设计模式

/**
 * 观察者模块
 * @author swt
 */
public interface Observer {

    /**
     * 更新数据
     */
    void update();
}

　　

/**
 * 显示观察者
 * @author swt
 */
public class DisplayObserver implements Observer {
    @Override
    public void update() {
        System.out.println("DisplayObserver.update");
    }
}

　　

/**
 * 主题对象有3个基础操作：注册新的观察者、通知所有观察者更新数据、移除取消观察的观察者对象。
 * @author swt
 */
public interface Subject {

    /**
     * 注册观察者
     * @param observer  观察者
     */
    void addObserver(Observer observer);

    /**
     * 移除观察者
     * @param observer  观察者
     */
    void removeObserver(Observer observer);

    /**
     * 通知所有观察者
     */
    void notifyObservers();

}

　　

/**
 * 状态主题
 * @author swt
 */
public class RelayStateSubject implements Subject {

    private List<Observer> observerList;

    private RelayStateSubject(){
        observerList = new ArrayList<>();
        System.out.println("RelayStateSubject.RelayStateSubject");
    }

    static class RelayStateSubjectHolder {
        private static final RelayStateSubject INSTANCE = new RelayStateSubject();
    }

    public static RelayStateSubject getInstance() {
        return RelayStateSubjectHolder.INSTANCE;
    }

    @Override
    public void addObserver(Observer observer) {
        observerList.add(observer);
        System.out.println("RelayStateSubject.addObserver");
    }

    @Override
    public void removeObserver(Observer observer) {
        observerList.remove(observer);
        System.out.println("RelayStateSubject.removeObserver");
    }

    @Override
    public void notifyObservers() {
        for (Observer o : observerList) {
            if(null == o) {
                continue;
            }
            o.update();
        }
        System.out.println("RelayStateSubject.notifyObservers");
    }

    /**
     * 改变状态
     */
    public void changeState() {
        notifyObservers();
    }
}

　　

public class Main {
    public static void main(String[] args) {
        RelayStateSubject relayStateSubject = RelayStateSubject.getInstance();
        relayStateSubject.addObserver(new DisplayObserver());
        relayStateSubject.changeState();
        System.out.println("Main.main");
    }
}

　输出

RelayStateSubject.RelayStateSubject
RelayStateSubject.addObserver
DisplayObserver.update
RelayStateSubject.notifyObservers
Main.main