Chapter 12 Event-Driven Programming

In event-drivent programming, code is executed when an event occurs - such as a button click or a mouse movement.

Example: Assume program A has a button X.

Prior to any user action, button X is created.
Along with the button, a listener is created which knows how to handle buttons (see Listeners section below).
The listener is registered so that when button X is pressed, this specific listener is notified (see Listeners section below).
The program then enters a loop waiting for events. If the user presses button X, the listener is informed.

Events and Event Source

An event is a signal to the program that something has happened. The program can choose to respond to or ignore the event. The component on which an event is generated (e.g., a button which is pressed) is called the source object. The table below shows the types of events generated by various source objects.

User Action	Source Object	Event Type
Click button	JButton	ActionEvent
Press return on text field	JTextField	ActionEvent
Select new item	JComboBox	ItemEvent, ActionEvent
Select item(s)	JList	ListSelectionEvent
Click check box	JCheckBox	ItemEvent, ActionEvent
Click radio button	JRadioButton	ItemEvent, ActionEvent
Select menu item	JMenuItem	ActionEvent
Move scroll bar	JScrollBar	AdjustmentEvent
Window open, close, iconify, deiconify, close	Window	WindowEvent
Component add or remove from container	Container	ContainerEvent
Component moved, resized, hidden or shown	Component	ComponentEvent
Component gained or lost focus	Component	FocusEvent
Key released or pressed	Component	KeyEvent
Mouse pressed, released, clicked, entered or exited	Component	MouseEvent
Mouse moved or dragged	Component	MouseEvent

An event is an instance of an event class. The root class of the event clases is java.util.EventObject. Figure 12.1 in the text shows the hierarchical relationships of the event classes.

An event object contains properties pertinent to the event. You can identify the source using getSource(). Most event classes are in java.awt.event except ListSelectionEvent, which is in javax.swing.event.

Listeners, Registrations, and Handling Events

When an event is triggered, an interested object receives the event. This object is called a listener. The listener must implement the corresponding event-listener interface. The naming convention is generally XListener for XEvent, except for MouseMotionListener. The interface specifies which methods must be implemented.

Event Class	Listener Interface	Methods (handlers)
ActionEvent	ActionListener	actionPerformed(ActionEvent e)
ItemEvent	ItemListener	itemStateChanged(ItemEvent e)
WindowEvent	WindowListener	windowClosing(WindowEvent e)
		windowOpened(WindowEvent e)
		windowIconified(WindowEvent e)
		windowDeiconified(WindowEvent e)
		windowClosed(WindowEvent e)
		windowActivated(WindowEvent e)
		windowDeactivated(WindowEvent e)
ContainerEvent	ContainerListener	componentAdded(ContainerEvent e)
		componentRemoved(ContainerEvent e)
ComponentEvent	ComponentListener	componentMoved(ComponentEvent e)
		componentHidden(ComponentEvent e)
		componentResized(ComponentEvent e)
		componenetShown(ComponentEvent e)
FocusEvent	FocusListener	focusGained(FocusEvent e)
		focusLost(FocusEvent e)
TextEvent	TextListener	textValueChanged(TextEvent e)
KeyEvent	KeyListener	keyPressed(KeyEvent e)
		keyReleased(KeyEvent e)
		keyTyped(KeyEvent e)
MouseEvent	MouseListener	mousePressed(MouseEvent e)
		mouseReleased(MouseEvent e)
		mouseEntered(MouseEvent e)
		mouseExited(MouseEvent e)
		mouseClicked(MouseEvent e)
	MouseMotionListener	mouseDragged(MouseEvent e)
		MouseMoved(MouseEvent e)
AdjustmentEvent	AdjustmenetListener	adjustmentValueChanged(AdjustmentEvent e)

Sample button listener:

class ButtonListener implements ActionListener {
/** This method will be invoked when a button is clicked */
    public void actionPerformed(ActionEvent e) {
       if (e.getActionCommand().equals("OK")) {
          System.out.println("The OK button is clicked");
    }
    else if (e.getActionCommand().equals("Cancel")) {
          System.out.println("The Cancel button is clicked");
   }
}
}

By default, the action command is the text of the button. You could also use e.getSource() to return the reference of the source object and determine which button was clicked.

To "register" this class as your listener:
// create buttons
private JButton jbtOK = new JButton("OK");
private JButton jbtCancel = new JButton("Cancel");
// create button listener
ButtonListener btListener = new ButtonListener();
jbtOk.addActionListener(btListener);
jbtCancel.addActionListener(btListener);

It is also possible for the class itself to implement ActionListener. In that case, the class should include the appropriate handler method (actionPerformed in this example). To register the handler, the line of code would be:
jbtOk.addActionListener(this);

NOTE: If a listener is registered with a source twice, the handler will be invoked twice when an event occurs.

Mouse Events

A mouse event is generated when a mouse is clicked, released, moved or dragged on a component. The event captures the location (x and y coordinate) and the number of clicks. Java provides two listener interfaces for mouse events: MouseListener and MouseMotionListener.

Keyboard Events

Keyboard events capture input from the keyboard. Every event has an associated key character or key code that is returned by the getKeyChar() or getKeyCode method in KeyEvent.