Java square not moving

Question

I am creating a program in Java where you move a square. However, the square does not move. I have tried a lot of things, but none of them work. It seems to be a problem with repaint. How could I solve this?

package movingSquare;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.WindowConstants;

public class MovingSquare extends JComponent {
    private static final long serialVersionUID = -3778627464016140311L;
    public static JFrame f = new JFrame("Moving Square");
    public static int x;
    public static int y;
    public static Rectangle r = new Rectangle(x, y, 20, 20);
    public static MovingSquare mv = new MovingSquare();
    public static KeyListener kl = new KeyListener() {
        @Override
        public void keyPressed(KeyEvent arg0) {
            // TODO Auto-generated method stub
            if(arg0.getKeyCode() == KeyEvent.VK_UP) {
                y += 1;
                r.setLocation(x, y);
                mv.repaint(r);
            }
            if(arg0.getKeyCode() == KeyEvent.VK_DOWN) {
                y -= 1;
                r.setLocation(x, y);
                mv.repaint();
            }
            if(arg0.getKeyCode() == KeyEvent.VK_LEFT) {
                x -= 1;
                r.setLocation(x, y);
                mv.repaint();
            }
            if(arg0.getKeyCode() == KeyEvent.VK_RIGHT) {
                x += 1;
                r.setLocation(x, y);
                mv.repaint();
            }
        }
        @Override
        public void keyReleased(KeyEvent arg0) {
            // TODO Auto-generated method stub

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

        }};
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        f.setBackground(Color.BLUE);
        f.setMinimumSize(new Dimension(720, 720));
        f.setResizable(false);
        f.setFocusable(true);
        f.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        f.addKeyListener(kl);
        f.add(new MovingSquare());
        f.pack();
        f.setVisible(true);
        boolean e = true;
        while(e) {
            System.out.println("x: " + x + " y: " + y);
        }
    }
    public void paint(Graphics g) {
        super.paint(g);
        Graphics2D g1 = (Graphics2D) g;
        g1.setColor(Color.BLUE);
        g1.fillRect(0, 0, 720, 720);
        g1.setColor(Color.RED);
        g1.fill(r);
    }
}

Answer 1

First of all custom painting is done by overriding paintComponent() not paint().

Get rid of all the static variables. The reason you create custom class is so you can define variables that contains the properties of the class.

There is no need for the frame variable as part of the class. The component only needs to know about itself, not the frame it belongs to.

f.add(new MovingSquare());

First you create a MovingSquare component and add it to the frame.

public static MovingSquare mv = new MovingSquare();

But then you create another MovingSquare component, but never add it do the frame so you will never be able to paint it.

Get rid of the above statement it is not needed since your class is already a MovingSquare.

mv.repaint();

As I said above that statement does nothing since that component is never added to the frame.

Instead the code should be:

repaint();

That is all you need to cause a component to repaint itself.

If you do use a KeyListener, then the KeyListener should be added to the component itself, not the frame. The component should be responsible for managing its own state.

So basically your design is wrong and needs to be fixed.

However, even that may not solve the problem as only the component with focus will receive events and as suggested in the comment you should be using Key Bindings.

So check out Motion Using the Keyboard for working examples to demonstrated the difference between KeyEvents and Key Bindings.