A Common Problem

class Milk {
  void drink() {
    System.out.println("You drink the milk.");
  }
}

class Oranges {
  void juggle() {
    System.out.println("You drop the oranges on the ground.");
  }
}

class Box {
  private Object contents;

  void add(Object thing) {
    if (contents == null) {
      contents = thing;
    } else {
      System.out.println("The box is full.");
    }
  }

  Object remove() {
    if (contents == null) {
      System.out.println("The box is empty.");
      return null;
    } else {
      Object thing = contents;
      contents = null;
      return thing;
    }
  }
}