Ball[] balls = new Ball[20];

void setup(){
  size(500,500);
  smooth();
  for(int i=0; i<balls.length;i++){
    balls[i]=new Ball();
  }
}

void draw(){
  background(255);
  for(int i=0; i<balls.length;i++){
    balls[i].display();
    balls[i].move();
  }
}


*****************
// The basic ball class

class Ball {
  //attributes or fields
  float radius; //size
  float x, y; //location
  float dx, dy;
  float speed = 5.0;
  float gravity = 0.1;
  float dampen = -0.9;
  color ballColor;

  // Constructor(s)
  Ball() {
    radius = 25;
    x=random(width);
    y=random(height);
    ballColor = color(121, 34, 230);
    dx=random(-3, 3);
    dy=random(-3, 3);
  }

  Ball(float r, color c) {
    radius = r;
    x=random(width);
    y=random(height);
    ballColor = c;
    dx=random(-3, 3);
    dy=random(-3, 3);
  }

  Ball(float r, color c, float x1, float y1) {
    radius = r;
    x=x1;
    y=y1;
    ballColor = c;
    dx=random(-3, 3);
    dy=random(-3, 3);
  }

  //Behaviors
  void move() {
    x=x+dx;
    y+=speed;
    speed+=gravity;
    bounce();
  } //move

    void bounce() {
    if ((x<=radius && dx<0)|| (x>=width-radius && dx>0)) {
      dx=-dx;
    }

    if (y<radius && dy<0) {
      y=radius; 
      dy=-dy;
    }

    if (y>height -radius) {
      y=height-radius;
      dy=-dy;
      speed *= dampen;
    }
  }//bounce

  void display() {
    noStroke();
    fill(ballColor);
    ellipse(x, y, 2*radius, 2*radius);
  }//display
}

***************************
// This class is a subclass of the ball class
// Notice use of super() to call the superclass constructor 

class ballInBox extends ball {
  // Inherits everything from ball class

  // additional attributes
  box myBox;

  ballInBox(float _x, float _y, float _r, color _c, box _b) {
    super(_x, _y, _r, _c);
    myBox = _b;
  } // ballInBox()

  // override the behavior of inherited bounce()
  void bounce() {
    if (x < radius) {
      x = radius;
      dx = -dx;
    }

    if (x > myBox.w-radius) {
      x = myBox.w-radius;
      dx = -dx;
    }

    if (y < radius) {
      y = radius;
      dy = -dy;
    }

    if (y > myBox.h - radius) {
      y = myBox.h-radius;
      dy = -dy;
      speed = speed*dampen;
    }
  } // bounce()
} // class ballInBox