Java AWT Event Handling: A Comprehensive Guide

Posted on May 4, 2024 in Computers

Java AWT Event Handling

Introduction to Checkboxes

This code demonstrates the use of checkboxes in Java AWT:

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

public class CheckBoxDemo extends Applet {
    Checkbox cb1 = new Checkbox("Java");
    Checkbox cb2 = new Checkbox("C++");

    public void init() {
        add(cb1);
        add(cb2);
    }
}

Displaying Messages on Applets

This code shows how to display a message on an applet:

import java.awt.*;
import java.applet.*;

public class FirstApplet extends Applet {
    public void paint(Graphics g) {
        g.drawString("Welcome to JavaApplet", 50, 30);
    }
}

Applet Parameters

Parameters can be passed to applets using the <PARAM> tag within the <APPLET> tag. The getParameter method retrieves these values.

Syntax:

<APPLET code="MyApplet.class" width=150 height=80>
    <PARAM name=parameter1 value=value1>
    <PARAM name=parameter2 value=value2>
</APPLET>

Code Example:

import java.applet.Applet;
import java.awt.Graphics;

public class WelcomeParam extends Applet {
    String msg = "";

    public void init() {
        msg = getParameter("Username");
        msg = "Welcome " + msg;
    }

    public void paint(Graphics g) {
        g.drawString(msg, 50, 50);
    }
}

Document Base and Code Base

The getDocumentBase() method retrieves the base URL of the document containing the applet, while getCodeBase() gets the base URL of the applet’s code.

Example:

import java.applet.*;
import java.net.*;

public class DocumentBaseExample extends Applet {
    public void init() {
        URL documentBase = getDocumentBase();
        URL codeBase = getCodeBase();
        System.out.println("Document Base: " + documentBase);
        System.out.println("Code Base: " + codeBase);
    }
}

Event Classes and Handling

Events represent changes in an object’s state. Event sources generate events, and event listeners handle them.

Common Event Classes:

ActionEvent: Triggered by component activation.
AdjustmentEvent: Generated when scrollbars are used.
TextEvent: Occurs when text is changed.
ContainerEvent: Triggered by adding/removing components.
ComponentEvent: Generated for resize, move, hide, or show actions.
ItemEvent: Occurs when an item is selected from a list.
FocusEvent: Triggered when a component gains keyboard focus.
KeyEvent: Generated for key presses and releases.

Event Delegation Model:

Event Source: Generates the event.
Event Object: Encapsulates event information.
Listener Interface: Defines event handling methods.
Listener Object: Implements the listener interface and processes events.

Adapter Classes

Adapter classes provide empty implementations for event listener interfaces, simplifying event handling. Examples include MouseAdapter, KeyAdapter, and WindowAdapter.

AWT Class Hierarchy

The AWT class hierarchy defines the relationships between graphical components:

Component: Base class for graphical objects.
Container: Manages component layout.
Window: Top-level window without borders.
Panel: Container without borders.
Frame: Top-level window with borders and a menu bar.

Java AWT Event Handling: A Comprehensive Guide