定义了一种一对多的依赖关系，让多个观察者对象同时监听某一个主题对象。这个主题对象在状态发生变化时，会通知所有观察者对象，使他们能够自己更新自己。
类图
实现
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| import java.util.ArrayList;
import java.util.List;
public abstract class Subject {
private List<Observer> observers = new ArrayList<>();
public void attach(Observer observer) {
observers.add(observer);
}
public void detach(Observer observer) {
observers.remove(observer);
}
public void notifyObservers() {
for (Observer observer : observers) {
observer.update();
}
}
}
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| public abstract class Observer {
public abstract void update();
}
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| public class ConcreteSubject extends Subject {
private String subjectState;
public String getSubjectState() {
return subjectState;
}
public void setSubjectState(String subjectState) {
this.subjectState = subjectState;
}
}
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| public class ConcreteObserver extends Observer {
private String name;
private ConcreteSubject concreteSubject;
public ConcreteObserver(ConcreteSubject concreteSubject, String name) {
this.name = name;
this.concreteSubject = concreteSubject;
this.concreteSubject.attach(this);
}
@Override
public void update() {
System.out.println("观察者" + name + "获取的主题状态是:" + concreteSubject.getSubjectState());
}
}
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| public class Client {
public static void main(String[] args) {
ConcreteSubject subject = new ConcreteSubject();
ConcreteObserver observer1 = new ConcreteObserver(subject, "X");
ConcreteObserver observer2 = new ConcreteObserver(subject, "Y");
ConcreteObserver observer3 = new ConcreteObserver(subject, "Z");
subject.setSubjectState("A");
subject.notifyObservers();
subject.detach(observer2);
subject.setSubjectState("B");
subject.notifyObservers();
}
}
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