List shapes = new ArrayList();

void setup()
{
 size(800, 800);
 smooth();
 for(int i = 0; i < 100; i++)
 if(i % 2 == 0)
 shapes.add(new Square());
 else
 shapes.add(new Circle());
}

void draw()
{
 background(255, 255, 255);
 Iterator i = shapes.iterator();
 while(i.hasNext())
 {
 ((Shape)i.next()).drawAndStep();
 }
}

abstract class Shape
{
 float x, y, r, xdir, ydir;
 color f, s;
 Shape()
 {
 x = random(width/4, width/2);
 y = random(height/4, height/2);
 r = random(20, 40);
 xdir = random(-10, 10);
 ydir = random(-10, 10);
 color rc = color((int)random(255), (int)random(255), (int)random(255));
 f = color(rc, 100);
 s = color(rc, 200);
 }

 void drawAndStep()
 {
 fill(f);
 stroke(s);
 strokeWeight(2);
 draw();
 x+=xdir;
 y+=ydir;
 if(x + r/2 >= width || x - r/2 <= 0)
 xdir *= -1;
 if(y + r/2 >= height || y - r/2 <= 0)
 ydir *= -1;
 }

 abstract void draw();
}

class Square extends Shape
{
 void draw()
 {
 rectMode(CENTER);
 rect(x, y, r, r);
 }
}

class Circle extends Shape
{
 void draw()
 {
 ellipse(x, y, r, r);
 }
}