ArrayList balls;
final int AMOUNT = 8;
final float G = 0.3;

void setup()
{
  size(500,500);
  background(255);
  smooth();
  noStroke();
  ellipseMode(CORNER);
  colorMode(HSB,100);
  balls = new ArrayList();
  for(int i=0;i<=AMOUNT;i++){
    balls.add(new Ball(0,0,i,0,i*10,i*10));
  }
}

void draw()
{
  background(100);
  for(int i=0;i<=AMOUNT;i++){
    Ball b = (Ball)balls.get(i);
    b.update();
    b.draw_ball();
  }
}

class Ball
{
  float xPos,yPos,Vx,Vy,rSize,gravity;
  int bColor;
  
  Ball(float x, float y,float vx, float vy, float size ,int ball_color ){
    xPos = x;
    yPos = y;
    Vx = vx;
    Vy = vy;
    rSize = size;
    bColor = ball_color;
    float r = random(2,5);
    gravity = G*r;
  }
  
  void draw_ball(){
    xPos += Vx;
    if(yPos+rSize>height-0.05)
      yPos = height-rSize;
    else
      yPos += Vy/2;    
    fill(bColor,20,99,80);
    ellipse(xPos,yPos,rSize,rSize);
  }
  
  void update(){
    if(xPos+rSize >width){
      xPos=2*(width-rSize)-xPos;
      Vx = -0.9*Vx;
    }
    else if(xPos<0){
      xPos = abs(xPos);
      Vx = -0.95*Vx;
    }
       
    if(yPos+rSize>height){
      yPos = 2*(height-rSize)-yPos;
      Vy = -0.9*Vy;
    }
     Vy +=gravity;
  }
}