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Thread: Why must declare all methods for abstract class.

  1. #1
    hqt is offline Member
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    Nov 2011
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    Question Why must declare all methods for abstract class.

    I want to add KeyboardPressed Event for my program. So here is my code:
    Java Code:
    public class Example extends Applet implements KeyListener{
        public void init(){
    /*Key Handle*/
        public void keyReleased(KeyEvent k){}
        public void keyTyped(KeyEvent k){}
        public void keyPressed(KeyEvent k){
            //Some code here 
    My question is: unlike when I program on .NET, in Java, when you want to add a KeyboardEvent, you must declare all of its method. (keyReleased, keyTyped, keyPressed) although I just want keyPressed do some works for me.
    maybe this make me hard to remember all (if I don't use Java IDE that can auto generate code for me)

    thanks :)

  2. #2
    DarrylBurke's Avatar
    DarrylBurke is offline Forum Police
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    Madgaon, Goa, India
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    Default Re: Why must declare all methods for abstract class.

    First, your topic title is wrong. The methods are declared in the interface (not Abstract class) and are required to be implemented in the concrete class.

    Implementing an interface in a class is often a bad design choice. In your example, the Example class is-a-Applet, but there's no reason it should be a KeyListener. Rather, it uses a KeyListener for its internal workings.

    The single line of code in your init() method is also wrong and won't compile. There's no Applet method addKeyListener() without a parameter.

    I would use a KeyListener declared as a field, local variable or directly in the add... method, extending KeyAdapter -- which has empty implementations of all KeyListener methods. For the last, the code would look something like
    Java Code:
        public void init(){
            addKeyListener(new KeyAdapter() {
                public void keyPressed(KeyEvent k){
                    //Some code here 
    That said, for listening for keystrokes in an Applet, you'd be better off using Swing (JApplet, not Applet, with all Swing components added) and Key Bindings.

    hqt likes this.
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