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

Java generics are not covariant.

See Java Theory and Practice: Generics Gotchas for further details.

The page shows a simple example that would havoc the type-system if it was covariant:

Imagine you could assign a List<Integer> to a List<Number>. Then the following code would allow you to put something that wasn't an Integer into a List<Integer>:

List<Integer> li = new ArrayList<Integer>();
List<Number> ln = li; // illegal
ln.add(new Float(3.1415)); // ERROR: Adds a float to li, which is a list of Integers!

Answer 2

Collection<? extends A> collecA

This fixes it. The problem is not the List extends Collection, but the generic types instead.

Answer 3

You can assign List<B> to Collection<B>, but not List<B> to Collection<A>.

Imagine what would happen if this were possible:

List<B> = new ArrayList<B>();
Collection<A> collecA = listB; //Assume that this line compiles
collecA.add(new A());
B item = listB.get(0); //ClassCastException!

As you see, we "fooled" the generics type system, by adding an instance of concrete type A to a collection that was supposed to have only objects of type B (or descendants). As a consequence, the last line which performs an implicit cast to B fails with a ClassCastException. What's wrong with it? The compiler can't guarantee type safety, and that's against one of the Java generics principles.

Therefore, it has been decided that List<B> is Collection<B>, but NOT List<A> (or Collection<A>).

As a side comment, it is interesting to note that arrays don't follow the same rules: String[] is an Object[], and assignments are legal.

Answer 4

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

"