Upcasting and Downcasting

Upcasting is casting to a supertype, while downcasting is casting to a subtype. Upcasting is always allowed, but downcasting involves a type check and can throw a ClassCastException.

In your case, a cast from from Dog to an Animal is an upcast, because a Dog is-a Animal. In general, you can upcast whenever there is an is-a relationship between two classes.

 

Downcasting would be something like this:

Animal animal = new Dog();

Dog castedDog = (Dog) animal;

 

Basically what you're doing is telling the compiler that you know what the runtime type of the object really is. The compiler will allow the conversion, but will still insert a runtime sanity check to make sure that the conversion makes sense. In this case, the cast is possible because at runtime animalis actually a Dog even though the static type of animal is Animal.

 

However, if you were to do this:

Animal animal = new Animal();

Dog notADog = (Dog) animal;

 

You'd get a ClassCastException. The reason why is because animal's runtime type is Animal, and so when you tell the runtime to perform the cast it sees that animal isn't really a Dog and so throws a ClassCastException.

 

To call a superclass's method you can do super.method() or by performing the upcast.

 

To call a subclass's method you have to do a downcast. As shown above, you normally risk a ClassCastException by doing this; however, you can use the instanceof operator to check the runtime type of the object before performing the cast, which allows you to prevent ClassCastExceptions:

 

Animal animal = getAnimal();

// Maybe a Dog? Maybe a Cat? Maybe an Animal?

if (animal instanceof Dog)

{

// Guaranteed to succeed, barring classloader shenanigans

Dog castedDog = (Dog) animal;

}