float x,y,xD,yD;

void setup() {
  size(400,400);
  framerate(25);
  x=width/2;
  y=height/2;
  xD=random(50)-25;
  yD=random(50)-25;
  ellipseMode(CENTER_DIAMETER);
}

void loop() {
  background(200);
  ellipse(x,y, 20,20);
  y=y+yD;
  x=x+xD;
  if(y>height) yD=-yD;
  else if(y<0) yD=-yD;
  if(x>width) xD=-xD;
  else if(x<0) xD=-xD;
}