title: Drawing in JPanel vs JComponent
body: <p>I need some help understanding why the drawing works differently in JComponent vs JPanel.</p>
  
  <pre><code>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 &lt;= 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();
      }
  }
  </code></pre>
  
  <p>In this above example, the "particle" moves from point A to point B just fine.. </p>
  
  <p>But when I subclass Particle from JComponent to JPanel.. </p>
  
  <p>the drawing forms a line.. i.e. the rectangle never disappears from where it starts.. </p>
  
  <p>Why is this the case?</p>