基于UML图的观察者模式实现

题

我无法理解“可重用面向对象软件的元素”中观察者模式的以下 UML 图。谁能解释我为什么我的 Java 实现是错误的，我必须改变什么才能正确实现它？

UML-图

尝试（但错误）实施

public interface Subject {
    
    public static final List<Observer> observers = new ArrayList<Observer>();
    
    public void attach(Observer o);
    public void detach(Observer o);
    public void notifyObservers();
}

public interface Observer { 
    public void update();
}

public class ConcreteSubject implements Subject {
    
    private String subjectState;

    @Override
    public void attach(Observer o) {
        observers.add(o);   
    }

    @Override
    public void detach(Observer o) {
        observers.remove(o);
    }

    @Override
    public void notifyObservers() {
        for (Observer o : observers) {
            o.update();
        }       
    }
    
    public String getState() {
        return subjectState;
    }
    
    public void setState() {
        subjectState += "x";
    }
}

public class ConcreteObserver implements Observer {
    
    private ConcreteSubject subject;
    public String observerState;

    @Override
    public void update() {
        observerState = subject.getState();
    }
}

回答