Processing gist

dvd_box

// size of dvd box
int dvdW = 80;
int dvdH = 50;

// positive value = moving towards right
// negative value = moving towards left
int xspeed = 1;

// coordinates of dvd box
// remember the x coordinate is the leftmost side
int x;
// remember the y coordinate is the topmost side
int y;

void setup() {
  size(400, 300);

  // more about random function at https://processing.org/reference/random_.html
  // we generate random x coordinate between 0 and the width of the screen minus the width of the dvd box (so it doesn't start outside of the screen)
  // random() function returns float so we convert it back to integer with int() function
  x = int(random(0, width - dvdW));
  y = int(random(0, height - dvdH));
}

void draw() {
  background(0);

  fill(255);
  noStroke();
  rect(x, y, dvdW, dvdH);

  // if x coordinate is equal to or higher than the width of the screen minus the width of the dvd box,
  // it means the right side of the dvd box is touching the right side of the screen
  // we change the xspeed to negative value (as specified above, negative value means moving towards left)
  //
  // the same goes when x is less or equal to zero, it means the dvd box is touching the left side of the screen
  // we change the xspeed to positive value (as specified above, positive value means moving towards right)
  if(x >= width - dvdW) {
    xspeed = -1;
  } else if (x <= 0) {
    xspeed = 1;
  }

  x = x + xspeed;
}

// exercises:
//
// 1) Implement the same behaviour for y coordinate so the box moves along y axis and bounces of top and bottom sides as well.
//
// 2) Change color of the box each time it bounces from ANY side (using functions: fill(), random()).
//
// 3) If the box hits ANY corner, stop rendering (using function: noLoop()).