Incompatible types error with Map

Question

Here is an example of the code that has been confusing me.

If I try to create a collection of a bounded type and assign the same variable to another variable that is another collection of unbounded unknown type, the code works.

List<? extends Number> numberList = new ArrayList<>();
List<?> anotherList = numberList; // OK

If I try to do the same inside a Map I get the "incompatible types" error.

Map<String, List<? extends Number>> numberMap = new HashMap<>();
Map<String, List<?>> anotherMap = numberMap; // ERROR

The error says:

Map<String,List<? extends Number>> cannot be converted to Map<String,List<?>>

Isn't Map<String,List<? extends Number>> a type of Map<String,List<?>>?

Here's the whole class for your convenience https://ideone.com/AZgV9H

I'm trying to understand why this isn't working and how should I change it in order to make it work.

Generics are **not** covariant. Just like a `List` is not a `List`, a `Map>`is not a `Map`. — Boris the Spider, Apr 08 '16 at 20:35
[Is List a subclass of List? Why aren't Java's generics implicitly polymorphic?](http://stackoverflow.com/questions/2745265/is-listdog-a-subclass-of-listanimal-why-arent-javas-generics-implicitly-p) — Pshemo, Apr 08 '16 at 20:39

score 3 · Accepted Answer · answered Apr 08 '16 at 20:36

Just because a List<? extends Number> is a subtype of List<?> doesn't mean that Map<String, List<? extends Number>> is a subtype of Map<String, List<?>> for the same reason that a List<Number> is not a subtype of List<?> -- Java's generics are invariant.

To get List<Number> to be a subtype of List<?>, you had to add the wildcard with a bound. The solution is the same here -- add ? extends to List<?>.

Map<String, ? extends List<?>> anotherMap = numberMap;

Incompatible types error with Map

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