionListener(ActionListener a){}

• TextField

public void addActionListener(ActionListener a){}

public void addTextListener(TextListener a){}

• TextArea

public void addTextListener(TextListener a){}

• Checkbox

public void addItemListener(ItemListener a){}

• Choice

public void addItemListener(ItemListener a){}

• List

public void addActionListener(ActionListener a){}

public void addItemListener(ItemListener a){}

The event handling code can be placed in one of the following places:

1. Within class
2. Other class
3. Anonymous class

Ex1: Within class

import java.awt.*; 

import java.awt.event.*; 

class AEvent extends Frame implements ActionListener{ 

TextField tf; 

AEvent(){ 

 //create components 

tf=new TextField(); 

tf.setBounds(60,50,170,20); 

Button b=new Button("click me"); 

b.setBounds(100,120,80,30); 

//register listener 

b.addActionListener(this);//passing current instance 

//add components and set size, layout and visibility 

add(b);add(tf); 

setSize(300,300); 

setLayout(null); 

setVisible(true); 

} 

public void actionPerformed(ActionEvent e){ 

tf.setText("Welcome"); 

} 

public static void main(String args[]){ 

new AEvent(); 

} 

} 

Ex2: Java event handling by outer class

import java.awt.*; 

import java.awt.event.*; 

class AEvent2 extends Frame{ 

TextField tf; 

AEvent2(){ 

//create components  

tf=new TextField(); 

tf.setBounds(60,50,170,20); 

Button b=new Button("click me"); 

b.setBounds(100,120,80,30); 

//register listener 

Outer o=new Outer(this); 

b.addActionListener(o);  //passing outer class instance 

//add components and set size, layout and visibility 

add(b);add(tf); 

setSize(300,300); 

setLayout(null); 

setVisible(true); 

} 

public static void main(String args[]){ 

new AEvent2(); 

} 

} 

Outer class to handle event:

import java.awt.event.*; 

class Outer implements ActionListener{ 

AEvent2 obj; 

Outer(AEvent2 obj){ 

this.obj=obj; 

} 

public void actionPerformed(ActionEvent e){ 

obj.tf.setText("welcome"); 

} 

}