Why is it running twice?

[jep9727]

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.

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.

[ComputerJy]

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.