float xvel = 1;
float yvel = -2;
float x, y;
color c = color(255,180,0);
float radius = 50;

void setup() {
  size(600,600);
  x = random(width-2*radius) + radius;
  y = random(height-2*radius) + radius;
}

void draw() {
  background(120);
  x += xvel;
  if (x + radius > width || x < radius) {
    x -= xvel;
    xvel = -xvel;
  }
  y += yvel;
  if (y + radius > height || y < radius) {
    y -= yvel;
    yvel = -yvel;
  }
  fill(c);
  circle(x,y,2*radius);
}