Beginner Java Program-- cannot get key pressed to work

Question

I am starting out with Java and I have a problem. When people click "a" on the java applet, I want it to draw a yellow rectangle and if they press anything else it draws a black rectangle but nothing happens.

    import java.awt.*;
import java.applet.*;
import javax.swing.*;
import java.awt.event.*;
import java.awt.event.KeyListener;
import java.awt.event.KeyEvent;

public class guitarGame extends Applet implements ActionListener, KeyListener{

    Timer timer = new Timer (1000, this);
    String s = "";
    char a;
    int selection;

    public void keyReleased(KeyEvent ae){}

    public void keyPressed(KeyEvent ae){}

    public void keyTyped(KeyEvent ae){
        a = ae.getKeyChar();
    }

    public void init(){
        addKeyListener(this);
    }

    public void actionPerformed (ActionEvent ae)
    {
        if (a == a)
        {
            selection = 1;
        }
        else{
            selection = 2;
        }
    }

    public void paint (Graphics g)
    {
        if (selection == 1){
            g.setColor(Color.YELLOW);
            g.fillRect(100,100,100,100);
        }
        if (selection == 2){
            g.setColor(Color.YELLOW);
            g.fillRect(100,100,100,100);
        }
        repaint();
    }
}

Any help?

Should (a==a) be (a=='a')?

Hayden
– Hayden

2013-04-29 23:11:56 +00:00
Commented Apr 29, 2013 at 23:11 — Hayden
– Hayden, Commented Apr 29, 2013 at 23:11

Reimeus · Accepted Answer · 2013-04-30 01:42:41Z

1

You are not doing anything with your KeyListener methods. Your ActionListener will never be called as it hasnt been registered with any component.

public void keyPressed(KeyEvent ae){
   char keyChar = ae.getKeyChar();
   if (keyChar == 'a' ) {
      selection = 1;
   } else {
      selection = 2;
   }

   repaint();
}

Some suggestions:

Don't call repaint inside paint - This will cause the latter to loop indefinitely
Use enums rather than magic numbers for paint selection.
Call super.paint(g)
Consider using Swing which has much better performance over the old heavyweight AWT.

edited Apr 30, 2013 at 1:42

answered Apr 29, 2013 at 23:12

Reimeus

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1 Comment

user2333960 Over a year ago

Thank you very much! I am starting out with Java -- thank you for the tips, the applet compiles and runs fine now

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