public class Program {
  public static void main(String[] args) {
    // because cat and dog are subclasses of animal, they can be stored in an array of animals
    Animal[] animals = { 
      new Dog("buster"), 
      new Cat("mittens") 
    };

    for (Animal a : animals) {
      // when calling the make_sound function, the implementation of the actual class gets called (in this case cat or dog)
      a.make_sound();
    }
  }
}