class Ball {
  float x, y;          // position
  float r;             // radius
  // the following also provide initial values for the fields,
  // equivalent to putting that in the constructor
  float s = 5;         // speed, intialized to 5
  int dx = 1, dy = 1;  // direction, initialized to diagonal

  // Initialize a Ball at position (x0,y0), with speed s0 and radius r0
  Ball(float x0, float y0, float r0)
  {
    x = x0; 
    y = y0;
    r = r0;
  }

  void draw()
  {
    fill(255);
    ellipse(x,y,r*2,r*2);
  }

  void update()
  {
    // Move in the appropriate direction by the step size
    x+=s*dx;
    y+=s*dy;
    // If too close to the wall (consider the radius of the ball),
    // move back and change direction
    if (x > width-r) { 
      x = width-r; 
      dx = -1; 
    }
    else if (x < r) { 
      x = r; 
      dx = 1; 
    }
    if (y > height-r) { 
      y = height-r; 
      dy = -1; 
    }
    else if (y < r) { 
      y = r; 
      dy = 1; 
    }
  }
}