Java AWT and Event Handling Notes

Java Abstract Window Toolkit (AWT)

Introduction

AWT is a set of classes and APIs for creating GUIs in Java.
It is one of the oldest GUI toolkits in Java.
Event handling is essential in GUI programming. Events are actions like mouse clicks or key presses.

Java AWT

AWT stands for Abstract Window Toolkit, an API for creating GUIs in Java.
It enables Java programmers to develop window-based applications.
AWT provides components like buttons, labels, and checkboxes.
AWT components use OS resources, making them platform-dependent (look and feel changes based on the OS).
Classes for AWT are in the java.awt package.

Hierarchy

Component Class:
- At the top of the AWT hierarchy. An abstract class with properties for components visible on the screen.
- Contains information about foreground, background, and text colors.
Container:
- A component that can hold other components (buttons, text fields, labels, etc.).
- A subclass of the Component class.
Panel:
- A container to hold components.
- Does not have a title bar, menu bar, or border.
Window:
- A container without a border or menu bar; creates a top-level view.
- Requires a frame, dialog, or another window.
Frame:
- A subclass of Window.
- A container with components like buttons, text fields, labels, etc.
- AWT applications are often created using the frame container.

Platform Independence

AWT achieves platform independence by calling native platform (OS) subroutines to create API components (e.g., TextField, CheckBox, Button).
An AWT GUI with components will have a different look and feel on different platforms (Windows, macOS, Unix) because AWT directly calls the native subroutines.

Useful Methods of Component Class

public void add(Component c): Inserts a component on this component.
public void setSize(int width, int height): Sets the size (width and height) of the component.
public void setLayout(LayoutManager m): Defines the layout manager for the component.
public void setVisible(boolean status): Changes the visibility of the component; default is false.

Example

A simple example: Creating a button within a window frame.

import java.awt.*;

public class AwtProgram1 {
    public AwtProgram1() {
        Frame f = new Frame();
        Button btn = new Button("Hello World");
        btn.setBounds(80, 80, 100, 50);
        f.add(btn); // adding a new Button.
        f.setSize(300, 250); // setting size.
        f.setTitle("JavaTPoint"); // setting title.
        f.setLayout(null); // set default layout for frame.
        f.setVisible(true); // set frame visibility true.
    }

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        AwtProgram1 awt = new AwtProgram1(); // creating a frame.
    }
}

Another example: Creating a user form with text fields and labels.

import java.awt.*;

public class AwtApp extends Frame {
    AwtApp() {
        Label firstName = new Label("First Name");
        firstName.setBounds(20, 50, 80, 20);

        Label lastName = new Label("Last Name");
        lastName.setBounds(20, 80, 80, 20);

        Label dob = new Label("Date of Birth");
        dob.setBounds(20, 110, 80, 20);

        TextField firstNameTF = new TextField();
        firstNameTF.setBounds(120, 50, 100, 20);

        TextField lastNameTF = new TextField();
        lastNameTF.setBounds(120, 80, 100, 20);

        TextField dobTF = new TextField();
        dobTF.setBounds(120, 110, 100, 20);

        Button sbmt = new Button("Submit");
        sbmt.setBounds(20, 160, 100, 30);

        Button reset = new Button("Reset");
        reset.setBounds(120, 160, 100, 30);

        add(firstName);
        add(lastName);
        add(dob);
        add(firstNameTF);
        add(lastNameTF);
        add(dobTF);
        add(sbmt);
        add(reset);

        setSize(300, 300);
        setLayout(null);
        setVisible(true);
    }

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        AwtApp awt = new AwtApp();
    }
}

Java Event Listeners and Handling

Changing the state of an object is known as an event (e.g., button click, mouse dragging).
The java.awt.event package provides event classes and Listener interfaces for event handling.

Event Classes and Listener Interfaces

Event Class	Listener Interfaces
ActionEvent	ActionListener
MouseEvent	MouseListener, MouseMotionListener
MouseWheelEvent	MouseWheelListener
KeyEvent	KeyListener
ItemEvent	ItemListener
TextEvent	TextListener
AdjustmentEvent	AdjustmentListener
WindowEvent	WindowListener
ComponentEvent	ComponentListener
ContainerEvent	ContainerListener
FocusEvent	FocusListener

Steps to Perform Event Handling

Register the component with the Listener:
- Classes provide registration methods for associating components with listeners.
- Examples:
  - 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){}
Write Event Handling Code:
- Event handling code can be placed in one of three locations:
  1. Within the class
  2. In another class
  3. Using an anonymous class

Examples of Event Handling

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();
    }
}

The setBounds(int xaxis, int yaxis, int width, int height) method sets the position of the component.

