Drawing in JPanel vs JComponent

Question

I need some help understanding why the drawing works differently in JComponent vs JPanel.

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;

import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Particle extends JComponent implements Runnable{
    private int x = 45;
    private int y = 45;
    private int cx;
    private int cy;
    private int size;
    private Color color;
    private JFrame frame;

    public Color getColor(){
        return color = new Color(100,0,190);
    }

    public Particle(){
        frame = new JFrame();
        frame.setSize(400, 400);
        frame.setResizable(false);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(this);
        frame.setVisible(true);
    }

    public void update(){
        x+=1;
        y+=1;
    }

    public void paintComponent(Graphics g){
        Graphics2D g2d = (Graphics2D) g.create();
        g2d.setColor(getColor());

        g2d.fillRect(x, y, 4, 4);
    }

    public void startThread(){
        Thread thread = new Thread(this);
        thread.start();
    }

    @Override
    public void run() {
        for(int i = 0; i <= 200; i++){
            try{
                update();
                repaint();
                Thread.sleep(4);    
            }catch(Exception e){
                System.out.print("Exception at thread.start()");
            }
        }
    }

    public static void main(String[] args) {
        Particle particle = new Particle();
        particle.startThread();
    }
}

In this above example, the "particle" moves from point A to point B just fine..

But when I subclass Particle from JComponent to JPanel..

the drawing forms a line.. i.e. the rectangle never disappears from where it starts..

Why is this the case?

Answer 1

Call super.paintComponent(g) in paintComponent implementation.

public void paintComponent(Graphics g) {
  super.paintComponent(g);

  Graphics2D g2d = (Graphics2D) g.create();
  g2d.setColor(getColor());
  g2d.fillRect(x, y, 4, 4);

}