Event Listener Interfaces - Java

Event Listener Interfaces

As explained, the delegation event model has two parts: sources and listeners. As it relates to this chapter, listeners are created by implementing one or more of the interfaces defined by the java.awt.event package. When an event occurs, the event source invokes the appropriate method defined by the listener and provides an event object as its argument. Table 24-3 lists several commonly used listener interfaces and provides a brief description of the methods that they define. The following sections examine the specific methods that are contained in each interface.

Interface : Description

ActionListener : Defines one method to receive action events.

AdjustmentListener : Defines one method to receive adjustment events.

ComponentListener : Defines four methods to recognize when a component is hidden, moved, resized, or shown.

ContainerListener : Defines two methods to recognize when a component is added to or removed from a container.

FocusListener : Defines two methods to recognize when a component gains or loses keyboard focus.

ItemListener : Defines one method to recognize when the state of an item changes.

KeyListener : Defines three methods to recognize when a key is pressed, released, or typed.

MouseListener : Defines five methods to recognize when the mouse is clicked, enters a component, exits a component, is pressed, or is released.

MouseMotionListener : Defines two methods to recognize when the mouse is dragged or moved.

MouseWheelListener : Defines one method to recognize when the mouse wheel is moved.

TextListener : Defines one method to recognize when a text value changes.

WindowFocusListener : Defines two methods to recognize when a window gains or loses input focus.

WindowListener : Defines seven methods to recognize when a window is activated, closed, deactivated, deiconified, iconified, opened, or quit.

Table 24-3   Commonly Used Event Listener Interfaces

The ActionListener Interface

This interface defines the actionPerformed( ) method that is invoked when an action event occurs. Its general form is shown here:

void actionPerformed(ActionEvent ae)

The AdjustmentListener Interface

This interface defines the adjustmentValueChanged( ) method that is invoked when an adjustment event occurs. Its general form is shown here:

void adjustmentValueChanged(AdjustmentEvent ae)

The ComponentListener Interface

This interface defines four methods that are invoked when a component is resized, moved, shown, or hidden. Their general forms are shown here:

void componentResized(ComponentEvent ce) 

void componentMoved(ComponentEvent ce) 

void componentShown(ComponentEvent ce) 

void componentHidden(ComponentEvent ce)

The ContainerListener Interface

This interface contains two methods. When a component is added to a container, componentAdded( ) is invoked. When a component is removed from a container, componentRemoved( ) is invoked. Their general forms are shown here:

void componentAdded(ContainerEvent ce) 

void componentRemoved(ContainerEvent ce)

The FocusListener Interface

This interface defines two methods. When a component obtains keyboard focus, focusGained( ) is invoked. When a component loses keyboard focus, focusLost( ) is called. Their general forms are shown here:

void focusGained(FocusEvent fe) 

void focusLost(FocusEvent fe)

The ItemListener Interface

This interface defines the itemStateChanged( ) method that is invoked when the state of an item changes. Its general form is shown here:

void itemStateChanged(ItemEvent ie)

The KeyListener Interface

This interface defines three methods. The keyPressed( ) and keyReleased( ) methods are invoked when a key is pressed and released, respectively. The keyTyped( ) method is invoked when a character has been entered.

For example, if a user presses and releases the a key, three events are generated in sequence: key pressed, typed, and released. If a user presses and releases the home key, two key events are generated in sequence: key pressed and released.

The general forms of these methods are shown here:

void keyPressed(KeyEvent ke) 

void keyReleased(KeyEvent ke) 

void keyTyped(KeyEvent ke)

The MouseListener Interface

This interface defines five methods. If the mouse is pressed and released at the same point, mouseClicked( ) is invoked. When the mouse enters a component, the mouseEntered( ) method is called. When it leaves, mouseExited( ) is called. The mousePressed( ) and mouseReleased( ) methods are invoked when the mouse is pressed and released, respectively.

The general forms of these methods are shown here:

void mouseClicked(MouseEvent me) 

void mouseEntered(MouseEvent me) 

void mouseExited(MouseEvent me) 

void mousePressed(MouseEvent me) 

void mouseReleased(MouseEvent me)

The MouseMotionListener Interface

This interface defines two methods. The mouseDragged( ) method is called multiple times as the mouse is dragged. The mouseMoved( ) method is called multiple times as the mouse is moved. Their general forms are shown here:

void mouseDragged(MouseEvent me) 

void mouseMoved(MouseEvent me)

The MouseWheelListener Interface

This interface defines the mouseWheelMoved( ) method that is invoked when the mouse wheel is moved. Its general form is shown here:

void mouseWheelMoved(MouseWheelEvent mwe)

The TextListener Interface

This interface defines the textValueChanged( ) method that is invoked when a change occurs in a text area or text field. Its general form is shown here:

void textValueChanged(TextEvent te)

The WindowFocusListener Interface

This interface defines two methods: windowGainedFocus( ) and windowLostFocus( ). These are called when a window gains or loses input focus. Their general forms are shown here:

void windowGainedFocus(WindowEvent we) 

void windowLostFocus(WindowEvent we)

The WindowListener Interface

This interface defines seven methods. The windowActivated( ) and windowDeactivated( ) methods are invoked when a window is activated or deactivated, respectively. If a window is iconified, the windowIconified( ) method is called. When a window is deiconified,

the windowDeiconified( ) method is called. When a window is opened or closed, the windowOpened( ) or windowClosed( ) methods are called, respectively. The windowClosing( ) method is called when a window is being closed. The general forms of these methods are

void windowActivated(WindowEvent we) 

void windowClosed(WindowEvent we) 

void windowClosing(WindowEvent we) 

void windowDeactivated(WindowEvent we) 

void windowDeiconified(WindowEvent we) 

void windowIconified(WindowEvent we) 

void windowOpened(WindowEvent we)