Observer -對象行爲型模式

        1.意圖
            定義對象間的一種一對多的關係，當一個對象的狀態發生改變的時候，所有依賴
            它的對象都得到通知。
            2.參與者
            Subject -目標知道它的觀察者
                    -提供註冊和刪除它的觀察者
            Observer -爲那些在目標中發生改變時需獲得通知的對象
            ConcreteSubject - 將有關狀態存入ConcreteObserver對象
                            - 當它的狀態發生改變的時候，通知它的觀察者
            ConcreteObserver - 維護一個指向ConcreteSubject對象的引用
                             - 存儲相關的狀態，這些狀態和目標一致
                             - 實現Observer的更新接口使它的狀態和目標一致

3.結構

4.代碼

public interface Subject {
  void register(Observer o);
  void remove(Observer o);
}

public class ConcreteSubject implements Subject {
    private String weather;
    private List<Observer> list;
    ConcreteSubject(){
    	list = new ArrayList<Observer>();
    }
	@Override
	public void register(Observer o) {
		list.add(o);

	}

	public void setWeather(String weather) {
		this.weather = weather;
		for(Observer o:list){
			o.changWeather(weather);
		}
	}
	@Override
	public void remove(Observer o) {
		list.remove(o);
	}

}


public interface Observer {
    void changWeather(String weather);
}

public class ConcreteObserver implements Observer {
	private String weather;
	private String name;
	@Override
	public void changWeather(String weather) {
		setWeather(weather);
	}
	public String getWeather() {
		return weather;
	}
	public void setWeather(String weather) {
		this.weather = weather;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}

}

public class Client {
	public static void main(String[] args) {
		ConcreteSubject weatherT = new ConcreteSubject();
		ConcreteObserver o1 = new ConcreteObserver();
		o1.setName("天氣預報臺1");
		ConcreteObserver o2 = new ConcreteObserver();
		o2.setName("天氣預報臺2");
		ConcreteObserver o3 = new ConcreteObserver();
		o3.setName("天氣預報臺3");
		ConcreteObserver o4 = new ConcreteObserver();
		o4.setName("天氣預報臺4");
		weatherT.register(o1);
		weatherT.register(o2);
		weatherT.register(o3);
		weatherT.register(o4);
		
		weatherT.setWeather("北京氣溫零下5度，多晴轉雲");
		
		System.out.println(o1.getName()+":"+o1.getWeather());
		System.out.println(o2.getName()+":"+o2.getWeather());
		System.out.println(o3.getName()+":"+o3.getWeather());
		System.out.println(o4.getName()+":"+o4.getWeather());
	}
}