Repaint af speedometer

package april;

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.GridLayout;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class SpeedometerDemo extends JFrame {
    private static final long serialVersionUID = 1L;
    public static class Speedometer extends JPanel {
        private static final long serialVersionUID = 1L;
        private int dim;
        private double maxspeed;
        private double speed;
        public Speedometer(int dim, int maxspeed) {
            this.dim = dim;
            this.maxspeed = maxspeed;
            speed = 0;
            setPreferredSize(new Dimension(2*dim, dim));
        }
        @Override
        public void paintComponent(Graphics g) {
            int x1 = dim;
            int y1 = dim - 10;
            double a = (maxspeed - speed) / maxspeed * Math.PI;
            int x2 = (int) Math.round(x1 + dim * 0.95 * Math.cos(a));
            int y2 = (int) Math.round(y1 - dim * 0.95 * Math.sin(a));
            g.drawLine(x1, y1, x2, y2);
        }
        public void setSpeed(double speed) {
            this.speed = speed;
            getParent().repaint(); // this.repaint() messes up things
        }
        public void changeSpeed(double speedchange) {
            this.speed += speedchange;
            getParent().repaint(); // this.repaint() messes up things
        }
    }
    private Speedometer speed;
    public SpeedometerDemo() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setTitle("Speedometer");
        getContentPane().setLayout(new BorderLayout());
        speed = new Speedometer(400, 10);
        getContentPane().add(speed, BorderLayout.CENTER);
        // start simulate how to change speed
        JPanel btns = new JPanel();
        btns.setLayout(new GridLayout(1, 2));
        JButton down = new JButton("-");
        down.addActionListener((evt) -> speed.changeSpeed(-1));
        btns.add(down);
        JButton up = new JButton("+");
        up.addActionListener((evt) -> speed.changeSpeed(1));
        btns.add(up);
        getContentPane().add(btns, BorderLayout.SOUTH);
        // end simulate
        pack();
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                JFrame f = new SpeedometerDemo();
                f.setVisible(true);
            }
        });
    }
}