Question

If I have a class which has an interface definition like below;

public class FirstClass {

  private static onDragListener listener  

  public interface onDragListener{
      void doSomething();
  }

  //a static method to set on my drag listener
  public static void setOnDragListener(onDragListener listener){
           FirstClass.listener = listener
  }

}

As you see above, there is a setOnDragListener(onDragListener listener) static method to set on the listener.

Then, there is another class which implements the interface defined in the first class like below:

public class SecondClass implements onDragListener{
    public void initialize(){
          FirstClass.setOnDragListener(this); //my question is here about 'this'. 
    }
}

My question is why I can use this as the argument pass to the FirstClass.setOnDragListener(this); in the initialize() method above?

I mean the setOnDragListener(onDragListener listener) is expecting a onDragListener type argument to pass in, does this in the initialize method of second class means the real implementation of the interface or how can I understand it?

Answer 1

Inheritance is a is-a relationship.

SecondClass is-a onDragListener.

this is a SecondClass.

Ergo, this is a onDragListener.