generics - How to cast a list of inheriting objects to a collection of objects in Java?

Question

I've a collection type:

Collection<A> collecA

And I've a list in my object:

List<B> listB

Where B is extending A

class B extends A { ... }

But I can't do the following:

collecA = listB

I can't understand why since Collection is implemented by List.

Answer 1

Let's assume for a moment you could do what you describe:

class B extends A { ... }

Collection<A> collecA;
List<B> listB;

collecA = listB;  // normally an error, but lets pretend its allowed

collecA.add(new A()); // PROBLEM!

The method call collecA.add(new A()) appears okay since collecA is a collection that holds As. However, if the above assignment were allowed, then we have a problem becausecollecA is really reference to a List<B> instance - I just added an A into a list that can only hold Bs!

Asker also said:

I can't understand why since Collection is implemented by List.

It doesn't matter that Collection is a superclass of List. This assignment is illegal even if you used two lists.

class B extends A { ... }
List<A> listA;
List<B> listB;
listA = listB;  // still an error, still leads to the same problem

The key is that the List<A> variable can reference only Lists that can hold As. However, a List<B> instance cannot hold As. Therefore, a List<A> variable like listA cannot be assigned a reference to a List<B> instance referred to bylistB.

Or more generally speaking: B being a subclass of A does not imply that SomeGenericClass<B> is a subclass of SomeGenericClass<A> (JLS §4.10: Subtyping does not extend through generic types: T <: U does not imply that C<T> <: C<U>.)

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It was this example/analogy from the Java Generics Tutorial that helped me understand this:

http://java.sun.com/docs/books/tutorial/java/generics/subtyping.html

"Understanding why becomes much easier if you think of tangible objects — things you can actually picture — such as a cage:

// A cage is a collection of things, with bars to keep them in.
interface Cage<E> extends Collection<E>;
...
Cage<Lion> lionCage = ...;
Cage<Butterfly> butterflyCage = ...;

But what about an "animal cage"? English is ambiguous, so to be precise let's assume we're talking about an "all-animal cage":

Cage<Animal> animalCage = ...;

This is a cage designed to hold all kinds of animals, mixed together. It must have bars strong enough to hold in the lions, and spaced closely enough to hold in the butterflies.
...
Since a lion is a kind of animal (Lion is a subtype of Animal), the question then becomes, "Is a lion cage a kind of animal cage? Is Cage<Lion> a subtype of Cage<Animal>?". By the above definition of animal cage, the answer must be "no". This is surprising! But it makes perfect sense when you think about it: A lion cage cannot be assumed to keep in butterflies, and a butterfly cage cannot be assumed to hold in lions. Therefore, neither cage can be considered an "all-animal" cage:

animalCage = lionCage;  // compile-time error
animalCage = butterflyCage; // compile-time error

"