java_Observer Design Pattern

摘自：
    http://www.ntu.edu.sg/home/ehchua/programming/java/J4a_GUI.html

Creating Your Own Event, Source and Listener：
Suppose that we have a source called Light, with two states - on and off. The source is capable of notifying its registered listener(s), whenever its state changes.

    First, we define the LightEvent class (extends from java.util.EventObject).
    Next, we define a LightListener interface to bind the source and its listeners. This interface specifies the signature of the handlers, lightOn(LightEvent) and lightOff(LightEvent).
    In the source Light, we use an ArrayList to maintain its listeners, and create two methods: addLightListner(LightListener) and removeLightListener(LightListener). An method called notifyListener() is written to invoke the appropriate handlers of each of its registered listeners, whenever the state of the Light changes.
    A listener class called LightWatcher is written, which implements the LightListener interface and provides implementation for the handlers.

Event: LightEvent.java
/** LightEvent */
import java.util.EventObject;

public class LightEvent extends EventObject {
   public LightEvent (Object src) {
      super(src);
   }
}

Listener Interface: LightListener.java
/** The LightListener interface */
import java.util.EventListener;

public interface LightListener extends EventListener {
   public void lightOn(LightEvent evt);  // called-back upon light on
   public void lightOff(LightEvent evt); // called-back upon light off
}

Source: Light.java
/** The Light Source */
import java.util.*;

public class Light {
   // Status - on (true) or off (false)
   private boolean on;
   // Listener list
   private List<LightListener> listeners = new ArrayList<LightListener>();

   /** Constructor */
   public Light() {
      on = false;
      System.out.println("Light: constructed and off");
   }

   /** Add the given LightListener */
   public void addLightListener(LightListener listener) {
      listeners.add(listener);
      System.out.println("Light: added a listener");
   }

   /** Remove the given LightListener */
   public void removeLightListener(LightListener listener) {
      listeners.remove(listener);
      System.out.println("Light: removed a listener");
   }

   /** Turn on this light */
   public void turnOn() {
      if (!on) {
         on = !on;
         System.out.println("Light: turn on");
         notifyListener();
      }
   }

   /** Turn off this light */
   public void turnOff() {
      if (on) {
         on = !on;
         System.out.println("Light: turn off");
         notifyListener();
      }
   }

   /** Construct an LightEvent and notify all its registered listeners */
   private void notifyListener() {
      LightEvent evt = new LightEvent(this);
      for (LightListener listener : listeners) {
         if (on) {
            listener.lightOn(evt);
         } else {
            listener.lightOff(evt);
         }
      }
   }
}

Listener: LightWatcher.java
/** An implementation of LightListener class */
public class LightWatcher implements LightListener {
   private int id;  // ID of this listener

   /** Constructor */
   public LightWatcher(int id) {
      this.id = id;
      System.out.println("LightWatcher-" + id + ": created");
   }

   /** Implementation of event handlers */
   @Override
   public void lightOn(LightEvent evt) {
      System.out.println("LightWatcher-" + id
         + ": I am notified that light is on");
   }

   @Override
   public void lightOff(LightEvent evt) {
      System.out.println("LightWatcher-" + id
         + ": I am notified that light is off");
   }
}

A Test Driver: TestLight.java
/** A Test Driver */
public class TestLight {
   public static void main(String[] args) {
      Light light = new Light();
      LightWatcher lw1 = new LightWatcher(1);
      LightWatcher lw2 = new LightWatcher(2);
      LightWatcher lw3 = new LightWatcher(3);
      light.addLightListener(lw1);
      light.addLightListener(lw2);
      light.turnOn();
      light.addLightListener(lw3);
      light.turnOff();
      light.removeLightListener(lw1);
      light.removeLightListener(lw3);
      light.turnOn();
   }
}

Below are the expected output:
Light: constructed and off
LightWatcher-1: created
LightWatcher-2: created
LightWatcher-3: created
Light: added a listener
Light: added a listener
Light: turn on
LightWatcher-1: I am notified that light is on
LightWatcher-2: I am notified that light is on
Light: added a listener
Light: turn off
LightWatcher-1: I am notified that light is off
LightWatcher-2: I am notified that light is off
LightWatcher-3: I am notified that light is off
Light: removed a listener
Light: removed a listener
Light: turn on
LightWatcher-2: I am notified that light is on