Anonymous class with interface

Question

I am confused about the concept of interface when dealing with anonymous inner class. As far as I know that you can't instantiate an interface in Java, so the following statement would have a compile error

     ActionListener action = new ActionListener();  // compile error

What happen when it deals with anonymous class? why does it allow to use new? For example:

     JButton button = new JButton("A");
     button.addActionListener(new ActionListener(){    //this is fine
           @Override
           public void actionPerformed(ActionEvent e){

           }
     };

Does the compiler just make a class and implement ActionListener behind the scene? How does it work?

Bill the Lizard · Accepted Answer · 2012-10-08 16:58:26Z

5

It allows you to create a new anonymous class that implements ActionListener because you're providing the implementation, you're just not giving it a class name.

answered Oct 8, 2012 at 16:58

Bill the Lizard

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3 Comments

peter Over a year ago

So what instance are we actually creating ?

Bill the Lizard Over a year ago

@user1389813 The instance that you're passing to the addActionListener method. It doesn't really need an explicitly-declared type because you're never going to use it again. The compiler just fills in some placeholder name. It's just the class that implements ActionListener with the override that you defined inline. It's still safe to say that it IS-A ActionListener, since that's what the reference type would be.

Victor Mukherjee Over a year ago

a new anonymous inner class object that implements ActionListener. with anonymous inner classes, it is obvious that you will be implementing some interface, so i feel the implements keyword is not necessary.

Renato Lochetti · Accepted Answer · 2012-10-08 16:59:01Z

1

When you create an inner-class, you are instantiating an anonymous class that implements the interface.

In your case, The effect is the same of: public class Foo implements ActionListener

answered Oct 8, 2012 at 16:59

Renato Lochetti

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Comments

Samuel · Accepted Answer · 2012-10-08 17:10:33Z

1

You're defining an inner class with a sequentially assigned name like 1, 2, 3 etc. At the same time you're instantiating the inner class with the new keyword. You don't care about the name of the inner class because you're using it anonymously. If you look in your bin directory you'll see a class file for each of the anonymous definitions. For example if you used an anonymous class in a class, Foo, you would have Foo.class and Foo$1.class created for you. I believe this means that you could instantiate more of the anonymous classes at a later date using reflection.

edited Oct 8, 2012 at 17:10

answered Oct 8, 2012 at 17:05

Samuel

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Comments

saum22 · Accepted Answer · 2012-10-08 17:00:22Z

0

You are creating a class and implementing the interface when working with anonymous class.you can override the methods or implement the method inside the anonymous class

    A a= new A(){ 

               };

Here a is a reference variable of type A which is referring not to A but to an object of class which implement A who doesn't have a name

answered Oct 8, 2012 at 17:00

saum22

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