float x, y;
float xv, yv;
float diam, radius;

void setup()
{
  size(500, 400);
  background(200);
  frameRate(10);

  x = random(width);
  y = random(height);
  xv = random(10)+1;
  yv = random(10)+1;
  diam = 50;
  radius = diam/2.0;
}

void draw()
{
  background(200);

  fill(0, 0, 200);
  noStroke();
  ellipseMode(CORNER);
  ellipse(x, y, diam, diam);

  // move the ball
  x += xv;
  y += yv;

  // handle right wall
  if (x>width) {
    xv = -xv;
    x = width;
  }

  // handle left wall
  if (x<0) {
    xv = -xv;
    x = 0;
  }

  // handle bottom
  if (y>height-radius) {
    yv = -yv;
    y = height-radius;
  }

  // handle top
  if (y<0) {
    yv = -yv;
    y = 0;
  }

  // add cheap gravity
  yv += 1;
}