Java设计模式--观察者模式

这是一种很常见的模式，主要是在两种对象之间建立一定的联系，当一方（被观察者）发生改变时，另一方（观察者）可以及时获得通知。既可以一对一也可以一对多.简单实现：

观察者抽象类：

public abstract class Observer {
    public abstract void update(String msg);
}

观察者实现类

public class ObserverA extends Observer{
    @Override
    public void update(String msg) {
        System.out.println("ObserverA: " + msg);
    }
}

public class ObserverB extends Observer{
    @Override
    public void update(String msg) {
        System.out.println("ObserverB: " + msg);
    }
}

public class ObserverC extends Observer{
    @Override
    public void update(String msg) {
        System.out.println("ObserverC: " + msg);
    }
}

被观察者：

public class Observable {
    private ArrayList observers = new ArrayList<>();
    public void sendMsg(String msg) {
        notifyAll(msg);
    }

    public void addObserver(Observer observer) {
        observers.add(observer);
    }

    private void notifyAll(String msg) {
        for (Observer observer : observers) {
            observer.update(msg);
        }
    }
}

测试类

    public static void main(String[] args) {
        Observable observable = new Observable();

        ObserverA observerA = new ObserverA();
        ObserverB observerB = new ObserverB();
        ObserverC observerC = new ObserverC();

        observable.addObserver(observerA);
        observable.addObserver(observerB);
        observable.addObserver(observerC);

        observable.sendMsg("hello");
    }