Generics incompatible types

Question

Stumbled upon incompatible types error cause of which I don't understand.

Why is this piece of code wrong?

List<List<String>> a = new ArrayList<>();
List b = a; // is ok
List<List> c = a; // incompatible types

Answer 1

It is described here. Supertype compatibility works only on the 'outer' level, but not 'inside' across the type parameters. It is not intuitive, but that's how it works... In addition, List is a raw type, and it behaves slightly differently than List<Object> - which is described here.

Answer 2

Writing

List b = a;

Doesn't involves generics. It defines a raw List type named b which can take any object as it's element.

Don't compare it with

List<List> c = a;

as it involves generics and that's why compiler will enforce type compatibility checking here.