Java Observer模式如何实现动态添加观察者

在Java中，Observer模式是一种行为设计模式，它允许对象（称为主题或可观察者）在其状态发生变化时通知其他对象（称为观察者）。要实现动态添加观察者，可以使用Java内置的java.util.Observer接口和java.util.Observable类，或者使用Java的java.beans包中的PropertyChangeListener和PropertyChangeSupport类。下面是使用这两种方法的示例：

方法1：使用java.util.Observer接口和java.util.Observable类

1. 创建一个观察者类，实现Observer接口：

import java.util.Observer;
import java.util.Observable;

public class ConcreteObserver implements Observer {
    private String name;

    public ConcreteObserver(String name) {
        this.name = name;
    }

    @Override
    public void update(Observable o, Object arg) {
        System.out.println(name + " received update: " + arg);
    }
}

2. 创建一个可观察者类，继承Observable类：

import java.util.Observable;

public class ConcreteObservable extends Observable {
    public void changeState(String newState) {
        setChanged();
        notifyObservers(newState);
    }
}

3. 在主程序中动态添加观察者：

public class Main {
    public static void main(String[] args) {
        ConcreteObservable observable = new ConcreteObservable();

        ConcreteObserver observer1 = new ConcreteObserver("Observer1");
        ConcreteObserver observer2 = new ConcreteObserver("Observer2");

        observable.addObserver(observer1);
        observable.addObserver(observer2);

        observable.changeState("New State");

        observable.deleteObserver(observer1);

        observable.changeState("Another New State");
    }
}

方法2：使用PropertyChangeListener和PropertyChangeSupport类

1. 创建一个观察者类，实现PropertyChangeListener接口：

import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;

public class ConcretePropertyChangeListener implements PropertyChangeListener {
    private String name;

    public ConcretePropertyChangeListener(String name) {
        this.name = name;
    }

    @Override
    public void propertyChange(PropertyChangeEvent evt) {
        System.out.println(name + " received update: " + evt.getNewValue());
    }
}

2. 创建一个可观察者类，使用PropertyChangeSupport类：

import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeSupport;

public class ConcretePropertyChangeSubject {
    private PropertyChangeSupport support;

    public ConcretePropertyChangeSubject() {
        support = new PropertyChangeSupport(this);
    }

    public void addPropertyChangeListener(PropertyChangeListener pcl) {
        support.addPropertyChangeListener(pcl);
    }

    public void removePropertyChangeListener(PropertyChangeListener pcl) {
        support.removePropertyChangeListener(pcl);
    }

    public void changeState(String newState) {
        support.firePropertyChange("state", null, newState);
    }
}

3. 在主程序中动态添加观察者：

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

        ConcretePropertyChangeListener listener1 = new ConcretePropertyChangeListener("Listener1");
        ConcretePropertyChangeListener listener2 = new ConcretePropertyChangeListener("Listener2");

        subject.addPropertyChangeListener(listener1);
        subject.addPropertyChangeListener(listener2);

        subject.changeState("New State");

        subject.removePropertyChangeListener(listener1);

        subject.changeState("Another New State");
    }
}

这两种方法都可以实现动态添加观察者。使用哪种方法取决于您的需求和编程风格。