Object Listener

Question

I would like to create a listener for java.util.LinkedList, so that when ever something is added or removed an event is fired.
Does any one have a good way about tackling this problem???

Answer 1

If you don't care about using Swing classes then you can use EventListenerList wich is the most generic class extended by all the swing listeners. In fact if you are not using Swing then the best way is to copy this class to your application so you don't depend on Swing just to use a list of listeners (this class does not depend on any other swing class so you will not have any problem doing this).
The following is the source code of this class
package yourPackage;
import java.io.*;
import java.util.*;
import java.lang.reflect.Array;
* 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 responsiblity 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:
* <pre>
* 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==FooListener.class) {
* // Lazily create the event:
* if (fooEvent == null)
* fooEvent = new FooEvent(this);
* ((FooListener)listeners[i+1]).fooXXX(fooEvent);
* </pre>
* 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.
* <p>
* <strong>Warning:</strong>
* 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. A future release of Swing will provide support for
* long term persistence.
* @version 1.27 02/02/00
* @author Georges Saab
* @author Hans Muller
* @author James Gosling
public class EventListenerList implements Serializable {
/* A null array to be shared by all empty listener lists*/
private final static Object[] NULL_ARRAY = new Object[0];
/* The list of ListenerType - Listener pairs */
protected transient Object[] listenerList = NULL_ARRAY;
* 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.
public Object[] getListenerList() {
     return listenerList;
* Return an array of all the listeners of the given type.
* @returns all of the listeners of the specified type.
* @since 1.3
public EventListener[] getListeners(Class t) {
     Object[] lList = listenerList;
     int n = getListenerCount(t);
EventListener[] result = (EventListener[])Array.newInstance(t, n);
     int j = 0;
     for (int i = lList.length-2; i>=0; i-=2) {
     if (lList[i] == t) {
          result[j++] = (EventListener)lList[i+1];
     return result;
* Returns the total number of listeners for this listener list.
public int getListenerCount() {
     return listenerList.length/2;
* Returns the total number of listeners of the supplied type
* for this listener list.
public int getListenerCount(Class t) {
     int count = 0;
     Object[] lList = listenerList;
     for (int i = 0; i < lList.length; i+=2) {
     if (t == (Class)lList[i])
          count++;
     return count;
* Adds the listener as a listener of the specified type.
* @param t the type of the listener to be added
* @param l the listener to be added
public synchronized void add(Class t, EventListener l) {
     if (l==null) {
     // In an ideal world, we would do an assertion here
     // to help developers know they are probably doing
     // something wrong
     return;
     if (!t.isInstance(l)) {
     throw new IllegalArgumentException("Listener " + l +
                         " is not of type " + t);
     if (listenerList == NULL_ARRAY) {
     // if this is the first listener added,
     // initialize the lists
     listenerList = new Object[] { t, l };
     } else {
     // Otherwise copy the array and add the new listener
     int i = listenerList.length;
     Object[] tmp = new Object[i+2];
     System.arraycopy(listenerList, 0, tmp, 0, i);
     tmp[i] = t;
     tmp[i+1] = l;
     listenerList = tmp;
* Removes the listener as a listener of the specified type.
* @param t the type of the listener to be removed
* @param l the listener to be removed
public synchronized void remove(Class t, EventListener l) {
     if (l ==null) {
     // In an ideal world, we would do an assertion here
     // to help developers know they are probably doing
     // something wrong
     return;
     if (!t.isInstance(l)) {
     throw new IllegalArgumentException("Listener " + l +
                         " is not of type " + t);
     // Is l on the list?
     int index = -1;
     for (int i = listenerList.length-2; i>=0; i-=2) {
     if ((listenerList[i]==t) && (listenerList[i+1].equals(l) == true)) {
          index = i;
          break;
     // If so, remove it
     if (index != -1) {
     Object[] tmp = new Object[listenerList.length-2];
     // Copy the list up to index
     System.arraycopy(listenerList, 0, tmp, 0, index);
     // Copy from two past the index, up to
     // the end of tmp (which is two elements
     // shorter than the old list)
     if (index < tmp.length)
          System.arraycopy(listenerList, index+2, tmp, index,
                    tmp.length - index);
     // set the listener array to the new array or null
     listenerList = (tmp.length == 0) ? NULL_ARRAY : tmp;
// Serialization support.
private void writeObject(ObjectOutputStream s) throws IOException {
     Object[] lList = listenerList;
     s.defaultWriteObject();
     // Save the non-null event listeners:
     for (int i = 0; i < lList.length; i+=2) {
     Class t = (Class)lList[i];
     EventListener l = (EventListener)lList[i+1];
     if ((l!=null) && (l instanceof Serializable)) {
          s.writeObject(t.getName());
          s.writeObject(l);
     s.writeObject(null);
private void readObject(ObjectInputStream s)
     throws IOException, ClassNotFoundException {
listenerList = NULL_ARRAY;
     s.defaultReadObject();
     Object listenerTypeOrNull;
     while (null != (listenerTypeOrNull = s.readObject())) {
     EventListener l = (EventListener)s.readObject();
     add(Class.forName((String)listenerTypeOrNull), l);
* Returns a string representation of the EventListenerList.
public String toString() {
     Object[] lList = listenerList;
     String s = "EventListenerList: ";
     s += lList.length/2 + " listeners: ";
     for (int i = 0 ; i <= lList.length-2 ; i+=2) {
     s += " type " + ((Class)lList[i]).getName();
     s += " listener " + lList[i+1];
     return s;
bye