title: How do I force a repaint in Java using Swing JPanel?
body: <p>Why won't my image repaint for a simple animation?  I call repaint() from two different methods and one causes repaint but the other doesn't.  The method that does force the repaint is generated from an event listener.  The one that doesn't is a timed animation thread.  I know the animation thread is running correctly, and as long as I continuously slide the slider, it displays perfectly.  Help please!<br>
  PS yes, I've seen many similar problems here, and I've tried validate, revalidate, and using paint vs paintComponent.  Four classes that comprise the code follow:</p>
  
  <pre><code>import javax.swing.*;
  
  public class gutz{
  
      public static int windowWidth = 640;
      public static int windowHeight = 480;
  
      public static void main(String[] args){
  
          hBod hb1 = new hBod(50, 30, 21, 111, 7, -11);   //mass, radius, xpos, ypos, xvel, yvel
          Thread t1 = new Thread(hb1);
  
          windowmakr w = new windowmakr();
          w.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          w.setSize(windowWidth, windowHeight);
          w.setVisible(true);
  
          t1.start();
      }
  }
  
  import java.awt.*;
  import javax.swing.*;
  import javax.swing.event.*;
  
  public class windowmakr extends JFrame {
  
      private JSlider slider;
      private drawr panel;
  
      public windowmakr(){
          super("Orbit Explorer");
          panel = new drawr();
          panel.setBackground(Color.BLACK);
  
          slider = new JSlider(SwingConstants.HORIZONTAL, 0, 200, 10);
          slider.setMajorTickSpacing(10);
          slider.setPaintTicks(true);
  
          slider.addChangeListener(
                  new ChangeListener(){
                      public void stateChanged(ChangeEvent e){
                          panel.setSpeed(slider.getValue());
                      }
                  }
          );
  
          add(slider, BorderLayout.SOUTH);
          add(panel, BorderLayout.CENTER);
  
      }
  
  }
  
  import java.awt.*;
  import javax.swing.*;
  
  public class drawr extends JPanel{
  
      private int diameter = 10;
      private static int rad;
      private static int xpos;
      private static int ypos;
  
      public void paintComponent(Graphics g){
          super.paintComponent(g);
          g.setColor(Color.ORANGE);
          g.fillOval((gutz.windowWidth-diameter)/2, ((gutz.windowHeight-diameter)/2)-40, diameter, diameter);
          g.setColor(Color.CYAN);
          g.fillOval(xpos, ypos, rad, rad);
  
      }
  
  /*  public void paint(Graphics g){
          super.paint(g);
          g.setColor(Color.ORANGE);
          g.fillOval((gutz.windowWidth-diameter)/2, ((gutz.windowHeight-diameter)/2)-40, diameter, diameter);
          g.setColor(Color.CYAN);
          g.fillOval(xpos, ypos, rad, rad);   
      }
  */  
      public void setSpeed(int newD){
          diameter = (newD &gt;= 0 ? newD : 10);
          repaint();
      }
  
      public void renderImage(int r, int xp, int yp){
          rad=r;
          xpos=xp;
          ypos=yp;
          repaint();
      }
  
      public Dimension getPreferredSize(){
          return new Dimension(200,200);
      }
  
      public Dimension getMinimumSize(){
          return getPreferredSize();
      }
  }
  
  public class hBod implements Runnable{
  
      private int mass;
      private int rad;
      private int xpos;
      private int ypos;
      private double xvel;
      private double yvel;
  
      public drawr render;
  
      public hBod(int m, int r, int xp, int yp, double xv, double yv){
          mass=m;
          rad=r;
          xpos=xp;
          ypos=yp;
          xvel=xv;
          yvel=yv;
  
          render = new drawr();
      }
  
      public void run(){
          try{
              while(true){
                  xpos+=xvel;
                  ypos+=yvel;
                  yvel+=1;
                  System.out.printf("rad - %d, xpos - %d, ypos - %d\n", rad, xpos, ypos);
                  render.renderImage(rad, xpos, ypos);
  
                  Thread.sleep(1000);
              }
          }catch(Exception e){}
      }
  }
  </code></pre>