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();
    }
}

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");
    }
}

Anonymous Class:

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

class AEvent3 extends Frame {
    TextField tf;

    AEvent3() {
        tf = new TextField();
        tf.setBounds(60, 50, 170, 20);
        Button b = new Button("click me");
        b.setBounds(50, 120, 80, 30);

        b.addActionListener(new ActionListener() {
            public void actionPerformed() {
                tf.setText("hello");
            }
        });
        add(b);add(tf);
        setSize(300, 300);
        setLayout(null);
        setVisible(true);
    }

    public static void main(String args[]) {
        new AEvent3();
    }
}

Java ActionListener

The ActionListener interface is notified whenever a button or menu item is clicked.
It is found in the java.awt.event package.
It contains only one method: actionPerformed().
actionPerformed() is invoked automatically when the registered component is clicked.
Syntax:
- public abstract void actionPerformed(ActionEvent e);

How to write ActionListener

Implement the ActionListener interface in the class:
- public class ActionListenerExample implements ActionListener
Register the component with the Listener:
- component.addActionListener(instanceOfListenerclass);
Override the actionPerformed() method:
- java public void actionPerformed(ActionEvent e) { //Write the code here }

ActionListener Examples

On Button Click:

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

//1st step
public class ActionListenerExample implements ActionListener {
    public static void main(String[] args) {
        Frame f = new Frame("ActionListener Example");
        final TextField tf = new TextField();
        tf.setBounds(50, 50, 150, 20);
        Button b = new Button("Click Here");
        b.setBounds(50, 100, 60, 30);
        //2nd step
        b.addActionListener(this);
        f.add(b);f.add(tf);
        f.setSize(400, 400);
        f.setLayout(null);
        f.setVisible(true);
    }

    //3rd step
    public void actionPerformed(ActionEvent e) {
        tf.setText("Welcome to Javatpoint.");
    }
}

Using Anonymous Class:

b.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        tf.setText("Welcome to Javatpoint.");
    }
});

Full Code:

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

public class ActionListenerExample {
    public static void main(String[] args) {
        Frame f = new Frame("ActionListener Example");
        final TextField tf = new TextField();
        tf.setBounds(50, 50, 150, 20);
        Button b = new Button("Click Here");
        b.setBounds(50, 100, 60, 30);

        b.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                tf.setText("Welcome to Javatpoint.");
            }
        });

        f.add(b);f.add(tf);
        f.setSize(400, 400);
        f.setLayout(null);
        f.setVisible(true);
    }
}

Java MouseListener

The MouseListener is notified whenever the state of the mouse changes.
It is found in the java.awt.event package.
It has five methods:

Methods of MouseListener Interface

public abstract void mouseClicked(MouseEvent e);
public abstract void mouseEntered(MouseEvent e);
public abstract void mouseExited(MouseEvent e);
public abstract void mousePressed(MouseEvent e);
public abstract void mouseReleased(MouseEvent e);

MouseListener Example

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

public class MouseListenerExample extends Frame implements MouseListener {
    Label l;

    MouseListenerExample() {
        addMouseListener(this);
        l = new Label();
        l.setBounds(20, 50, 100, 20);
        add(l);
        setSize(300, 300);
        setLayout(null);
        setVisible(true);
    }

    public void mouseClicked(MouseEvent e) {
        l.setText("Mouse Clicked");
    }

    public void mouseEntered(MouseEvent e) {
        l.setText("Mouse Entered");
    }

    public void mouseExited(MouseEvent e) {
        l.setText("Mouse Exited");
    }

    public void mousePressed(MouseEvent e) {
        l.setText("Mouse Pressed");
    }

    public void mouseReleased(MouseEvent e) {
        l.setText("Mouse Released");
    }

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

Java MouseMotionListener

The MouseMotionListener is notified whenever the mouse is moved or dragged.
It is found in the java.awt.event package.
It has two methods:

Methods of MouseMotionListener Interface

public abstract void mouseDragged(MouseEvent e);
public abstract void mouseMoved(MouseEvent e);

MouseMotionListener Example

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

public class MouseMotionListenerExample extends Frame implements MouseMotionListener {
    MouseMotionListenerExample() {
        addMouseMotionListener(this);

        setSize(300, 300);
        setLayout(null);
        setVisible(true);
    }

    public void mouseDragged(MouseEvent e) {
        Graphics g = getGraphics();
        g.setColor(Color.BLUE);
        g.fillOval(e.getX(), e.getY(), 20, 20);
    }

    public void mouseMoved(MouseEvent e) {}

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

Java ItemListener

The ItemListener is notified whenever a checkbox is clicked.
It is found in the java.awt.event package.
It has one method: itemStateChanged().

