监听器的原理是观察者模式。就像明星（事件源）聚拢了一群粉丝（观察者）。当明星有啥举动的时候会通过粉丝们报道出去。

订阅信息、计算器button都是该原理的应用。

以下写了一个监听器的小样例：

package listener;
import java.util.EventObject;
/**
 * 定义事件对象,用于标记当前进行的动作。必须继承EventObject。
 * @author zyj
 *
 */
public class StateEvent extends EventObject {

private static final long serialVersionUID = 5323292975415079206L;

private Enum<ActionEnum> action;

public StateEvent(Object source, Enum<ActionEnum> action) {
super(source);
this.action = action;
}

public Enum<ActionEnum> getAction() {
return action;
}

public void setAction(Enum<ActionEnum> action) {
this.action = action;
}

}

package listener;
public enum ActionEnum {
wake,sleep;
}

package listener;
import java.util.EventListener;
/**
 * 定义监听接口，负责监听StateEvent事件。

必须继承EventListener。

* @author zyj
 *
 */
public interface StateListener extends EventListener {
void handleState(StateEvent event);
}

package listener;
public class WakeListener implements StateListener {

@Override
public void handleState(StateEvent event) {
if(event.getAction() != null && event.getAction().equals(ActionEnum.wake)){
System.out.println("你醒了。");
}
}

}

package listener;
public class SleepListener implements StateListener {

@Override
public void handleState(StateEvent event) {
if(event.getAction() != null && event.getAction().equals(ActionEnum.sleep)){
System.out.println("你睡了！");
}
}

}

package listener;
import java.util.ArrayList;
import java.util.List;

public class StateManager {
private List<StateListener> list = new ArrayList<StateListener>();

public boolean addListener(StateListener listener){
if(listener == null)
return false;
return list.add(listener);
}

public boolean removeListener(StateListener listener){
if(listener == null)
return false;
return list.remove(listener);
}

private void notifyAllListeners(StateEvent event){
for(StateListener listener : list){
listener.handleState(event);
}
}

public void wake(){
StateEvent event = new StateEvent(this, ActionEnum.wake);
notifyAllListeners(event);
}

public void sleep(){
StateEvent event = new StateEvent(this, ActionEnum.sleep);
notifyAllListeners(event);
}

}

package listener;
public class ListenerTest {
public static void main(String[] args) {
StateManager manager = new StateManager();
manager.addListener(new WakeListener());
manager.addListener(new SleepListener());
manager.wake();
System.out.println("-----------------");
manager.sleep();
}
}