Ball Class. Using processing.js library.

ball-class.js

Ball[] population = new Ball[400];

void setup() {
  size(1024, 576);
  smooth();
  frameRate(30);
  for(int i=0; i<400; i++) {
    population[i] = new Ball(random(0, width), random(0, 100));
  }
}

void draw() {
  background(0);
  for(int i=0; i<400; i++){
    population[i].ops();
  }
}

// Constructor
class Ball { // global variable
  float x  =  0;
  float y  =  0;
  float xv = 16;
  float yv =  9;

  Ball(float _x, float _y) {
    x = _x;
    y = _y;
  }

  void ops() {
    display (); // display ellipse
    velocity(); // velocity vector
    invertV (); // invert velocity
    force   (); // simulate gravity
  }

  void force() {
    yv += random(-0.6, 0.6); // separate objects
  }

  void invertV() { // invert velocity vector
    if(x > width || x < 0) {
      xv = xv * -1;
    }
    else if(y > height || y < 0) {
      yv = yv * -1;
    }
  }

  void display() { // display ellipse
    fill(random(0, 255), random(0, 255), random(0, 255));
    ellipse(x, y, 20, 20);
  }

  void velocity() { // velocity vector
    x += xv;
    y += yv;
  }
}