Java中的MouseMotionListener如何工作？

Question

I am getting confused how the MouseMotionListener is used behind the scenes

• It is an Interface inside java awt
• But here we are not calling the methods it defines but we implement it and describe our own definition

For example this is what i think interfaces do

Interface A {
    method work();
}

class ConcreteA {

 method work(){ //implementation goes here } 

}

then a client code can call to any class that implements the A interface and call its work method

classImplementingA.work()

But here we implement the MouseMotionListener and add our own defition to the methods it exposes MouseDragged, mouseMoved.

So i want to know

Does internally java calls this interface methods in turn calling our defined methods OR something else

ie java internal works like client code for this interface and we defined the behind the scene implementation.

So plz somebody show me how java is handling the mouse movements and making this interface work with it in an easy way.

Answer 1

Listeners in general are based on the observable/observer pattern.

Let's take a simpler example: ActionListener . You want to "observe" a button. You are thus the observer, and the button is the observable. You create an instance of ActionListener , and thus override its actionPerformed method. You then add this listener to the button. And now, the button, each time it's clicked, will call back (notify) the listener by calling its actionPerformed method.

A MouseMotionListener uses the same principle: you create an instance of MouseMotionListener and add it to a Swing component. Then, each time the mouse is moved inside this component, the component calls back (notifies) the listener by calling one of its methods ( mouseDragged or mouseMoved , depending on whether a mouse button is pressed or not).

So, yes, you're right. The client of the interface, in this case, is not your own code, but the Swing components. They are the callers of the interface implementations you define.