Module java.desktop
Package java.awt

Class AWTEventMulticaster

java.lang.Object
java.awt.AWTEventMulticaster
All Implemented Interfaces:
ActionListener, AdjustmentListener, ComponentListener, ContainerListener, FocusListener, HierarchyBoundsListener, HierarchyListener, InputMethodListener, ItemListener, KeyListener, MouseListener, MouseMotionListener, MouseWheelListener, TextListener, WindowFocusListener, WindowListener, WindowStateListener, EventListener

public class AWTEventMulticaster
extends Object
implements ComponentListener, ContainerListener, FocusListener, KeyListener, MouseListener, MouseMotionListener, WindowListener, WindowFocusListener, WindowStateListener, ActionListener, ItemListener, AdjustmentListener, TextListener, InputMethodListener, HierarchyListener, HierarchyBoundsListener, MouseWheelListener
AWTEventMulticaster implements efficient and thread-safe multi-cast event dispatching for the AWT events defined in the java.awt.event package.

The following example illustrates how to use this class:


 public myComponent extends Component {
     ActionListener actionListener = null;

     public synchronized void addActionListener(ActionListener l) {
         actionListener = AWTEventMulticaster.add(actionListener, l);
     }
     public synchronized void removeActionListener(ActionListener l) {
         actionListener = AWTEventMulticaster.remove(actionListener, l);
     }
     public void processEvent(AWTEvent e) {
         // when event occurs which causes "action" semantic
         ActionListener listener = actionListener;
         if (listener != null) {
             listener.actionPerformed(new ActionEvent());
         }
     }
 }
 
The important point to note is the first argument to the add and remove methods is the field maintaining the listeners. In addition you must assign the result of the add and remove methods to the field maintaining the listeners.

AWTEventMulticaster is implemented as a pair of EventListeners that are set at construction time. AWTEventMulticaster is immutable. The add and remove methods do not alter AWTEventMulticaster in anyway. If necessary, a new AWTEventMulticaster is created. In this way it is safe to add and remove listeners during the process of an event dispatching. However, event listeners added during the process of an event dispatch operation are not notified of the event currently being dispatched.

All of the add methods allow null arguments. If the first argument is null, the second argument is returned. If the first argument is not null and the second argument is null, the first argument is returned. If both arguments are non-null, a new AWTEventMulticaster is created using the two arguments and returned.

For the remove methods that take two arguments, the following is returned:

  • null, if the first argument is null, or the arguments are equal, by way of ==.
  • the first argument, if the first argument is not an instance of AWTEventMulticaster.
  • result of invoking remove(EventListener) on the first argument, supplying the second argument to the remove(EventListener) method.

Swing makes use of EventListenerList for similar logic. Refer to it for details.

Since:
1.1
See Also:
EventListenerList
  • Field Details

    • a

      protected final EventListener a
      A variable in the event chain (listener-a)
    • b

      protected final EventListener b
      A variable in the event chain (listener-b)
  • Constructor Details

    • AWTEventMulticaster

      protected AWTEventMulticaster​(EventListener a, EventListener b)
      Creates an event multicaster instance which chains listener-a with listener-b. Input parameters a and b should not be null, though implementations may vary in choosing whether or not to throw NullPointerException in that case.
      Parameters:
      a - listener-a
      b - listener-b
  • Method Details

    • remove

      protected EventListener remove​(EventListener oldl)
      Removes a listener from this multicaster.

      The returned multicaster contains all the listeners in this multicaster with the exception of all occurrences of oldl. If the resulting multicaster contains only one regular listener the regular listener may be returned. If the resulting multicaster is empty, then null may be returned instead.

      No exception is thrown if oldl is null.

