EventListenerList (Java SE 25 & JDK 25)
Description: declaration: module: java.desktop, package: javax.swing.event, class: EventListenerList
Keywords: javax.swing.event.EventListenerList class

# Class EventListenerList

java.lang.Object
javax.swing.event.EventListenerList

All Implemented Interfaces:
  `Serializable`

public class EventListenerList extends Object implements Serializable
A class that holds a list of EventListeners. A single instance can be used to hold all listeners (of all types) for the instance using the list. It is the responsibility of the class using the EventListenerList to provide type-safe API (preferably conforming to the JavaBeans spec) and methods which dispatch event notification methods to appropriate Event Listeners on the list. The main benefits that this class provides are that it is relatively cheap in the case of no listeners, and it provides serialization for event-listener lists in a single place, as well as a degree of MT safety (when used correctly). Usage example: Say one is defining a class that sends out FooEvents, and one wants to allow users of the class to register FooListeners and receive notification when FooEvents occur. The following should be added to the class definition:

    EventListenerList listenerList = new EventListenerList();
    FooEvent fooEvent = null;

    public void addFooListener(FooListener l) {
        listenerList.add(FooListener.class, l);
    }

    public void removeFooListener(FooListener l) {
        listenerList.remove(FooListener.class, l);
    }

    // Notify all listeners that have registered interest for
    // notification on this event type.  The event instance
    // is lazily created using the parameters passed into
    // the fire method.

    protected void fireFooXXX() {
        // Guaranteed to return a non-null array
        Object[] listeners = listenerList.getListenerList();
        // Process the listeners last to first, notifying
        // those that are interested in this event
        for (int i = listeners.length-2; i>=0; i-=2) {
            if (listeners[i]==FooListener.class) {
                // Lazily create the event:
                if (fooEvent == null)
                    fooEvent = new FooEvent(this);
                ((FooListener)listeners[i+1]).fooXXX(fooEvent);
            }
        }
    }

foo should be changed to the appropriate name, and fireFooXxx to the appropriate method name. One fire method should exist for each notification method in the FooListener interface.

Warning: Serialized objects of this class will not be compatible with future Swing releases. The current serialization support is appropriate for short term storage or RMI between applications running the same version of Swing. As of 1.4, support for long term storage of all JavaBeans has been added to the `java.beans` package. Please see `XMLEncoder`.

- ## Field Summary

  Fields
  Modifier and Type
  Field
  Description
  `protected Object[]`
  `listenerList`
  The list of ListenerType - Listener pairs

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `EventListenerList()`
  Constructs a `EventListenerList`.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `<T extends EventListener>
  void`
  `add(Class<T> t, T l)`
  Adds the listener as a listener of the specified type.
  `int`
  `getListenerCount()`
  Returns the total number of listeners for this listener list.
  `int`
  `getListenerCount(Class<?> t)`
  Returns the total number of listeners of the supplied type for this listener list.
  `Object[]`
  `getListenerList()`
  Passes back the event listener list as an array of ListenerType-listener pairs.
  `<T extends EventListener>
  T[]`
  `getListeners(Class<T> t)`
  Return an array of all the listeners of the given type.
  `<T extends EventListener>
  void`
  `remove(Class<T> t, T l)`
  Removes the listener as a listener of the specified type.
  `String`
  `toString()`
  Returns a string representation of the EventListenerList.

  ### Methods declared in class Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

- ## Field Details

  - ### listenerList

    protected transient volatile Object[] listenerList
    The list of ListenerType - Listener pairs

- ## Constructor Details

  - ### EventListenerList

    public EventListenerList()
    Constructs a `EventListenerList`.

- ## Method Details

  - ### getListenerList

    public Object[] getListenerList()
    Passes back the event listener list as an array of ListenerType-listener pairs. Note that for performance reasons, this implementation passes back the actual data structure in which the listener data is stored internally! This method is guaranteed to pass back a non-null array, so that no null-checking is required in fire methods. A zero-length array of Object should be returned if there are currently no listeners. WARNING!!! Absolutely NO modification of the data contained in this array should be made -- if any such manipulation is necessary, it should be done on a copy of the array returned rather than the array itself.
    Returns:
      array of ListenerType-listener pairs

  - ### getListeners

    public <T extends EventListener> T[] getListeners(Class<T> t)
    Return an array of all the listeners of the given type.
    Type Parameters:
      `T` - the type of `EventListener` to search for
    Parameters:
      `t` - the type of `EventListener` classes to be returned
    Returns:
      all of the listeners of the specified type.
    Throws:
      `ClassCastException` - if the supplied class is not assignable to EventListener
    Since:
      1.3

  - ### getListenerCount

    public int getListenerCount()
    Returns the total number of listeners for this listener list.
    Returns:
      an integer count of total number of listeners

  - ### getListenerCount

    public int getListenerCount(Class<?> t)
    Returns the total number of listeners of the supplied type for this listener list.
    Parameters:
      `t` - the type of listeners to count
    Returns:
      the number of listeners of type `t`

  - ### add

    public <T extends EventListener> void add(Class<T> t, T l)
    Adds the listener as a listener of the specified type.
    Type Parameters:
      `T` - the type of `EventListener` to add
    Parameters:
      `t` - the type of the `EventListener` class to add
      `l` - the listener to be added

  - ### remove

    public <T extends EventListener> void remove(Class<T> t, T l)
    Removes the listener as a listener of the specified type.
    Type Parameters:
      `T` - the type of `EventListener`
    Parameters:
      `t` - the type of the listener to be removed
      `l` - the listener to be removed

  - ### toString

    public String toString()
    Returns a string representation of the EventListenerList.
    Overrides:
      `toString` in class `Object`
    Returns:
      a string representation of the object