Custom Event Handler in Java

An Event Class which will be produced by producers and listened by listeners.

public class CustomEvent implements java.io.Serializable{
  
  private int eventType = 0;
  private String eventDetail = "";
  
  public String getEventDetail () {
    return eventDetail;
  }

  public int getEventType () {
    return eventType;
  }

  public void setEventDetail (String eventDetail) {
    this.eventDetail = eventDetail;
  }

  public void setEventType (int eventType) {
    this.eventType = eventType;
  }
}

An Interface which is a contract between event producers and listeners.

public interface EventListenerIntf {
  public void handleEvent (CustomEvent event) throws Exception;
}

An Event Listener

public class EventListener implements EventListenerIntf{
  public EventListener () {
    EventProducer.addListener (this);
  }

  private String name  = null;
  
  public void handleEvent (CustomEvent event)  {
    System.out.println ("Event handled by "+ this.name+" : EventType: "+event.getEventType()+" EventDetail :"+event.getEventDetail());
  }

  public String getName () {
    return name;
  }

  public void setName (String name) {
    this.name = name;
  }

}

A Producer which produces events and have its listener list.

public class EventProducer {
  private static List listenerList = new java.util.ArrayList ();

  public static synchronized void addListener (EventListener listener) {
    listenerList.add (listener);
  }

  public static synchronized void produceEvents () {
    CustomEvent event1 = new CustomEvent ();
    event1.setEventType (1);
    event1.setEventDetail ("event1 detail");
    notifyEventListeners (event1);
    
    CustomEvent event2 = new CustomEvent ();
    event2.setEventType (2);
    event2.setEventDetail ("event2 detail");
    notifyEventListeners (event2);
    
    CustomEvent event3 = new CustomEvent ();
    event3.setEventType (3);
    event3.setEventDetail ("event3 detail");
    notifyEventListeners (event3);
    
    CustomEvent event4 = new CustomEvent ();
    event4.setEventType (4);
    event4.setEventDetail ("event4 detail");
    notifyEventListeners (event4);
  }

  public static void notifyEventListeners (CustomEvent event) {
    for (EventListener listener : listenerList) {
      listener.handleEvent (event);
    }
  }
}

Lets make a simple test these with a simple scenario.

public class Test {
  public static void main (String[] args) {
    //Listeners getting up !
    EventListener amy = new EventListener ();
    amy.setName ("Amy");
    EventListener april = new EventListener ();
    april.setName ("April");
    EventListener eli = new EventListener ();
    eli.setName ("Eli");

    // Producer will create some events
    EventProducer.produceEvents();
    
    System.out.println (" good bye");
    System.exit (0);
  }
}

And out output is :

Event handled by Amy : EventType: 1 EventDetail :event1 detail
Event handled by April : EventType: 1 EventDetail :event1 detail
Event handled by Eli : EventType: 1 EventDetail :event1 detail
Event handled by Amy : EventType: 2 EventDetail :event2 detail
Event handled by April : EventType: 2 EventDetail :event2 detail
Event handled by Eli : EventType: 2 EventDetail :event2 detail
Event handled by Amy : EventType: 3 EventDetail :event3 detail
Event handled by April : EventType: 3 EventDetail :event3 detail
Event handled by Eli : EventType: 3 EventDetail :event3 detail
Event handled by Amy : EventType: 4 EventDetail :event4 detail
Event handled by April : EventType: 4 EventDetail :event4 detail
Event handled by Eli : EventType: 4 EventDetail :event4 detail
good bye