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Thread: Timer class countdown

  1. #1
    Junior Member p0oint's Avatar
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    Question 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.


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: Timer class countdown

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

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    p0oint (August 25th, 2010)

  4. #3
    Junior Member p0oint's Avatar
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    Default Re: Timer class countdown

    Quote Originally Posted by Norm View Post
    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?

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    Super Moderator Norm's Avatar
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    Default Re: Timer class countdown

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

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    p0oint (August 26th, 2010)

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    Administrator copeg's Avatar
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    Default Re: Timer class countdown

    The ActionListener the code creates is called an anonymous inner class - a quick way to define a class or implement an interface, an advantage being its easy and has access to the outer class variables (via this.outerclassname.variable). Quite often the reference isn't needed, and the class defined when needed

      timer = new Timer(300, new ActionListener() {
          public void actionPerformed(ActionEvent e) {
            System.out.println("Counter = " + counter);
     
            if (++counter > 10) {
              timer.stop();
              System.exit(0);
            }
          }
        });

    The anonymous class is just that: no name class. The compiler is smart enough to treat this as a normal class, with a default constructor being called upon instantiation. If you need a constructor you may be better off defining either another class or inner class. See Inner Class Example (The Java™ Tutorials > Learning the Java Language > Classes and Objects)
    Last edited by copeg; August 25th, 2010 at 10:19 PM.

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    p0oint (August 26th, 2010)

  9. #6
    Junior Member p0oint's Avatar
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    Default Re: Timer class countdown

    Thank you. I understand now what really is.

    So it is like
    class Anonymous implements ActionListener
    {
    public void actionPerformed(ActionEvent e) {
            System.out.println("Counter = " + counter);
     
            if (++counter > 10) {
              timer.stop();
              System.exit(0);
            }
          }
        }
    }
    With definition of the function actionPerformed(ActionEvent e)

    and then new Anonymous is put into the constructor of the Timer (300,new Anonymous), right?
    Last edited by p0oint; August 26th, 2010 at 04:43 AM.

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