Anonymous Class

You can declare an inner class without naming it.

    button.addActionListener(new ActionListener() {
        int xyz;    //instance variable(s)
        ....   
        public void actionPerformed(ActionEvent e) {  //override method(s)
            ....
        }
    });

Anonymous classes can make code difficult to read. You should limit their use to those classes that are very small (no more than a method or two) and whose use is well-understood (like the Swing event-handling adapter classes).