JPanel repaint() not working

Question

I have searched few several questions with similar titles to this (for example how to use jpanel with paint (or repaint)) but I can't find a reason for my code not to work. I'm a graphics with JPanels newbie so my first attempt at this sort of thing is to make a triangle that one can move with the arrow keys. I got the triangle drawn fine, with no problems. However, when I tried to move the triangle with the arrows, it didn't move. Can anyone find why it won't repaint?

JPanel code (I also have a separate class for the JFrame itself which only makes the frame and adds the panel, if you need to see that tell me and I'll edit it in):

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class Drawpanel extends JPanel implements KeyListener{

    int[] xcoords = {-20,0,20};//Triangle x coordinates
    int[] ycoords = {20,-20,20};//Triangle y coordinates
    int x = 100;//X coordinate for drawing the triangle
    int y = 100;//Y coordinate for drawing the triangle
    Graphics2D g2d;
    Polygon tri = new Polygon(xcoords,ycoords,xcoords.length);//triangle

    public void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        g2d = (Graphics2D)g;
        g2d.translate(x,y);//draw triangle at x and y coordinates
        g2d.setColor(new Color(234,22,72));
        g2d.fill(tri);
    }
    public void keyTyped(KeyEvent k){}
    public void keyReleased(KeyEvent k){}
    public void keyPressed(KeyEvent k)
    {
        int keycode = k.getKeyCode();
        switch(keycode)
        {
        case KeyEvent.VK_DOWN://if down arrow pressed, decrease ship y coordinate by 10
            y -= 10;
        case KeyEvent.VK_UP:
            y += 10;
        case KeyEvent.VK_LEFT:
            x -= 10;
        case KeyEvent.VK_RIGHT:
            x += 10;
        }   
        repaint();//screen doesn't repaint
    }   

}

Answer 1

Don't use a KeyListener. Swing was designed to be used with Key Bindings.

The probable problem is that the panel doesn't have focus and doesn't respond to the KeyEvents. See Motion Using the Keyboard for more information and a solution using Key Bindings.

Another possible problem is that you don't override the getPreferredSize() method of your panel to return an appropriate Dimension, so the size is 0 so Swing thinks there is nothing to paint.