title: How to keep a method running continuously in background until the program ends?
body: <p>I want to know, how to keep a method running in the background. ie. this method starts when the program starts and keeps executing its statements until the program is closed.
  For ex. lets say i have a method "gravity()" which continuously decrements a certain value while the program is running. 
  Right now for trying this out im using the following program, wherein im trying to pull the duke down (gravity) when no keys are pressed. but is not happening.</p>
  
  <pre><code>    import java.awt.*;
  import java.awt.event.*;
  import java.awt.image.BufferedImage;
  import java.io.IOException;
  import java.net.*;
  import java.util.HashMap;
  import java.util.Map;
  import javax.imageio.ImageIO;
  import javax.swing.*;
  
  public class MoveIcon extends JPanel {
  
      private static final long serialVersionUID = 1L;
      private static final String IMAGE_PATH = "http://duke.kenai.com/misc/Bullfight.jpg";
      private static final String IMAGE_PATH_PLAYER = "http://duke.kenai.com/iconSized/duke4.gif";
      public static final int STEP = 3;
      private static final int TIMER_DELAY = STEP * 8;
      private BufferedImage bkgrndImage = null;
      private BufferedImage playerImage = null;
      private Map&lt;Direction, Boolean&gt; directionMap = new HashMap&lt;Direction, Boolean&gt;();
      private int playerX = 0;
      private int playerY = 0;
  
      enum Direction {
  
          UP(KeyEvent.VK_UP, 0, -1), DOWN(KeyEvent.VK_DOWN, 0, 1),
          LEFT(KeyEvent.VK_LEFT, -1, 0), RIGHT(KeyEvent.VK_RIGHT, 1, 0);
          private int keyCode;
          private int xDirection;
          private int yDirection;
  
          private Direction(int keyCode, int xDirection, int yDirection) {
              this.keyCode = keyCode;
              this.xDirection = xDirection;
              this.yDirection = yDirection;
          }
  
          public int getKeyCode() {
              return keyCode;
          }
  
          public int getXDirection() {
              return xDirection;
          }
  
          public int getYDirection() {
              return yDirection;
          }
      }
  
      public MoveIcon() {
          try {
              URL bkgrdImageURL = new URL(IMAGE_PATH);
              URL playerImageURL = new URL(IMAGE_PATH_PLAYER);
              bkgrndImage = ImageIO.read(bkgrdImageURL);
              playerImage = ImageIO.read(playerImageURL);
              setPreferredSize(new Dimension(bkgrndImage.getWidth(), bkgrndImage.getHeight()));
          } catch (MalformedURLException e) {
              e.printStackTrace();
          } catch (IOException e) {
              e.printStackTrace();
          }
          for (Direction direction : Direction.values()) {
              directionMap.put(direction, false);
          }
          setKeyBindings();
          Timer timer = new Timer(TIMER_DELAY, new TimerListener());
          timer.start();
      }
  
      private void setKeyBindings() {
          InputMap inMap = getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW);
          ActionMap actMap = getActionMap();
          for (final Direction direction : Direction.values()) {
              KeyStroke pressed = KeyStroke.getKeyStroke(direction.getKeyCode(), 0, false);
              KeyStroke released = KeyStroke.getKeyStroke(direction.getKeyCode(), 0, true);
              inMap.put(pressed, direction.toString() + "pressed");
              inMap.put(released, direction.toString() + "released");
              actMap.put(direction.toString() + "pressed", new AbstractAction() {
  
                  private static final long serialVersionUID = 1L;
  
                  @Override
                  public void actionPerformed(ActionEvent e) {
                      directionMap.put(direction, true);
                  }
              });
              actMap.put(direction.toString() + "released", new AbstractAction() {
  
                  private static final long serialVersionUID = 1L;
  
                  @Override
                  public void actionPerformed(ActionEvent e) {
                      directionMap.put(direction, false);
                  }
              });
          }
  
      }
  
      @Override
      protected void paintComponent(Graphics g) {
          super.paintComponent(g);
          if (bkgrndImage != null) {
              g.drawImage(bkgrndImage, 0, 0, null);
          }
          if (playerImage != null) {
              g.drawImage(playerImage, playerX, playerY, null);
          }
      }
  
      private class TimerListener implements ActionListener {
  
          @Override
          public void actionPerformed(ActionEvent e) {
              boolean moved = false;
              for (Direction direction : Direction.values()) {
                  if (directionMap.get(direction)) {
                      playerX += STEP * direction.getXDirection();
                      playerY += STEP * direction.getYDirection();
                      moved = true;
                  }
              }
              if (moved) {
                  int x = playerX - 2 * STEP;
                  int y = playerY - 2 * STEP;
                  int w = playerImage.getWidth() + 4 * STEP;
                  int h = playerImage.getHeight() + 4 * STEP;
                  MoveIcon.this.repaint(x, y, w, h); // !! repaint just the player
              }
          }
      }
  
      private static void createAndShowUI() {
          JFrame frame = new JFrame("MoveIcon");
          frame.getContentPane().add(new MoveIcon());
          frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          frame.pack();
          frame.setLocationRelativeTo(null);
          frame.setVisible(true);
      }
  
      public static void main(String[] args) {
          java.awt.EventQueue.invokeLater(new Runnable() {
  
              @Override
              public void run() {
                  createAndShowUI();
              }
          });
      }
  }
  </code></pre>
  
  <p>Thank you.</p>