AWT-Abstract Window Toolkit

Event Delegation Modal

• Java supports Event Driven Programming.
• For Event Driven Programming java uses Event Delegation Modal.
• This model defines standard to generate and process events.
• The event delegation model contains two types of elements:
• Event sources
• Event listeners

• An event source is a GUI component like Button etc. that generates an event.
• The event is generated by any type of user interaction.
For ex:
if a user clicks a Button, if user resize a window etc.

• These are the objects who are interested in events of Event Sources.
• Event listeners receive notification of an event.
• To receive notification of an event, the listener must be registered with the Event Source by using Event Source method.
• To subscribe to the event source, the event listener has to implement the appropriate listener interface.
For ex:btn.addActionListener(x);

Interface ActionListener [ java.awt.event ]

• This Listener Interface must be implemented by the listener object who is interested in event of buttons, menu items etc.

• public abstract void actionPerformed(ActionEvent e);
• This method will be automatically called of the listener when a button or menu item is clicked.

 import java.awt.*;
 import java.awt.event.*;
 class MyFrame extends Frame implements ActionListener
 {
    Button btn;
    MyFrame()
    {
        setLayout(new FlowLayout());
        btn=new Button("OK");
        btn.addActionListener(this);
        add(btn);
    }
    public void actionPerformed(ActionEvent e)
    {
        setTitle("Welcome to AWT Events");
    }
 }
 class demo
 {
    public static void main(String args[])
    {
        MyFrame frm=new MyFrame();
        frm.setVisible(true);
        frm.setSize(400,400);
    }
 }

Class ActionEvent [ java.awt.event ]

• The ActionEvent is generated when button or menu item is clicked.
• This Object contains information about the button or menuitem due to which the event was generated.

• String getActionCommand()
• Returns the command string associated with this EventSource.
• Object getSource()
• Returns the ID of EventSource due to which the event has generated.

 import java.awt.*;
 import java.awt.event.*;
 class MyFrame extends Frame implements ActionListener
 {
    Button b1,b2;
    public MyFrame()
    { 
        setLayout(new FlowLayout());
        b1=new Button("RED");
        b2=new Button("BLUE");
        b1.addActionListener(this);
        b2.addActionListener(this);
        add(b1);
        add(b2);
    }
    public void actionPerformed(ActionEvent e)
    {
        if(e.getSource()==b1)
            setBackground(Color.RED);
        else
            setBackground(Color.BLUE);
    }
 }
 class demo
 {
    public static void main(String args[])
    {
        MyFrame frm=new MyFrame();
        frm.setSize(400,300);
        frm.setVisible(true);
    }
 }