      Parameters:
      oldl - the listener to be removed
      Returns:
      resulting listener
    • componentResized

      public void componentResized​(ComponentEvent e)
      Handles the componentResized event by invoking the componentResized methods on listener-a and listener-b.
      Specified by:
      componentResized in interface ComponentListener
      Parameters:
      e - the component event
    • componentMoved

      public void componentMoved​(ComponentEvent e)
      Handles the componentMoved event by invoking the componentMoved methods on listener-a and listener-b.
      Specified by:
      componentMoved in interface ComponentListener
      Parameters:
      e - the component event
    • componentShown

      public void componentShown​(ComponentEvent e)
      Handles the componentShown event by invoking the componentShown methods on listener-a and listener-b.
      Specified by:
      componentShown in interface ComponentListener
      Parameters:
      e - the component event
    • componentHidden

      public void componentHidden​(ComponentEvent e)
      Handles the componentHidden event by invoking the componentHidden methods on listener-a and listener-b.
      Specified by:
      componentHidden in interface ComponentListener
      Parameters:
      e - the component event
    • componentAdded

      public void componentAdded​(ContainerEvent e)
      Handles the componentAdded container event by invoking the componentAdded methods on listener-a and listener-b.
      Specified by:
      componentAdded in interface ContainerListener
      Parameters:
      e - the component event
    • componentRemoved

      public void componentRemoved​(ContainerEvent e)
      Handles the componentRemoved container event by invoking the componentRemoved methods on listener-a and listener-b.
      Specified by:
      componentRemoved in interface ContainerListener
      Parameters:
      e - the component event
    • focusGained

      public void focusGained​(FocusEvent e)
      Handles the focusGained event by invoking the focusGained methods on listener-a and listener-b.
      Specified by:
      focusGained in interface FocusListener
      Parameters:
      e - the focus event
    • focusLost

      public void focusLost​(FocusEvent e)
      Handles the focusLost event by invoking the focusLost methods on listener-a and listener-b.
      Specified by:
      focusLost in interface FocusListener
      Parameters:
      e - the focus event
    • keyTyped

      public void keyTyped​(KeyEvent e)
      Handles the keyTyped event by invoking the keyTyped methods on listener-a and listener-b.
      Specified by:
      keyTyped in interface KeyListener
      Parameters:
      e - the key event
    • keyPressed

      public void keyPressed​(KeyEvent e)
      Handles the keyPressed event by invoking the keyPressed methods on listener-a and listener-b.
      Specified by:
      keyPressed in interface KeyListener
      Parameters:
      e - the key event
    • keyReleased

      public void keyReleased​(KeyEvent e)
      Handles the keyReleased event by invoking the keyReleased methods on listener-a and listener-b.
      Specified by:
      keyReleased in interface KeyListener
      Parameters:
      e - the key event
    • mouseClicked

      public void mouseClicked​(MouseEvent e)
      Handles the mouseClicked event by invoking the mouseClicked methods on listener-a and listener-b.
      Specified by:
      mouseClicked in interface MouseListener
      Parameters:
      e - the mouse event
    • mousePressed

      public void mousePressed​(MouseEvent e)
      Handles the mousePressed event by invoking the mousePressed methods on listener-a and listener-b.
      Specified by:
      mousePressed in interface MouseListener
      Parameters:
      e - the mouse event
    • mouseReleased

      public void mouseReleased​(MouseEvent e)
      Handles the mouseReleased event by invoking the mouseReleased methods on listener-a and listener-b.
      Specified by:
      mouseReleased in interface MouseListener
      Parameters:
      e - the mouse event
    • mouseEntered

      public void mouseEntered​(MouseEvent e)
      Handles the mouseEntered event by invoking the mouseEntered methods on listener-a and listener-b.
      Specified by:
      mouseEntered in interface MouseListener
      Parameters:
      e - the mouse event
    • mouseExited

      public void mouseExited​(MouseEvent e)
      Handles the mouseExited event by invoking the mouseExited methods on listener-a and listener-b.
      Specified by:
      mouseExited in interface MouseListener
      Parameters:
      e - the mouse event
    • mouseDragged

      public void mouseDragged​(MouseEvent e)
      Handles the mouseDragged event by invoking the mouseDragged methods on listener-a and listener-b.
      Specified by:
      mouseDragged in interface MouseMotionListener
      Parameters:
      e - the mouse event
    • mouseMoved

      public void mouseMoved​(MouseEvent e)
      Handles the mouseMoved event by invoking the mouseMoved methods on listener-a and listener-b.
      Specified by:
      mouseMoved in interface MouseMotionListener
      Parameters:
      e - the mouse event
    • windowOpened

      public void windowOpened​(WindowEvent e)
      Handles the windowOpened event by invoking the windowOpened methods on listener-a and listener-b.
      Specified by:
      windowOpened in interface WindowListener
      Parameters:
      e - the window event
    • windowClosing

