Why is it running twice?

I'm working on a homework that probably everyone that has taken java ends up doing. I have to get a program to draw 10 triangles. The problem I have is that my program is drawing 10 triangles. Then putting up a black background and then drawing 10. If I don't sleep the thread you cannot tell that the first 10 were ever drawn, but it bugs me and I cannot understand why it is doing this.

Code:

---

import java.awt.*;
import java.awt.geom.*;
import java.util.Random;
import javax.swing.*;


public class HW1a extends JFrame {
    int i,z;
    int x[] = new int[3];
    int y[] = new int[3];
    Random rand = new Random();
 
  public HW1a()
  {
      super( "Drawing Triangles" );

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

 
  public void paint( Graphics g )
  {
     
      super.paint( g ); 
      getContentPane().setBackground(Color.BLACK);
      Graphics2D g2d = ( Graphics2D ) g;

      for(z=0; z<10; z++){  //needs to have 10 triangles
     
      GeneralPath triangle = new GeneralPath(); 
   


      for (i=0; i<3; i++) {
        x[i] = rand.nextInt(600);
        y[i] = rand.nextInt(600);
      }

     
      triangle.moveTo( x[ 0 ], y[ 0 ] );

     
      for ( int count = 1; count < x.length; count++ )
        triangle.lineTo( x[ count ], y[ count ] );

      triangle.closePath(); 

   
      g2d.setColor( new Color(rand.nextInt(256),rand.nextInt(256),rand.nextInt(256)));
      g2d.fill( triangle ); 
      try
      {
      // sleep for 50 ms.
      Thread.sleep( 50 );
      }
      catch ( Exception e )
      {}

     
     
      }
  }

 
  public static void main(String[] args)
  {
      HW1a application = new HW1a();
      application.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
  }

}

---

Thank you in advance for any explanations.

Code:

---

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.GeneralPath;
import java.util.Random;
import javax.swing.JFrame;

public class Test extends JFrame
{
        private static final long serialVersionUID = 3864411708478015289L;
        int i, z;
        int x[] = new int[3];
        int y[] = new int[3];
        Random rand = new Random();

        public Test()
        {
                super("Drawing Triangles");
                setSize(600, 600);
                setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                setVisible(true);
        }

        @Override
        public void paint(Graphics g)
        {
                super.paint(g);
                g.setColor(Color.BLACK);
                g.fillRect(0, 0, this.getWidth(), this.getHeight());
                Graphics2D g2d = (Graphics2D) g;
                for (z = 0; z < 10; z++)
                {  //needs to have 10 triangles

                        GeneralPath triangle = new GeneralPath();
                        for (i = 0; i < 3; i++)
                        {
                                x[i] = rand.nextInt(600);
                                y[i] = rand.nextInt(600);
                        }
                        triangle.moveTo(x[ 0], y[ 0]);
                        for (int count = 1; count < x.length; count++)
                                triangle.lineTo(x[count], y[count]);
                        triangle.closePath();
                        g2d.setColor(new Color(rand.nextInt(256), rand.nextInt(256), rand.nextInt(256)));
                        g2d.fill(triangle);
                        try
                        {
                                // sleep for 50 ms.
                                Thread.sleep(50);
                        } catch (Exception e)
                        {
                        }
                }
        }

        public static void main(String[] args)
        {
                new Test();
        }
}

---

The "getContentPane().setBackground(Color.BLACK);" Causes the content pane to refresh with a black background after the painting of the JFrame is done.