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Thread: Error on KeyListener

  1. #1
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    Default Error on KeyListener

    Hi,

    i'm starting studing java, i have found a tutorial on youtube where explain to intercept pressions from Keyboard. I try to do this but seems that's something wrong (made by me) somewhere but i can't find the problem..
    this is the code:

     
     
     
    import java.awt.Graphics;
    import java.awt.event.KeyAdapter;
    import java.awt.event.KeyEvent;
     
    import javax.swing.JFrame;
     
     
    public class JavaGame extends JFrame {
     
    	int x;
    	int y;
     
    	public class AL extends KeyAdapter{
    		public void KeyPressed(KeyEvent e){
    			int KeyCode = e.getKeyCode();
     
    			if (KeyCode == e.VK_UP){
    				y--;
    				System.out.println(y);
    			}
     
    			if (KeyCode == e.VK_DOWN){
    				y++;
    				System.out.println(y);
    			}
     
    			if (KeyCode == e.VK_LEFT){
    				x--;
    				System.out.println(x);
    			}
     
    			if (KeyCode == e.VK_RIGHT){
    				x++;
    				System.out.println(x);
    			}
     
    		}
     
    		public void KeyReleased(KeyEvent e){
     
    		}
     
    		public void paint (Graphics g){
    			g.fillOval(x, y, 15, 15);
     
    			repaint();
    		}
    	}
     
    	public JavaGame(){
    		addKeyListener(new AL());
    		setTitle("JavaGame");
    		setSize(400,400);
     
    		setResizable(false);
    		setVisible(true);
    		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     
    		x = 150;
    		y = 150;
    	}
     
     
    	public static void main(String[] args) {
     
    		new JavaGame();
     
    	}
     
    }

    with this code i make a window, then with keybord i should paint on this window an oval that follow up down left right buttons on keyboard.
    when running, the code write the window, but there isn't interception of keys...


  2. #2
    Super Moderator pbrockway2's Avatar
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    Default Re: Error on KeyListener

    You should call repaint() in the event handler because that is where you know the repaint is needed. And not in the painting code.

    You should also use standard Java coding conventions and start all methods and variables with a lowercase letter. This makes the code more readable and will help avoid problems. KeyAdapter has a method keyPressed() that will be called when a key is pressed, not KeyPressed().

  3. The Following User Says Thank You to pbrockway2 For This Useful Post:

    Colino (January 9th, 2012)

  4. #3
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    Default Re: Error on KeyListener

    many thanks, now with lower case seems to intercept keys but still doesn't paint... not enter in paint method

  5. #4
    Super Moderator Norm's Avatar
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    Default Re: Error on KeyListener

    Add a println statement to the paint method to see if it is being called.
    What color will the fillOval() use?

  6. #5
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    Default Re: Error on KeyListener

    Quote Originally Posted by Norm View Post
    Add a println statement to the paint method to see if it is being called.
    What color will the fillOval() use?
    hi,
    i have already done with println but seems that there isn't the call to that method...
    i tried the same code with eclipse into ubuntu and xp , in ubuntu it is printed black, in xp the oval not appear..

  7. #6
    Super Moderator Norm's Avatar
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    Default Re: Error on KeyListener

    there isn't the call to that method...
    That's strange. There should be calls to the paint() method.
    Try adding an @Override statement just before the paint method to see if the compiler thinks that your paint mehtod is an override or not.
    in ubuntu it is printed black,
    This is confusing, if the method is not called, then how is the oval drawn?

  8. #7
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    Default Re: Error on KeyListener

    i noticed another thing

    this is the code:
     
     
    import java.awt.*;
    import java.awt.Graphics;
    import java.awt.event.KeyAdapter;
    import java.awt.event.KeyEvent;
     
    import javax.swing.JFrame;
     
    public class JavaGame extends JFrame {
     
    	int x;
    	int y;
     
     
    	public class AL extends KeyAdapter{
    		public void keyPressed(KeyEvent e){
    			int keyCode = e.getKeyCode();
     
    			if (keyCode == e.VK_UP){
    				y--;
     
    				System.out.println(y);
    			}
     
    			if (keyCode == e.VK_DOWN){
    				y++;
     
    				System.out.println(y);
    			}
     
    			if (keyCode == e.VK_LEFT){
    				x--;
    				System.out.println(x);
    			}
     
    			if (keyCode == e.VK_RIGHT){
    				x++;
    				System.out.println(x);
     
    			}
     
     
    		}
     
    		public void keyReleased(KeyEvent e){
     
    		}
     
     
     
    		public void paint(Graphics g){
     
    			System.out.println("I\'m here");
    			g.fillOval(x, y, 15, 15);
    			repaint();
     
    		}
     
    	}
     
    	public JavaGame(){
    		addKeyListener(new AL());
    		setTitle("JavaGame");
    		setSize(400,400);
    		setBackground(new Color(255,155,0));
    		setResizable(false);
    		setVisible(true);
    		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     
    		x = 150;
    		y = 150;
     
    	}
     
     
    	public static void main(String[] args) {
     
    		new JavaGame();
     
    	}
     
    }

    the statemant
    setBackground(new Color(255,155,0));
    doesn't make the background of that color...
    and
    System.out.println("I\'m here");
    isn't printed in console..

  9. #8
    Super Moderator Norm's Avatar
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    Default Re: Error on KeyListener

    Did you add the @Override statement to ask the compiler to check your code?

  10. #9
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    Default Re: Error on KeyListener

    i put @Override on paint mathod and eclipse show this error msg
    "The method paint(Graphics) of type JavaGame.AL must override or implement a supertype method"

  11. #10
    Super Moderator Norm's Avatar
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    Default Re: Error on KeyListener

    That says your paint method is not overriding a method in the AL class.
    Does the AL class have a paint method that you can override?

  12. The Following User Says Thank You to Norm For This Useful Post:

    Colino (January 9th, 2012)

  13. #11
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    Default Re: Error on KeyListener

    i put paint method outside AL class, now works..
    many thanks norm, for the help!

  14. #12
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    Default Re: Error on KeyListener

    I think you have to implements the KeyListener interface..

  15. #13
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    Default Re: Error on KeyListener

    Quote Originally Posted by RaoPatek View Post
    I think you have to implements the KeyListener interface..
    i tried but eclipse underline it in yellow and tells me that isn't used.. and so i delete that implement

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