define a class as Observable and Observer both

Question

technically it seems OK to define a Class as both Observable and Observer using the following code:

public class Data extends Observable implements Observer

however, trying to implement it, it's not working.

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);
    }

}

with the main function as below:

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

it should prints some "SC" but it's not working. Can anyone describes why?

Answer 1

If you don't .setChanged(), then .notifyObservers() has no effect. This is the case both if you have separate classes defining Observable and Observers, or if you have a single class, as in your example.

Try changing your setters as follows:

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

From the documentation of 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.