观察者模式

简介

当对象间存在一对多关系时，则使用观察者模式（Observer Pattern）。比如，当一个对象被修改时，则会自动通知依赖它的对象。观察者模式属于行为型模式。

实现

1. 创建Subject类

public class Subject {
    private List<Observer> observers = new ArrayList<>();
    private int state;

    public int getState(){
        return state;
    }

    public void setState(int state){
        this.state = state;
        notifyAllObservers();
    }

    private void notifyAllObservers() {
        for (Observer observer : observers) {
            observer.update();
        }
    }

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

2. 创建Observer类

public abstract class Observer {
    protected Subject subject;
    public abstract void update();
}

3. 创建实体观察者类

public class BinaryObserver extends Observer{
    public BinaryObserver(Subject subject) {
        this.subject = subject;
        this.subject.attach(this);
    }

    @Override
    public void update() {
        System.out.println("二进制: "+Integer.toBinaryString(subject.getState()));
    }
}

public class OctalObserver extends Observer{
    public OctalObserver(Subject subject) {
        this.subject = subject;
        this.subject.attach(this);
    }

    @Override
    public void update() {
        System.out.println("八进制: "+Integer.toOctalString(subject.getState()));
    }
}

public class HexObserver extends Observer {
    public HexObserver(Subject subject) {
        this.subject = subject;
        this.subject.attach(this);
    }

    @Override
    public void update() {
        System.out.println("十六进制: "+Integer.toHexString(subject.getState()));
    }
}

4. 使用Subject和实体类观察者对象

public class ObserverPatternDemo {
    public static void main(String[] args) {
        Subject subject = new Subject();

        new HexObserver(subject);
        new OctalObserver(subject);
        new BinaryObserver(subject);

        System.out.println("第一次设置15");
        subject.setState(15);

        System.out.println("=======================");
        System.out.println("第二次设置10");
        subject.setState(10);
    }
}

类图

代码

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