//抽象观察者角色
public interface Watcher
{
  public void update(String str);
} 

//抽象主题角色，watched：被观察
public interface Watched
{
  public void addWatcher(Watcher watcher);
  public void removeWatcher(Watcher watcher);
  public void notifyWatchers(String str);
} 

public class ConcreteWatcher implements Watcher
{
  @Override
  public void update(String str)
  {
    System.out.println(str);
  }
} 

import java.util.ArrayList;
import java.util.List;
public class ConcreteWatched implements Watched
{
  // 存放观察者
  private List<Watcher> list = new ArrayList<Watcher>();
  @Override
  public void addWatcher(Watcher watcher)
  {
    list.add(watcher);
  }
  @Override
  public void removeWatcher(Watcher watcher)
  {
    list.remove(watcher);
  }
  @Override
  public void notifyWatchers(String str)
  {
    // 自动调用实际上是主题进行调用的
    for (Watcher watcher : list)
    {
      watcher.update(str);
    }
  }
} 

public class Test
{
  public static void main(String[] args)
  {
    Watched girl = new ConcreteWatched();
    Watcher watcher1 = new ConcreteWatcher();
    Watcher watcher2 = new ConcreteWatcher();
    Watcher watcher3 = new ConcreteWatcher();
    girl.addWatcher(watcher1);
    girl.addWatcher(watcher2);
    girl.addWatcher(watcher3);
    girl.notifyWatchers("开心");
  }
}