processing4/java/examples/Topics/Motion/Bounce/applet/Bounce.java

import processing.core.*; 

import java.applet.*; 
import java.awt.*; 
import java.awt.image.*; 
import java.awt.event.*; 
import java.io.*; 
import java.net.*; 
import java.text.*; 
import java.util.*; 
import java.util.zip.*; 
import java.util.regex.*; 

public class Bounce extends PApplet {

/**
 * Bounce. 
 * 
 * When the shape hits the edge of the window, it reverses its direction. 
 */
 
int size = 60;       // Width of the shape
float xpos, ypos;    // Starting position of shape    

float xspeed = 2.8f;  // Speed of the shape
float yspeed = 2.2f;  // Speed of the shape

int xdirection = 1;  // Left or Right
int ydirection = 1;  // Top to Bottom


public void setup() 
{
  size(640, 200);
  noStroke();
  frameRate(30);
  smooth();
  // Set the starting position of the shape
  xpos = width/2;
  ypos = height/2;
}

public void draw() 
{
  background(102);
  
  // Update the position of the shape
  xpos = xpos + ( xspeed * xdirection );
  ypos = ypos + ( yspeed * ydirection );
  
  // Test to see if the shape exceeds the boundaries of the screen
  // If it does, reverse its direction by multiplying by -1
  if (xpos > width-size || xpos < 0) {
    xdirection *= -1;
  }
  if (ypos > height-size || ypos < 0) {
    ydirection *= -1;
  }

  // Draw the shape
  ellipse(xpos+size/2, ypos+size/2, size, size);
}

  static public void main(String args[]) {
    PApplet.main(new String[] { "Bounce" });
  }
}