void move() {
    //moves by 1 unit in either direction
    this.coordinateX += RANDOM.nextInt(3) - 1;
    this.coordinateY += RANDOM.nextInt(3) - 1;
  }
@Test
  void moveTest() {
    var b = new Bubble(10, 10, 1, 2);
    var initialX = b.coordinateX;
    var initialY = b.coordinateY;
    b.move();
    //change in x and y < |2|
    assertTrue(b.coordinateX - initialX < 2 && b.coordinateX - initialX > -2);
    assertTrue(b.coordinateY - initialY < 2 && b.coordinateY - initialY > -2);
  }