04. AWT Event Handling

1. Event Delegation Model

Java AWT uses the Delegation Event Model.

  • Event Source: The component where the event occurs (e.g., Button).
  • Event Listener: The code to be executed when an event happens (Implementation of an Interface).
  • Event Object: Information about the occurred event (e.g., clicked button, mouse coordinates).

In other words, you register a listener (add) to an event source, and when an event occurs, the listener’s method is called.

2. Major Events and Listeners

Component Event Class Listener Interface Major Method
Button ActionEvent ActionListener actionPerformed(ActionEvent e)
List, Choice ItemEvent ItemListener itemStateChanged(ItemEvent e)
TextField TextEvent TextListener textValueChanged(TextEvent e)
Frame (Window) WindowEvent WindowListener windowClosing(WindowEvent e)
Mouse MouseEvent MouseListener mouseClicked, mousePressed, …
Keyboard KeyEvent KeyListener keyPressed, keyReleased, …

3. How to Handle Events

Method 1: Implementing the Listener Interface Directly

This method involves making the class itself act as a listener.

import java.awt.*;
import java.awt.event.*;

public class EventExam implements ActionListener {
    Frame f;
    Button b;

    public EventExam() {
        f = new Frame("Event Test");
        f.setLayout(new FlowLayout());

        b = new Button("Click Me");
        b.addActionListener(this); // Register the listener (myself) to the button

        f.add(b);
        f.setSize(300, 200);
        f.setVisible(true);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        System.out.println("Button Clicked!");
    }

    public static void main(String[] args) {
        new EventExam();
    }
}

Method 2: Using Anonymous Inner Classes

This method implements the listener on the spot without creating a separate class implementation. It is the most common approach.

import java.awt.*;
import java.awt.event.*;

public class AnonymousEventExam {
    public static void main(String[] args) {
        Frame f = new Frame("Anonymous Event");
        f.setSize(300, 200);
        f.setLayout(new FlowLayout());

        Button b = new Button("Close Window");

        // 1. Handling Button Click (ActionListener)
        b.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                System.out.println("Close button clicked");
                System.exit(0);
            }
        });

        // 2. Handling Window 'X' Button (WindowListener -> WindowAdapter)
        // Since WindowListener has 7 methods, we use the Adapter class to override only what we need.
        f.addWindowListener(new WindowAdapter() {
            @Override
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        });

        f.add(b);
        f.setVisible(true);
    }
}

* Adapter Class

When it is cumbersome to implement all methods of an interface (e.g., WindowListener has 7 methods), you can inherit from an Adapter Class (like WindowAdapter) which has empty implementations of these methods, and override only the necessary methods.

  • WindowListener -> WindowAdapter
  • MouseListener -> MouseAdapter
  • KeyListener -> KeyAdapter
  • (Since ActionListener has only one method, it does not have an adapter.)
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