Static (Compile-time) Polymorphism

Polymorphism (the idea of "having many forms") is closely related to type substitution. You have already seen a flavor of it in action: you can pass an argument of type GradStudent to a method like add that takes a parameter of type Student because the compiler honors type substitution and implicitly casts from the subtype (GradStudent) to the base type (Student). This is called static or compile-time polymorphism.

Exercise Make up an example to showcase compile-time polymorphism for the following type hierarchy.

public class Shelter {
  private Animal[] animals;
  private int numAnimals;

  // Constructor not shown to save space

  public add(Animal a) {
    animals[numAnimals++] = a;
  }
}