Event Handling in Java

Handling Events with an Anonymous Class

If an event handler is specific to a component (that is, not shared by other components), there is no need to declare a class to handle the event. The event handler can be implemented using an anonymous inner class. This example demonstrates an anonymous inner class to handle key events for a component.

component.addKeyListener(new KeyAdapter() {

public void keyPressed(KeyEvent evt) {

}

});

Handling Action Events

Action events are fired by subclasses of AbstractButton and includes buttons, checkboxes, and menus.

AbstractButton button = new JButton(quot;OK");

button.addActionListener(new MyActionListener());

public class MyActionListener

implements ActionListener {

public void actionPerformed(ActionEvent evt) {

// Determine which abstract

// button fired the event.

AbstractButton button =

(AbstractButton)evt.getSource();

}

}

Handling Key Presses

You can get the key that was pressed either as a key character (which is a Unicode character) or as a key code (a special value representing a particular key on the keyboard).

component.addKeyListener(new MyKeyListener());

public class MyKeyListener extends KeyAdapter {

public void keyPressed(KeyEvent evt) {

// Check for key characters.

if (evt.getKeyChar() == 'a') {

process(evt.getKeyChar());

}

// Check for key codes.

if (evt.getKeyCode() == KeyEvent.VK_HOME) {

process(evt.getKeyCode());

}

}

}

Handling Mouse Clicks

component.addMouseListener(

new MyMouseListener());

public class MyMouseListener

extends MouseAdapter {

public void mouseClicked(MouseEvent evt) {

if ((evt.getModifiers() &

InputEvent.BUTTON1_MASK) != 0) {

processLeft(evt.getPoint());

}

if ((evt.getModifiers() &

InputEvent.BUTTON2_MASK) != 0) {

processMiddle(evt.getPoint());

}

if ((evt.getModifiers() &

InputEvent.BUTTON3_MASK) != 0) {

processRight(evt.getPoint());

}

}

}

Handling Mouse Motion

component.addMouseMotionListener(

new MyMouseMotionListener());

public class MyMouseMotionListener

extends MouseMotionAdapter {

public void mouseMoved(MouseEvent evt) {

// Process current position of cursor

// while all mouse buttons are up.

process(evt.getPoint());

}

public void mouseDragged(MouseEvent evt) {

// Process current position of cursor

// while mouse button is pressed.

process(evt.getPoint());

}

}

Detecting Double and Triple Clicks

component.addMouseListener(

new MyMouseListener());

public class MyMouseListener extends MouseAdapter {

public void mouseClicked(MouseEvent evt) {

if (evt.getClickCount() == 3) {

// triple-click

} else if (evt.getClickCount() == 2) {

// double-click

}

}

}

Handling Focus Changes

component.addFocusListener(

new MyFocusListener());

public class MyFocusListener

extends FocusAdapter {

public void focusGained(FocusEvent evt) {

// The component gained the focus.

}

public void focusLost(FocusEvent evt) {

// The component lost the focus.

}

}