List<List<?>> cannot be assigned to List<List<?>> when type parameter is involved

Question

In the following code, get() is called and it's result assigned to a variable whose type is List<List<?>>. get() returns a List<List<T>> and is called on an instance whose type parameter T is set to ?, so it should fit.

import java.util.List;

class Test {
    void foo(NestedListProducer<?> test) {
        List<List<?>> a = test.get();
    }

    interface NestedListProducer<T> {
        List<List<T>> get();
    }
}

But both IntelliJ IDEA and Oracle's javac version 1.7.0_45 reject my code as invalid. This is the error message of 'javac':

java: incompatible types
  required: java.util.List<java.util.List<?>>
  found:    java.util.List<java.util.List<capture#1 of ?>>

Why is this code invalid, i.e. what could go wrong if it was allowed?

Answer 1

The ? is a wildcard meaning any type. One ? cannot be the same as another ?, because another ? could be any other type, and they don't match. You must use generics to say that the types are the same:

// Make this generic
<A> void foo(NestedListProducer<A> test) {
    List<List<A>> a = test.get();
}