int numCircles = 60;            // number of circles

Circle[] theCircles = new Circle[numCircles];

void setup()
{
  size(500,400);
  smooth();                     // anti-alias
  noStroke();                   // no outlines
  frameRate(30);
  
  for (int i=0; i < numCircles; i++) 
  {
    theCircles[i] = new Circle();
  }
  
}

void draw()
{
  background(255);
  
  for (int i=0; i < numCircles; i++) 
  {
    theCircles[i].drawCircle();                      // draw the circles
  }
}


class Circle
{
 int r;
 int g;
 int b;
 int xpos;
 int ypos;
 int xdir;
 int ydir;
 int size;
 
 Circle()
 {
   
   r = int(random(255));                     // random colors 
   b = int(random(255));
   g = int(random(255));
   
   xpos = int(random(width));               // random location
   ypos = int(random(height));
   xdir = int(random(1,7));                 // random velocities
   ydir = int(random(1,7));
   size = int(random(30,90));               // random diameter
 }
 
 void moveCircle()
 {
   if (xpos > width || xpos < 0)
   {
     xdir = -xdir;
   }
   
   if (ypos > height || ypos < 0)
   {
     ydir = -ydir;
   }
   
  xpos = xpos + xdir; 
  ypos = ypos + ydir; 
 }
 
 void drawCircle()
 {
   moveCircle();
   fill (r, g, b, 128);                  //   alpha not random!
   ellipse(xpos,ypos,size,size);
 }
 
}