java - 将一个类定义为 Observable 和 Observer 两者

Question

从技术上讲，使用以下代码将 Class 定义为 Observable 和 Observer 似乎没问题:

public class Data extends Observable implements Observer

但是，尝试实现它，它不起作用。

public class Data extends Observable implements Observer {

    @Override
    public void update(Observable o, Object o1) {
        System.out.println("SC");        
    }

    Integer A;
    String B;
    Float C;

    public Data() {
        this.addObserver(this);
    }

    public void setA(Integer A) {
        this.A = A;
        notifyObservers();
    }

    public void setB(String B) {
        this.B = B;
        notifyObservers();
    }

    public void setC(Float C) {
        this.C = C;
        notifyObservers(this.C);
    }

}

main 函数如下:

public static void main(String[] args) {
    Data d = new Data();
    d.setA(5);
    d.setB("Hi");
    d.setC(2.0f);
}

它应该打印一些“SC”，但它不工作。任何人都可以描述为什么吗？

Answer 1

如果您不使用 .setChanged()，则 .notifyObservers() 无效。如果您有单独的类定义 Observable 和 Observers，或者如果您只有一个类，如您的示例，则属于这种情况。

尝试按如下方式更改您的 setter :

public void setC(Float C) {
  this.C = C;
  setChanged();  // <-- add this line
  notifyObservers(this.C);
}

来自的文档Observable ,

setChanged() Marks this Observable object as having been changed; the hasChanged method will now return true.

notifyObservers(Object arg) If this object has changed, as indicated by the hasChanged method, then notify all of its observers and then call the clearChanged method to indicate that this object has no longer changed.