modifying all of the instances in an array at once

[numballs]

okay, here's the deal. I'm still quite fresh to programming. My task is to create a program, that can draw any number of balls, and then make them bounce across the canvas. The part of the task I am stuck on is converting convert a method that draws two balls to a canvass and makes them bounce, into a method that draws an number of balls and makes them bounce.

My problem, is that I can get any specified number of balls drawn, but I cant get them to all move at once..here is the code for just making two balls bounce:

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public void bounce()
    {
        int ground = 400;   // position of the ground line

        myCanvas.setVisible(true);

        // draw the ground
        myCanvas.drawLine(50, ground, 550, ground);

        // crate and show the balls
        BouncingBall ball = new BouncingBall(50, 50, 16, Color.blue, ground, myCanvas);
        ball.draw();
        BouncingBall ball2 = new BouncingBall(70, 80, 20, Color.red, ground, myCanvas);
        ball2.draw();

        // make them bounce
        boolean finished =  false;
        while(!finished) {
            myCanvas.wait(50);           // small delay
            ball.move();
            ball2.move();
            // stop once ball has travelled a certain distance on x axis
            if(ball.getXPosition() >= 550 && ball2.getXPosition() >= 550)
                finished = true;
        }
        ball.erase();
        ball2.erase();
    }

this works fine...so far I have ammended the code to this....

public void bounce(int numb)

{
        int ball;
        int ground = 400;   // position of the ground line

        myCanvas.setVisible(true);

        // draw the ground
        myCanvas.drawLine(50, ground, 550, ground);

        // crate and show the balls
           int xpos = 20;

       allBalls = new BouncingBall[numb];
  for (ball = 0; allBalls.length < numb; ball++)
       {    
           allBalls[ball] = new BouncingBall(xpos, 20, 16, Color.blue, ground, myCanvas);
           xpos = (xpos + 20);
           allBalls[ball].draw();}
           myCanvas.wait(50);

 for (ball = 0; ball < numb; ball++)
       {   allBalls[ball].move();     


// make them bounce
        while(allBalls[ball].getXPosition() >= 550) 
        {
            allBalls[ball].erase();

        }}
}
}

and the all I can get is nullpointer exception... I know that a null pointer is normally thrown when objects haven't been initialized, but this is twisting my melon a bit. I did have it drawing the blls, without moving them, but i started getting outofbounds exceptions so I messed with it a bit more, and now this. can somebody please help me? COming to a forum is a last resort after having spentabout a week on the projct as a whole, but then this one task has so far taken 2 days to try to figure out.

many thanks in advance