Question

I’ve been coming across this code several times and I would like to know what it means or what it is equivalent to:

A a = new A() {
  // declare some methods and whatever
};

What does the above mean? What is it equivalent to (if it is equivalent to anything)?

Answer 1

They’re declaring an anonymous class. Class A is (likely) either an interface or an abstract class that must have implemented methods in order to be a subclass of A. Java allows you to define those methods on the fly with anonymous classes.

For example, there is only one method to implement for ActionListener, which is actionPerformed(ActionEvent). Many times it’s easier to define that method in an anonymous class like so:

ActionListener myListener = new ActionListener() {
  @Override
  public void actionPerformed(ActionEvent e) {
    Object source = e.getSource();
    // do other stuff for action performed
  }
}

Edit in response to the OP’s comment

You could certainly create a class file for an ActionListener and create a new instance of your class instead of creating an anonymous class. That is, assuming you could do everything you needed to do in a separate class. Many times anonymous and inner classes are defined so that you can have access to the outer class’s fields and methods. It is also many times more object oriented to define an inner or anonymous class if the the inner class “belongs” to the outer class and to no one else.