Usually, when we create an array or an ArrayList, the list items all need to be the same type. But polymorphism puts a new spin on what is considered the same type…

In fact, we can put instances of different classes that share a parent class together in an array or ArrayList! For example, let’s say we have a Monster parent class with a few child classes: Vampire, Werewolf, and Zombie. We can set up an array with instances of each:

Monster dracula, wolfman, zombie1;

dracula = new Vampire();
wolfman = new Werewolf();
zombie1 = new Zombie();

Monster[] monsters = {dracula, wolfman, zombie1};

We can even iterate through the list of items — regardless of subclass — and perform the same action with each item:

for (Monster monster : monsters) {

  monster.attack();

}

In the code above, we were able to call attack() on each monster in monsters despite the fact that, in the for-each loop, monster is declared as the parent class type Monster.