Question

How does one make a method in an interface that’s not abstract? I know that it CAN be done, I just don’t know how.

I’ll give an example of my confusion. In Java you can implement java.awt.event.MouseListener, then you must call the method addMouseListener(Object)… passing your class as a parameter, so the MouseListener knows what object to cast from. How is the addMouseListener(Object) method possible?

Someone said I was confusing the way anonymous classes work with interfaces having non-abstract methods. How would you implement an anonymous class within a interface so the implementer can call its methods? I’m very new and still a ‘noob’ at OOP.

Answer 1

If you’re interested in anonymous inner classes, they work as shown below.

We’ll continue with the MouseListener example.

The interface for java.awt.event.MouseListener looks something like this:

public interface  MouseListener {
  void mouseClicked(MouseEvent e);
  void mouseEntered(MouseEvent e);
  void mouseExited(MouseEvent e);
  void mousePressed(MouseEvent e);
  void mouseReleased(MouseEvent e);
}

Somewhere in your app you may want to respond to mouse events, so using an anonymous inner class you could do something like this.

component.addMouseListener(new MouseListener() {
  public void mouseClicked(MouseEvent e){/*implementation goes here...*/}
  public void mouseEntered(MouseEvent e){/*implementation goes here...*/}
  public void mouseExited(MouseEvent e){/*implementation goes here...*/}
  public void mousePressed(MouseEvent e){/*implementation goes here...*/}
  public void mouseReleased(MouseEvent e){/*implementation goes here...*/}
});

What you’ve done here is created a new class (without name, hence anonymous) that implements the MouseListener interface. You have not, as suggested above, created a non-abstract method on an interface.

You could have also just created a new named class (“named class” means regular old class):

class MyMouseListener implements MouseListener {
  public void mouseClicked(MouseEvent e){/*implementation goes here...*/}
  public void mouseEntered(MouseEvent e){/*implementation goes here...*/}
  public void mouseExited(MouseEvent e){/*implementation goes here...*/}
  public void mousePressed(MouseEvent e){/*implementation goes here...*/}
  public void mouseReleased(MouseEvent e){/*implementation goes here...*/}
}

Then somewhere else you would do component.addMouseListener(new MyMouseListener());

See the difference?

I hope this helps. Good luck.

P.S. – Read up on inheritance, interfaces, inner classes and anonymous inner classes in Java for a deeper understanding.