Java Generics: Cannot cast List to List<

Question

This question already has an answer here:

• Is List<Dog> a subclass of List<Animal>? Why are Java generics not implicitly polymorphic? 16 answers

Just come across with this problem:

List<DataNode> a1 = new ArrayList<DataNode>();
List<Tree> b1 = a1;  // compile error: incompatible type

Where the type DataNode is a subtype of Tree.

public class DataNode implements Tree

To my surprise, this works for array:

DataNode[] a2 = new DataNode[0];
Tree[] b2 = a2;   // this is okay

This likes a bit strange. Can anyone give an explanation on this?

Answer 1

DataNode might be a subtype of Tree, but List DataNode is not a subtype of List Tree.

https://docs.oracle.com/javase/tutorial/extra/generics/subtype.html

Answer 2

Well, I'll be honest here: lazy genericity implementation.

There's no semantic reason not to allow your first affectation.

Incidentally, though I adored templating in C++, generics, together with the kind of silly limitation we have here, are the main reason why I gave up on Java.

Answer 3

If you do have to cast from List<DataNode> to List<Tree>, and you know it is safe to do so, then an ugly way to achieve this is to do a double-cast:

List<DataNode> a1 = new ArrayList<DataNode>();

List<Tree> b1 = (List<Tree>) (List<? extends Tree>) a1;

Answer 4

It is the answer from C#, but I think it doesn't actually matter here, as the reason is the same.

"In particular, unlike array types, constructed reference types do not exhibit “covariant” conversions. This means that a type List<B> has no conversion (either implicit or explicit) to List<A> even if B is derived from A. Likewise, no conversion exists from List<B> to List<object>.

The rationale for this is simple: if a conversion to List<A> is permitted, then apparently one can store values of type A into the list. But this would break the invariant that every object in a list of type List<B> is always a value of type B, or else unexpected failures may occur when assigning into collection classes."

http://social.msdn.microsoft.com/forums/en-US/clr/thread/22e262ed-c3f8-40ed-baf3-2cbcc54a216e