float x, y, a;

void setup() {
  size(300, 200);
  x = width/2;
  y = height/2;
  a = 50;
}

void draw() {
  background(0);
  fill(255);
  if (keyPressed && (key == CODED)) {
    if (keyCode == LEFT) x--;
    if (keyCode == RIGHT) x++;
    if (keyCode == UP) y--;
    if (keyCode == DOWN) y++;
  }
  if (x > width - a) x = width - a;
  if (x < 0) x = 0;
  if (y > height - a) y = height - a;
  if (y < 0) y = 0;
  rect(x, y, a, a);
}