Moving element by KeyListener in Swing

Question

I have problem with KeyListener, I've been searching solution but it should be working. Unfortunately, it doesn't and i have no idea why. When i type up arrow, nothing happens. I was reading about "Focus", but I don't know how this works, maybe you can give me some example, unnecessary to my problem.

public class Trawa extends JPanel implements KeyListener {
int wysokosc = 200;
public Trawa(){
    addKeyListener(this);
    setSize(200,600);
    setBackground(Color.GREEN);
}
public void paint(Graphics g) {
            super.paint(g);
            Graphics2D g2d = (Graphics2D) g;

            Color c1 = new Color(36,217,36);
            g2d.setColor(c1);
            g2d.fillRect(10, wysokosc, 100, 100);
            c1 = new Color(0,0,0);
            g2d.setColor(c1);
            g2d.fillRect(10, wysokosc, 30, 30);
            g2d.fillRect(80, wysokosc, 30, 30);
            c1 = new Color(252,3,0);
            g2d.setColor(c1);
            g2d.fillRect(40, wysokosc+60 ,30,30);
}
@Override
public void keyPressed(KeyEvent arg0) {
    int key = arg0.getKeyCode();
    if(key == KeyEvent.VK_UP)
        wysokosc-=100;
    repaint();  
}
@Override
public void keyReleased(KeyEvent arg0) {


}
@Override
public void keyTyped(KeyEvent arg0) {
    // TODO Auto-generated method stub

}
}

Here is my main class with frame. Maybe the problem is here.

public class GUI extends JFrame{
public GUI(){
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setResizable(false);
    add(new Trawa());
    setLayout(new GridLayout(1,5));
    pack();
    setSize(1300, 600);
    setLocation(40, 100);
    setVisible(true);
}

public static void main(String[] args) {
    new GUI();
  }
}

You can also attach a key listener to your entire Frame: http://stackoverflow.com/a/1379517/1003886 — kajacx, Jun 11 '14 at 14:18

score 2 · Answer 1 · answered Jun 11 '14 at 14:16

2

You have to understand how focus works before using a KeyListener.

Try calling setFocusable(true) from your JPanel's constructor.

answered Jun 11 '14 at 14:16

Kevin Workman

41,537
9
68
107

Shailesh Aswal · Accepted Answer · 2014-06-11T14:42:47.957

0

You have done everything correctly, but you forgot to bind the key listener with your frame.

try this version of you GUI class

class GUI extends JFrame {
    public GUI() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setResizable(false);
        Trawa t = new Trawa();//this is a JPanel as well as a KeyListner
        add(t);
        this.addKeyListener(t);
        setLayout(new GridLayout(1, 5));
        pack();
        setSize(500, 400);
        setLocation(40, 100);
        setVisible(true);

    }
}

as pointed by Kevin: the minimal change required is to set focus on the panel

public Trawa() {
    addKeyListener(this);
    setSize(200, 600);
    setBackground(Color.GREEN);     
    setFocusable(true);     
}

edited Jun 11 '14 at 14:42

answered Jun 11 '14 at 14:20

Shailesh Aswal

6,632
1
20
27

1

All this does is move the KeyListener to the JFrame instead of the JPanel. Instead, you should call setFocusable(true) inside the JPanel so it receives the focus. – Kevin Workman Jun 11 '14 at 14:27
correctly said, this works because the frame already has focus on it. Appreciate your comment. – Shailesh Aswal Jun 11 '14 at 14:41

Moving element by KeyListener in Swing

2 Answers2