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.