Thread: Help with code- horizontal to vertical bouncing balls

Hi all, I'm new to java and I need help changing the direction of these balls from a horizontal axis to a vertical axis bouncing in the center. I've tried reversing the integers but there seems to be something i've missed.

 package package_bouncingball;
 
import java.applet.*; 
import java.awt.*; 
import javax.swing.*;
import javax.swing.event.*;
 
public class Class_bouncingball extends Applet implements Runnable 
 
{
 
 
  int x_pos1 = 150; 
  int y_pos1 = 200; 
  int radius1 = 20; 
 
  int x_pos2 = 250; 
  int y_pos2 = 200; 
  int radius2 = 20;
 
  private float ballspeedx1 = -3;   
  private float ballspeedy1 = 0;
 
  private float ballspeedx2 = 3;   
  private float ballspeedy2 = 0; 
 
 
  public void init() {} 
 
  public void start() { 
  Thread th = new Thread (this); 
   th.start (); } 
   public void stop() {} 
  public void destroy() {} 
  public void run () {
Thread.currentThread().setPriority(Thread.MIN_PRIORITY); 
while (true) 
{ 
  x_pos1 += ballspeedx1;
  y_pos1 += ballspeedy1;
  x_pos2 += ballspeedx2;
  y_pos2 += ballspeedy2;  
 
 
  repaint();
 
  if (x_pos1  < 100) {
    ballspeedx1 = -ballspeedx1; 
    x_pos1 = 100; 
  } 
  if (x_pos2  < 100) {
    ballspeedx2 = -ballspeedx2; 
    x_pos2 = 100; 
  }  
 
  if (x_pos1  > 300) {
    ballspeedx1 = -ballspeedx1; 
    x_pos1 = 300; 
  } 
  if (x_pos2  > 300) {
    ballspeedx2 = -ballspeedx2; 
    x_pos2 = 300; 
  }                                       
 
if (Math.abs(x_pos2-x_pos1)<radius1+radius2){
    ballspeedx1 = -ballspeedx1;
    ballspeedx2 = -ballspeedx2;
  }
 
  try { Thread.sleep (20); } 
 
 
 
  catch (InterruptedException ex) {} 
 
Thread.currentThread().setPriority(Thread.MAX_PRIORITY); }} 
 
public void paint (Graphics g) {
 
g.setColor (Color.red); 
g.fillOval (x_pos1 - radius1, y_pos1 - radius1, 2 * radius1, 2 * radius1); 
 
g.setColor (Color.blue); 
g.fillOval (x_pos2 - radius2, y_pos2 - radius2, 2 * radius2, 2 * radius2); 
 
 
g.setColor(Color.black);
g.drawLine(80,80,80,320); // left
g.drawLine(320,80,320,320); // right
g.drawLine(80,80,320,80);  // 
g.drawLine(80,320,320,320); // 
  }
 
}

Quick question. What happens to a number when you multiply it by -1?