设计模式——观察者模式Observer

Q：观察者模式属于哪一类设计模式

A：观察者模式属于行为学模式

Q：什么是观察者模式

A：当一个对象的状态发生改变时，所有依赖它的对象都得到通知，并自动更新

观察者模式解析：报纸类维护了一个订阅者数组，我们可以通过报纸类遍历订阅者数组来通知所有订阅者

订阅者类

public interface Subscriber { void update(String message); }

订阅者实现类

public class SubscriberImpl implements Subscriber {
    private String name;

    @Override
    public void update(String message) {
        System.out.println(name + "---接收到消息:" + message);
    }
}

报纸类

public interface NewsPaper {
    void addSubscriber(Subscriber subscriber);//添加订阅者
    void removeSubscriber(Subscriber subscriber);//移除订阅者
    void notifySubscriber(String message);//通知订阅者
}

报纸实现类

public class NewsPaperImpl implements NewsPaper {
    private List<Subscriber> subscribers = new ArrayList<>();

    @Override
    public void addSubscriber(Subscriber subscriber) { subscribers.add(subscriber); }

    @Override
    public void removeSubscriber(Subscriber subscriber) { subscribers.remove(subscriber); }

    @Override
    public void notifySubscriber(String message) {
        for (Subscriber s : subscribers) {
            s.update(message);
        }
    }
}

演示类

public class Demo {
    public static void main(String[] args) {
        NewsPaperImpl newsPaper = new NewsPaperImpl();
        SubscriberImpl subscriber1 = new SubscriberImpl("李老头");
        SubscriberImpl subscriber2 = new SubscriberImpl("王奶奶");

        newsPaper.addSubscriber(subscriber1);
        newsPaper.addSubscriber(subscriber2);

        newsPaper.notifySubscriber("今天的报纸收到了！");

        newsPaper.removeSubscriber(subscriber1);

        newsPaper.notifySubscriber("明天的报纸还是这个点到！");
    }
}

演示结果