`itemStateChanged()` Method

Invoked automatically when the registered checkbox component is clicked or unclicked.
Syntax:
- public abstract void itemStateChanged(ItemEvent e);

ItemListener Example

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

public class ItemListenerExample implements ItemListener {
    Checkbox checkBox1, checkBox2;
    Label label;

    ItemListenerExample() {
        Frame f = new Frame("CheckBox Example");
        label = new Label();
        label.setAlignment(Label.CENTER);
        label.setSize(400, 100);
        checkBox1 = new Checkbox("C++");
        checkBox1.setBounds(100, 100, 50, 50);
        checkBox2 = new Checkbox("Java");
        checkBox2.setBounds(100, 150, 50, 50);
        f.add(checkBox1); f.add(checkBox2); f.add(label);
        checkBox1.addItemListener(this);
        checkBox2.addItemListener(this);
        f.setSize(400, 400);
        f.setLayout(null);
        f.setVisible(true);
    }

    public void itemStateChanged(ItemEvent e) {
        if (e.getSource() == checkBox1)
            label.setText("C++ Checkbox: "
                + (e.getStateChange() == 1 ? "checked" : "unchecked"));
        if (e.getSource() == checkBox2)
            label.setText("Java Checkbox: "
                + (e.getStateChange() == 1 ? "checked" : "unchecked"));
    }

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

Java KeyListener

The KeyListener is notified whenever the state of a key changes.
It is found in the java.awt.event package.
It has three methods.

Interface Declaration

public interface KeyListener extends EventListener

Methods of KeyListener Interface

Sr. no.	Method name	Description
1.	`public abstract void keyPressed (KeyEvent e);`	Invoked when a key has been pressed.
2.	`public abstract void keyReleased (KeyEvent e);`	Invoked when a key has been released.
3.	`public abstract void keyTyped (KeyEvent e);`	Invoked when a key has been typed.

Methods inherited

This interface inherits methods from the following interface:
- java.awt.EventListener

KeyListener Example: Count Words & Characters

The example counts words and characters from a TextArea using the keyReleased() method.

// importing the necessary libraries
import java.awt.*;
import java.awt.event.*;

// class which inherits Frame class and implements KeyListener interface
public class KeyListenerExample2 extends Frame implements KeyListener {
    // object of Label and TextArea
    Label l;
    TextArea area;

    // class constructor
    KeyListenerExample2() {
        // creating the label
        l = new Label();
        // setting the location of label
        l.setBounds(20, 50, 200, 20);
        // creating the text area
        area = new TextArea();
        // setting location of text area
        area.setBounds(20, 80, 300, 300);
        // adding KeyListener to the text area
        area.addKeyListener(this);
        // adding label and text area to frame
        add(l);add(area);
        // setting size, layout and visibility of frame
        setSize(400, 400);
        setLayout(null);
        setVisible(true);
    }

    // even if we do not define the interface methods, we need to override them
    public void keyPressed(KeyEvent e) {}

    // overriding the keyReleased() method of KeyListener interface
    public void keyReleased(KeyEvent e) {
        // defining a string which is fetched by the getText() method of TextArea class
        String text = area.getText();
        // splitting the string in words
        String words[] = text.split("\\s");
        // printing the number of words and characters of the string
        l.setText("Words: " + words.length + " Characters:" + text.length());
    }

    public void keyTyped(KeyEvent e) {}

    // main method
    public static void main(String[] args) {
        new KeyListenerExample2();
    }
}

Java AWT and Event Handling Notes

Java Abstract Window Toolkit (AWT)

Introduction

Java AWT

Hierarchy

Platform Independence

Useful Methods of Component Class

Example

Java Event Listeners and Handling

Event Classes and Listener Interfaces

Steps to Perform Event Handling

Examples of Event Handling

Java ActionListener

How to write ActionListener

ActionListener Examples

Java MouseListener

Methods of MouseListener Interface

MouseListener Example

Java MouseMotionListener

Methods of MouseMotionListener Interface

MouseMotionListener Example

Java ItemListener

itemStateChanged() Method

ItemListener Example

Java KeyListener

Interface Declaration

Methods of KeyListener Interface

Methods inherited

KeyListener Example: Count Words & Characters

`itemStateChanged()` Method