Event Handling and Listeners - potatoscript/JavaSwing GitHub Wiki

⚡ Event Handling and Listeners in Java Swing ⚡

📝 What is Event Handling in Java Swing?

In Java Swing, Event Handling refers to the process of responding to user actions, such as clicks, typing, or mouse movement. These actions are called events, and Java provides a way to listen for these events and respond accordingly.

For instance, when a user clicks a button, an event is generated, and the program can handle that event by executing a specific action.

🎯 Key Concepts:

  1. Event: An object that encapsulates the details of a user interaction (like a mouse click or key press).
  2. Listener: A special interface that listens for events and defines what happens when an event occurs.
  3. Event Source: The component (such as a button or text field) that generates the event.
  4. Event Listener: A class or object that listens for and responds to the event generated by the event source.

📚 Types of Events in Java Swing

🖱️ Mouse Events:

  • MouseListener: Used for handling mouse events like clicks, entering, and exiting components.
    • mousePressed(), mouseReleased(), mouseClicked(), mouseEntered(), mouseExited()

⌨️ Keyboard Events:

  • KeyListener: Used for handling keyboard events like key presses and releases.
    • keyPressed(), keyReleased(), keyTyped()

🖱️ Action Events:

  • ActionListener: Used for handling actions like button clicks or menu selections.
    • actionPerformed()

🔄 Window Events:

  • WindowListener: Used for handling window events like opening, closing, and resizing windows.
    • windowOpened(), windowClosing(), windowClosed()

📚 Step 1: ActionListener and Button Click

📝 Example: Using ActionListener with a Button

In this example, we’ll create a simple JButton that, when clicked, displays a message in a JLabel.

import javax.swing.*;
import java.awt.event.*;

public class ActionListenerExample {
    public static void main(String[] args) {
        // Create the main frame
        JFrame frame = new JFrame("ActionListener Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(300, 200);
        frame.setLayout(null);

        // Create a button
        JButton button = new JButton("Click Me!");
        button.setBounds(50, 50, 200, 40);

        // Create a label
        JLabel label = new JLabel("Message will appear here");
        label.setBounds(50, 120, 200, 30);

        // Add an ActionListener to the button
        button.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                label.setText("Button was clicked!");
            }
        });

        // Add components to the frame
        frame.add(button);
        frame.add(label);

        // Show the frame
        frame.setVisible(true);
    }
}

🎨 Explanation:

  1. JButton: A button labeled "Click Me!" is created.
  2. JLabel: A label that initially displays "Message will appear here".
  3. ActionListener: The addActionListener method is used to listen for button clicks. When the button is clicked, the actionPerformed method is triggered, changing the text of the label to "Button was clicked!".

👩‍🎨 Output:

  • When the button is clicked, the label's text changes to "Button was clicked!".

📚 Step 2: MouseListener and Mouse Events

📝 Example: Using MouseListener to Track Mouse Events

In this example, we will create a JPanel and listen for mouse clicks, entry, and exit events.

import javax.swing.*;
import java.awt.event.*;

public class MouseListenerExample {
    public static void main(String[] args) {
        // Create the main frame
        JFrame frame = new JFrame("MouseListener Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(300, 200);
        frame.setLayout(null);

        // Create a panel to listen for mouse events
        JPanel panel = new JPanel();
        panel.setBounds(50, 50, 200, 100);
        panel.setBackground(Color.CYAN);

        // Add a MouseListener to the panel
        panel.addMouseListener(new MouseAdapter() {
            public void mouseClicked(MouseEvent e) {
                System.out.println("Mouse clicked at: " + e.getPoint());
            }

            public void mouseEntered(MouseEvent e) {
                System.out.println("Mouse entered the panel");
            }

            public void mouseExited(MouseEvent e) {
                System.out.println("Mouse exited the panel");
            }
        });

        // Add the panel to the frame
        frame.add(panel);

        // Show the frame
        frame.setVisible(true);
    }
}

🎨 Explanation:

  1. JPanel: A panel is created with a cyan background.
  2. MouseListener: The panel listens for mouse events. When the mouse is clicked, entered, or exited, the corresponding methods (mouseClicked, mouseEntered, and mouseExited) are triggered and print messages to the console.

👩‍🎨 Output:

  • When the mouse clicks on the panel, it prints the click location.
  • When the mouse enters or exits the panel, it prints appropriate messages to the console.

📚 Step 3: KeyListener and Keyboard Events

📝 Example: Using KeyListener to Track Key Presses

In this example, we’ll create a JTextField that listens for key presses and displays the pressed key in a JLabel.

import javax.swing.*;
import java.awt.event.*;

public class KeyListenerExample {
    public static void main(String[] args) {
        // Create the main frame
        JFrame frame = new JFrame("KeyListener Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(300, 200);
        frame.setLayout(null);

        // Create a text field
        JTextField textField = new JTextField();
        textField.setBounds(50, 50, 200, 30);

        // Create a label
        JLabel label = new JLabel("Pressed key will appear here");
        label.setBounds(50, 100, 200, 30);

        // Add a KeyListener to the text field
        textField.addKeyListener(new KeyAdapter() {
            public void keyPressed(KeyEvent e) {
                char key = e.getKeyChar();
                label.setText("Key Pressed: " + key);
            }
        });

        // Add components to the frame
        frame.add(textField);
        frame.add(label);

        // Show the frame
        frame.setVisible(true);
    }
}

🎨 Explanation:

  1. JTextField: A text field is created where the user can type.
  2. JLabel: A label that will display the key pressed.
  3. KeyListener: The addKeyListener method is used to listen for key presses. The keyPressed method is triggered when a key is pressed, and it updates the label with the key that was pressed.

👩‍🎨 Output:

  • When the user types in the text field, the label updates to show the pressed key.

📚 Step 4: WindowListener and Window Events

📝 Example: Using WindowListener to Track Window Events

In this example, we’ll create a JFrame and listen for window events such as opening, closing, and resizing.

import javax.swing.*;
import java.awt.event.*;

public class WindowListenerExample {
    public static void main(String[] args) {
        // Create the main frame
        JFrame frame = new JFrame("WindowListener Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(300, 200);
        
        // Add a WindowListener
        frame.addWindowListener(new WindowAdapter() {
            public void windowOpened(WindowEvent e) {
                System.out.println("Window opened!");
            }

            public void windowClosing(WindowEvent e) {
                System.out.println("Window closing!");
            }
        });

        // Show the frame
        frame.setVisible(true);
    }
}

🎨 Explanation:

  1. WindowListener: The addWindowListener method is used to listen for window events like opening or closing. The windowOpened and windowClosing methods are triggered accordingly.

👩‍🎨 Output:

  • When the window opens, it prints "Window opened!".
  • When the window is about to close, it prints "Window closing!".