      public void windowClosing​(WindowEvent e)
      Handles the windowClosing event by invoking the windowClosing methods on listener-a and listener-b.
      Specified by:
      windowClosing in interface WindowListener
      Parameters:
      e - the window event
    • windowClosed

      public void windowClosed​(WindowEvent e)
      Handles the windowClosed event by invoking the windowClosed methods on listener-a and listener-b.
      Specified by:
      windowClosed in interface WindowListener
      Parameters:
      e - the window event
    • windowIconified

      public void windowIconified​(WindowEvent e)
      Handles the windowIconified event by invoking the windowIconified methods on listener-a and listener-b.
      Specified by:
      windowIconified in interface WindowListener
      Parameters:
      e - the window event
      See Also:
      Window.setIconImage(java.awt.Image)
    • windowDeiconified

      public void windowDeiconified​(WindowEvent e)
      Handles the windowDeiconified event by invoking the windowDeiconified methods on listener-a and listener-b.
      Specified by:
      windowDeiconified in interface WindowListener
      Parameters:
      e - the window event
    • windowActivated

      public void windowActivated​(WindowEvent e)
      Handles the windowActivated event by invoking the windowActivated methods on listener-a and listener-b.
      Specified by:
      windowActivated in interface WindowListener
      Parameters:
      e - the window event
    • windowDeactivated

      public void windowDeactivated​(WindowEvent e)
      Handles the windowDeactivated event by invoking the windowDeactivated methods on listener-a and listener-b.
      Specified by:
      windowDeactivated in interface WindowListener
      Parameters:
      e - the window event
    • windowStateChanged

      public void windowStateChanged​(WindowEvent e)
      Handles the windowStateChanged event by invoking the windowStateChanged methods on listener-a and listener-b.
      Specified by:
      windowStateChanged in interface WindowStateListener
      Parameters:
      e - the window event
      Since:
      1.4
    • windowGainedFocus

      public void windowGainedFocus​(WindowEvent e)
      Handles the windowGainedFocus event by invoking the windowGainedFocus methods on listener-a and listener-b.
      Specified by:
      windowGainedFocus in interface WindowFocusListener
      Parameters:
      e - the window event
      Since:
      1.4
    • windowLostFocus

      public void windowLostFocus​(WindowEvent e)
      Handles the windowLostFocus event by invoking the windowLostFocus methods on listener-a and listener-b.
      Specified by:
      windowLostFocus in interface WindowFocusListener
      Parameters:
      e - the window event
      Since:
      1.4
    • actionPerformed

      public void actionPerformed​(ActionEvent e)
      Handles the actionPerformed event by invoking the actionPerformed methods on listener-a and listener-b.
      Specified by:
      actionPerformed in interface ActionListener
      Parameters:
      e - the action event
    • itemStateChanged

      public void itemStateChanged​(ItemEvent e)
      Handles the itemStateChanged event by invoking the itemStateChanged methods on listener-a and listener-b.
      Specified by:
      itemStateChanged in interface ItemListener
      Parameters:
      e - the item event
    • adjustmentValueChanged

      public void adjustmentValueChanged​(AdjustmentEvent e)
      Handles the adjustmentValueChanged event by invoking the adjustmentValueChanged methods on listener-a and listener-b.
      Specified by:
      adjustmentValueChanged in interface AdjustmentListener
      Parameters:
      e - the adjustment event
    • inputMethodTextChanged

      public void inputMethodTextChanged​(InputMethodEvent e)
      Handles the inputMethodTextChanged event by invoking the inputMethodTextChanged methods on listener-a and listener-b.
      Specified by:
      inputMethodTextChanged in interface InputMethodListener
      Parameters:
      e - the item event
    • caretPositionChanged

      public void caretPositionChanged​(InputMethodEvent e)
      Handles the caretPositionChanged event by invoking the caretPositionChanged methods on listener-a and listener-b.
      Specified by:
      caretPositionChanged in interface InputMethodListener
      Parameters:
      e - the item event
    • hierarchyChanged

      public void hierarchyChanged​(HierarchyEvent e)
      Handles the hierarchyChanged event by invoking the hierarchyChanged methods on listener-a and listener-b.
      Specified by:
      hierarchyChanged in interface HierarchyListener
      Parameters:
      e - the item event
      Since:
      1.3
      See Also:
      HierarchyEvent.getChangeFlags()
    • ancestorMoved

