Tengo mi programa que puede dibujar rectángulos. Tengo dos problemas que no puedo resolver. Después de dibujar el rectángulo no quedará. El único código que tengo que borra el lienzo de debajo de la pintura, se vuelve a pintar solo cuando se arrastra el mouse. ¿Por qué, cuando suelto el mouse o muevo el mouse, mi lienzo está limpio? La segunda cosa no es tanto un problema, pero algo que no puedo entender, cuando la altura o el ancho de mi rectángulo es negativo, el rectángulo se llena de negro.

package pracpapp2;


import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class MouseTracker4July extends JFrame
   implements MouseListener, MouseMotionListener {


  private static final long serialVersionUID = 1L;
  private JLabel mousePosition;
  int x, y;
  int x1, x2, y1, y2;
  int w, h;
  private JLabel recStart;
  private JLabel recStop;
  private JLabel cords;
   // set up GUI and register mouse event handlers
   public MouseTracker4July()
   {
      super( "Rectangle Drawer" );

      mousePosition = new JLabel();
      mousePosition.setHorizontalAlignment(SwingConstants.CENTER);
      getContentPane().add( mousePosition, BorderLayout.CENTER );


      JLabel text1 = new JLabel();
      text1.setText( "At the center the mouse pointer's coordinates will be displayed." );
      getContentPane().add( text1, BorderLayout.SOUTH );

      recStart = new JLabel();
      getContentPane().add(recStart, BorderLayout.WEST);

      recStop = new JLabel();
      getContentPane().add(recStop, BorderLayout.EAST);

      cords = new JLabel();
      getContentPane().add(cords, BorderLayout.NORTH);


      addMouseListener( this );        // listens for own mouse and
      addMouseMotionListener( this );  // mouse-motion events

      setSize( 800, 600 );
      setVisible( true );
   }

   // MouseListener event handlers
   // handle event when mouse released immediately after press
   public void mouseClicked( MouseEvent event )
   {
      mousePosition.setText( "Clicked at [" + event.getX() +
         ", " + event.getY() + "]" );
   }

   // handle event when mouse pressed
   public void mousePressed( MouseEvent event )
   {

      mousePosition.setText( "Pressed at [" +(x1 = event.getX()) +
         ", " + (y1 = event.getY()) + "]" );

      recStart.setText( "Start:  [" + x1 +
         ", " + y1 + "]" );
   }

   // handle event when mouse released after dragging
   public void mouseReleased( MouseEvent event )
   {
     mousePosition.setText( "Released at [" +(x2 = event.getX()) +
         ", " + (y2 = event.getY()) + "]" );

     recStop.setText( "End:  [" + x2 +
         ", " + y2 + "]" );

   }

   // handle event when mouse enters area
   public void mouseEntered( MouseEvent event )
   {
      mousePosition.setText( "Mouse entered at [" + event.getX() +
         ", " + event.getY() + "]" );
   }

   // handle event when mouse exits area
   public void mouseExited( MouseEvent event )
   {
      mousePosition.setText( "Mouse outside window" );
   }

   // MouseMotionListener event handlers
   // handle event when user drags mouse with button pressed
   public void mouseDragged( MouseEvent event )
   {
      mousePosition.setText( "Dragged at [" + (x = event.getX()) + 
         ", " + (y = event.getY()) + "]" );
      // call repaint which calls paint
      repaint();

   }

   // handle event when user moves mouse
   public void mouseMoved( MouseEvent event )
   {
      mousePosition.setText( "Moved at [" + event.getX() +
         ", " + event.getY() + "]" );
   }

   public void paint(Graphics g)
   {
      super.paint(g); // clear the frame surface
      g.drawString("Start Rec Here", x1, y1);
      g.drawString("End Rec Here", x, y);

      w = x1 - x;
      h = y1 - y;
      w = w * -1;
      h = h * -1;

      g.drawRect(x1, y1, w, h);

      cords.setText( "w = " + w + ", h = " + h);
   }

   public static void main( String args[] )
   { 
      MouseTracker4July application = new MouseTracker4July();
      application.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
   }

} // end class MouseTracker