Ball[] balls = new Ball[1]; //array with only one element
float gravity = 0.1;

void setup() {
  size(400,400);
  smooth();
  frameRate(30);
  //initialize ball index 0
  balls[0] = new Ball(50,0,16);
}
void draw(){
  background(100);
  //update and display all balls
  for (int i = 0; i < balls.length; i++) { //whatever length of array, update and display all objects
    balls[i].gravity();
    balls[i].move();
    balls[i].display();
  }
}

void mousePressed() {
  //a new ball object
  Ball b = new Ball(mouseX, mouseY, 10);
  
  //append to array
  balls = (Ball[]) append(balls, b);
}