Thread: Timer class countdown

Hello!

I got the following code:

import javax.swing.JFrame;
import javax.swing.Timer;
 
class MainClass extends JFrame {
  Timer timer;
 
  int counter;
 
  MainClass(String title) {
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 
    ActionListener a = new ActionListener() {
      public void actionPerformed(ActionEvent e) {
        System.out.println("Counter = " + counter);
 
        if (++counter > 10) {
          timer.stop();
          System.exit(0);
        }
      }
    };
 
    timer = new Timer(300, a);
    timer.start();
 
    pack();
    setVisible(true);
  }
 
  public static void main(String[] args) {
    new MainClass("Timer Demo1");
  }
}

Now I got question. ActionListener is actually a interface. How is allowed to create instance of the interface ActionListener by stating ActionListener a = new ActionListener() ?

I got another question about the red part:

    ActionListener a = new ActionListener() [COLOR="Red"]{
      public void actionPerformed(ActionEvent e) {
        System.out.println("Counter = " + counter);
 
        if (++counter > 10) {
          timer.stop();
          System.exit(0);
        }
      }
    };[/COLOR]

What is actually the red part?

Thanks in advance.

The red part is the body of the class 'a' that implements the ActionListener interface.

Originally Posted by Norm

The red part is the body of the class 'a' that implements the ActionListener interface.

Thanks. Is it a short way of writing class a implements ActionListener ?

But how can you define class into constructor?

To have a constructor use the normal way of defining a class.