      public void ancestorMoved​(HierarchyEvent e)
      Handles the ancestorMoved event by invoking the ancestorMoved methods on listener-a and listener-b.
      Specified by:
      ancestorMoved in interface HierarchyBoundsListener
      Parameters:
      e - the item event
      Since:
      1.3
    • ancestorResized

      public void ancestorResized​(HierarchyEvent e)
      Handles the ancestorResized event by invoking the ancestorResized methods on listener-a and listener-b.
      Specified by:
      ancestorResized in interface HierarchyBoundsListener
      Parameters:
      e - the item event
      Since:
      1.3
    • mouseWheelMoved

      public void mouseWheelMoved​(MouseWheelEvent e)
      Handles the mouseWheelMoved event by invoking the mouseWheelMoved methods on listener-a and listener-b.
      Specified by:
      mouseWheelMoved in interface MouseWheelListener
      Parameters:
      e - the mouse event
      Since:
      1.4
      See Also:
      MouseWheelEvent
    • add

      Adds component-listener-a with component-listener-b and returns the resulting multicast listener.
      Parameters:
      a - component-listener-a
      b - component-listener-b
      Returns:
      the resulting listener
    • add

      Adds container-listener-a with container-listener-b and returns the resulting multicast listener.
      Parameters:
      a - container-listener-a
      b - container-listener-b
      Returns:
      the resulting listener
    • add

      public static FocusListener add​(FocusListener a, FocusListener b)
      Adds focus-listener-a with focus-listener-b and returns the resulting multicast listener.
      Parameters:
      a - focus-listener-a
      b - focus-listener-b
      Returns:
      the resulting listener
    • add

      public static KeyListener add​(KeyListener a, KeyListener b)
      Adds key-listener-a with key-listener-b and returns the resulting multicast listener.
      Parameters:
      a - key-listener-a
      b - key-listener-b
      Returns:
      the resulting listener
    • add

      public static MouseListener add​(MouseListener a, MouseListener b)
      Adds mouse-listener-a with mouse-listener-b and returns the resulting multicast listener.
      Parameters:
      a - mouse-listener-a
      b - mouse-listener-b
      Returns:
      the resulting listener
    • add

      Adds mouse-motion-listener-a with mouse-motion-listener-b and returns the resulting multicast listener.
      Parameters:
      a - mouse-motion-listener-a
      b - mouse-motion-listener-b
      Returns:
      the resulting listener
    • add

      public static WindowListener add​(WindowListener a, WindowListener b)
      Adds window-listener-a with window-listener-b and returns the resulting multicast listener.
      Parameters:
      a - window-listener-a
      b - window-listener-b
      Returns:
      the resulting listener
    • add

      Adds window-state-listener-a with window-state-listener-b and returns the resulting multicast listener.
      Parameters:
      a - window-state-listener-a
      b - window-state-listener-b
      Returns:
      the resulting listener
      Since:
      1.4
    • add

      Adds window-focus-listener-a with window-focus-listener-b and returns the resulting multicast listener.
      Parameters:
      a - window-focus-listener-a
      b - window-focus-listener-b
      Returns:
      the resulting listener
      Since:
      1.4
    • add

      public static ActionListener add​(ActionListener a, ActionListener b)
      Adds action-listener-a with action-listener-b and returns the resulting multicast listener.
      Parameters:
      a - action-listener-a
      b - action-listener-b
      Returns:
      the resulting listener
    • add

      public static ItemListener add​(ItemListener a, ItemListener b)
      Adds item-listener-a with item-listener-b and returns the resulting multicast listener.
      Parameters:
      a - item-listener-a
      b - item-listener-b
      Returns:
      the resulting listener
    • add

      Adds adjustment-listener-a with adjustment-listener-b and returns the resulting multicast listener.
      Parameters:
      a - adjustment-listener-a
      b - adjustment-listener-b
      Returns:
      the resulting listener
    • add

      public static TextListener add​(TextListener a, TextListener b)
      Adds text-listener-a with text-listener-b and returns the resulting multicast listener.
      Parameters:
      a - text-listener-a
      b - text-listener-b
      Returns:
      the resulting listener
    • add

      Adds input-method-listener-a with input-method-listener-b and returns the resulting multicast listener.
      Parameters:
      a - input-method-listener-a
      b - input-method-listener-b
      Returns:
      the resulting listener
    • add

