Arrays and ArrayList instances are polymorphic

We already know that we can put objects into arrays and ArrayList. But being polymorphic means they can handle objects of multiple distinct types as long as they have a common parent type all within the same array or ArrayList.

In Chapter 10, Object-Oriented Programming, we learned that polymorphism means different forms. But what does it mean to us in the context of arrays and ArrayList?

Boiled down to its simplest: any subclass can be used as part of the code that uses the superclass.

For example, if we have an array of Animal instances, we could put any object that is a type that is a subclass of Animal in the Animal array – perhaps Cat and Dog instances.

This means we can write code that is simpler and easier to understand, and easier to change:

// This code assumes we have an Animal class
// And we have a Cat and Dog class that extends Animal
Animal myAnimal =  new Animal();
Dog myDog = new Dog()...