定义

结构

代码案例

定义抽象观察者接口

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

定义微信用户类实现接口

@AllArgsConstructor
public class WeixinUser implements Observer{
    private String username;
    @Override
    public void update(String message) {
        System.out.println(this.username + "-" + message);
    }
}

定义抽象主题类 包含3个规定的方法

public interface Subject {
    //添加订阅者
    public void attach(Observer observer);

    //删除订阅者
    public void detach(Observer observer);

    //通知订阅者更新信息
    public void notify(String message);
}

定义微信公众号类 实现该接口

public class SubscriptionSubject implements Subject{
    //存储该公众号的用户
    private List<Observer> weixinUserList = new ArrayList<>();

    @Override
    public void attach(Observer observer) {
        weixinUserList.add(observer);
    }

    @Override
    public void detach(Observer observer) {
        weixinUserList.remove(observer);
    }

    @Override
    public void notify(String message) {
        for(Observer observer : weixinUserList){
            observer.update(message);
        }
    }
}

测试使用

public class Client {
    public static void main(String[] args) {
        SubscriptionSubject subscriptionSubject = new SubscriptionSubject();

        //创建微信用户
        WeixinUser weixinUser = new WeixinUser("小A");
        WeixinUser weixinUser2 = new WeixinUser("小B");
        WeixinUser weixinUser3 = new WeixinUser("小C");

        //订阅公众号
        subscriptionSubject.attach(weixinUser);
        subscriptionSubject.attach(weixinUser2);
        subscriptionSubject.attach(weixinUser3);

        //更新信息
        subscriptionSubject.notify("新消息!!");
    }
}

总结

使用JDK提供的观察者接口