      Adds hierarchy-listener-a with hierarchy-listener-b and returns the resulting multicast listener.
      Parameters:
      a - hierarchy-listener-a
      b - hierarchy-listener-b
      Returns:
      the resulting listener
      Since:
      1.3
    • add

      Adds hierarchy-bounds-listener-a with hierarchy-bounds-listener-b and returns the resulting multicast listener.
      Parameters:
      a - hierarchy-bounds-listener-a
      b - hierarchy-bounds-listener-b
      Returns:
      the resulting listener
      Since:
      1.3
    • add

      Adds mouse-wheel-listener-a with mouse-wheel-listener-b and returns the resulting multicast listener.
      Parameters:
      a - mouse-wheel-listener-a
      b - mouse-wheel-listener-b
      Returns:
      the resulting listener
      Since:
      1.4
    • remove

      public static ComponentListener remove​(ComponentListener l, ComponentListener oldl)
      Removes the old component-listener from component-listener-l and returns the resulting multicast listener.
      Parameters:
      l - component-listener-l
      oldl - the component-listener being removed
      Returns:
      the resulting listener
    • remove

      public static ContainerListener remove​(ContainerListener l, ContainerListener oldl)
      Removes the old container-listener from container-listener-l and returns the resulting multicast listener.
      Parameters:
      l - container-listener-l
      oldl - the container-listener being removed
      Returns:
      the resulting listener
    • remove

      public static FocusListener remove​(FocusListener l, FocusListener oldl)
      Removes the old focus-listener from focus-listener-l and returns the resulting multicast listener.
      Parameters:
      l - focus-listener-l
      oldl - the focus-listener being removed
      Returns:
      the resulting listener
    • remove

      public static KeyListener remove​(KeyListener l, KeyListener oldl)
      Removes the old key-listener from key-listener-l and returns the resulting multicast listener.
      Parameters:
      l - key-listener-l
      oldl - the key-listener being removed
      Returns:
      the resulting listener
    • remove

      public static MouseListener remove​(MouseListener l, MouseListener oldl)
      Removes the old mouse-listener from mouse-listener-l and returns the resulting multicast listener.
      Parameters:
      l - mouse-listener-l
      oldl - the mouse-listener being removed
      Returns:
      the resulting listener
    • remove

      public static MouseMotionListener remove​(MouseMotionListener l, MouseMotionListener oldl)
      Removes the old mouse-motion-listener from mouse-motion-listener-l and returns the resulting multicast listener.
      Parameters:
      l - mouse-motion-listener-l
      oldl - the mouse-motion-listener being removed
      Returns:
      the resulting listener
    • remove

      public static WindowListener remove​(WindowListener l, WindowListener oldl)
      Removes the old window-listener from window-listener-l and returns the resulting multicast listener.
      Parameters:
      l - window-listener-l
      oldl - the window-listener being removed
      Returns:
      the resulting listener
    • remove

      public static WindowStateListener remove​(WindowStateListener l, WindowStateListener oldl)
      Removes the old window-state-listener from window-state-listener-l and returns the resulting multicast listener.
      Parameters:
      l - window-state-listener-l
      oldl - the window-state-listener being removed
      Returns:
      the resulting listener
      Since:
      1.4
    • remove

      public static WindowFocusListener remove​(WindowFocusListener l, WindowFocusListener oldl)
      Removes the old window-focus-listener from window-focus-listener-l and returns the resulting multicast listener.
      Parameters:
      l - window-focus-listener-l
      oldl - the window-focus-listener being removed
      Returns:
      the resulting listener
      Since:
      1.4
    • remove

      public static ActionListener remove​(ActionListener l, ActionListener oldl)
      Removes the old action-listener from action-listener-l and returns the resulting multicast listener.
      Parameters:
      l - action-listener-l
      oldl - the action-listener being removed
      Returns:
      the resulting listener
    • remove

      public static ItemListener remove​(ItemListener l, ItemListener oldl)
      Removes the old item-listener from item-listener-l and returns the resulting multicast listener.
      Parameters:
      l - item-listener-l
      oldl - the item-listener being removed
      Returns:
      the resulting listener
    • remove

