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

List<List> 

is implicitly

List<List<Object>>

which is not a parent of

List<List<String>>

the reason why it succeeds in the first case is because of Type Inference. The compiler will essentially check which type is needed for the expression to make sense and it will generate

List<List<String>> a = b;

In the second case it will default to

List<List<Object>> a = b // which does not compile

Answer 3

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.

Answer 4

Short answer: because your c list contains a list with all kind of objects.
For example you can also add Integer objects.
And a list can contains only String objects.