Question

I am a bit confused on the purpose of declaring instances as specific types. For example,

Integer intOb1 = new Integer(3);
Object intOb2 = new Integer(4);

I know the type of intOb1 is Integer, and the type of intOb2 is Object, but what does declaring intOb2 as Object allow one to do? Use methods in Object? Can’t you already use those methods as an Integer? Or is the main purpose just to be able to “treat” intOb2 as an Object?

As you can see, I’m befuddled.

Answer 1

This actual isn’t called casting this is an example of polymorphism. Which allows variables to take on different types based on their inheritance relationships with other classes.

For example, suppose you were writing a program to simulate a zoo. You would have a class called Zoo and a class called Animal. There are also several classes that extend from the Animal class: Lion, Zebra, and Elephant.

It would be extremely useful to keep all of these objects grouped together in a single list but since they are of different types, i.e: Lion, Zebra, and Elephant, you can’t store them in a single list, you would have to maintain a separate list for each type of animal. This is where polymorphism comes into play.

Since each of the classes Lion, Zebra, and Elephant all extend from the Animal class we can simply store them in a list of type Animal.

code example:

public class Zoo
{
   private List<Animal> animals; 

   public Zoo()
   {
      this.animals = new ArrayList<>();
   }

   //notice this method takes an Animal object as a parameter
   public void add(Animal a)
   {
      this.animals.add(a);
   }
}

public abstract class Animal
{
   private String name;
   private String type;

   public Animal(String name, String type)
   {
      this.name = name;
      this.type = type;
   }

   //all subclasses must implement this method
   public abstract void speak();
}

public class Lion extends Animal
{
   private String animalType = "Lion";

   public Lion(String name)
   {
      super(name, animalType);
   }

   public void speak()
   {
      System.out.println("ROAR");
   }
}

//....etc for the other two animals

public class TestZoo
{
   public static void main(String[] args)
   {
      Zoo myZoo  = new Zoo();
      Lion l     = new Lion("Larry");
      Elephant e = new Elephant("Eli");
      Zebra    z = new Zebra("Zayne");

      myZoo.add(l);  //<-- note that here I don't pass Animal objects to the 
      myZoo.add(e);  //    add method but subclasses of Animal
      myZoo.add(z);
   }
}

Hope this helps, even with a silly example.