      public static AdjustmentListener remove​(AdjustmentListener l, AdjustmentListener oldl)
      Removes the old adjustment-listener from adjustment-listener-l and returns the resulting multicast listener.
      Parameters:
      l - adjustment-listener-l
      oldl - the adjustment-listener being removed
      Returns:
      the resulting listener
    • remove

      public static TextListener remove​(TextListener l, TextListener oldl)
      Removes the old text-listener from text-listener-l and returns the resulting multicast listener.
      Parameters:
      l - text-listener-l
      oldl - the text-listener being removed
      Returns:
      the resulting listener
    • remove

      public static InputMethodListener remove​(InputMethodListener l, InputMethodListener oldl)
      Removes the old input-method-listener from input-method-listener-l and returns the resulting multicast listener.
      Parameters:
      l - input-method-listener-l
      oldl - the input-method-listener being removed
      Returns:
      the resulting listener
    • remove

      public static HierarchyListener remove​(HierarchyListener l, HierarchyListener oldl)
      Removes the old hierarchy-listener from hierarchy-listener-l and returns the resulting multicast listener.
      Parameters:
      l - hierarchy-listener-l
      oldl - the hierarchy-listener being removed
      Returns:
      the resulting listener
      Since:
      1.3
    • remove

      Removes the old hierarchy-bounds-listener from hierarchy-bounds-listener-l and returns the resulting multicast listener.
      Parameters:
      l - hierarchy-bounds-listener-l
      oldl - the hierarchy-bounds-listener being removed
      Returns:
      the resulting listener
      Since:
      1.3
    • remove

      public static MouseWheelListener remove​(MouseWheelListener l, MouseWheelListener oldl)
      Removes the old mouse-wheel-listener from mouse-wheel-listener-l and returns the resulting multicast listener.
      Parameters:
      l - mouse-wheel-listener-l
      oldl - the mouse-wheel-listener being removed
      Returns:
      the resulting listener
      Since:
      1.4
    • addInternal

      protected static EventListener addInternal​(EventListener a, EventListener b)
      Returns the resulting multicast listener from adding listener-a and listener-b together. If listener-a is null, it returns listener-b; If listener-b is null, it returns listener-a If neither are null, then it creates and returns a new AWTEventMulticaster instance which chains a with b.
      Parameters:
      a - event listener-a
      b - event listener-b
      Returns:
      the resulting listener
    • removeInternal

      protected static EventListener removeInternal​(EventListener l, EventListener oldl)
      Returns the resulting multicast listener after removing the old listener from listener-l. If listener-l equals the old listener OR listener-l is null, returns null. Else if listener-l is an instance of AWTEventMulticaster, then it removes the old listener from it. Else, returns listener l.
      Parameters:
      l - the listener being removed from
      oldl - the listener being removed
      Returns:
      the resulting listener
    • saveInternal

      protected void saveInternal​(ObjectOutputStream s, String k) throws IOException
      Serialization support. Saves all Serializable listeners to a serialization stream.
      Parameters:
      s - the stream to save to
      k - a prefix stream to put before each serializable listener
      Throws:
      IOException - if serialization fails
    • save

      protected static void save​(ObjectOutputStream s, String k, EventListener l) throws IOException
      Saves a Serializable listener chain to a serialization stream.
      Parameters:
      s - the stream to save to
      k - a prefix stream to put before each serializable listener
      l - the listener chain to save
      Throws:
      IOException - if serialization fails
    • getListeners

      public static <T extends EventListener> T[] getListeners​(EventListener l, Class<T> listenerType)
      Returns an array of all the objects chained as FooListeners by the specified java.util.EventListener. FooListeners are chained by the AWTEventMulticaster using the addFooListener method. If a null listener is specified, this method returns an empty array. If the specified listener is not an instance of AWTEventMulticaster, this method returns an array which contains only the specified listener. If no such listeners are chained, this method returns an empty array.
      Type Parameters:
      T - the listener type
      Parameters:
      l - the specified java.util.EventListener
      listenerType - the type of listeners requested; this parameter should specify an interface that descends from java.util.EventListener
      Returns:
      an array of all objects chained as FooListeners by the specified multicast listener, or an empty array if no such listeners have been chained by the specified multicast listener
      Throws:
      NullPointerException - if the specified listenertype parameter is null
      ClassCastException - if listenerType doesn't specify a class or interface that implements java.util.EventListener
      Since:
      1.4