Question

I am trying to create a list of list of strings,

List<List<String>> string= new ArrayList<ArrayList<String>>();

and getting Type Mismatch error:cannot convert from ArrayList<ArrayList<String>> to List<List<String>>.

I know I can change

List<List<String>> to ArrayList<ArrayList<String>>

and it will work fine.

But I was just wondering why doesn’t it let it happen? Simple List<Object> can refer to ArrayList<Object> so what is different about this?

Answer 1

Because a List<List<String>> would accept other lists of strings that were not array lists, which is not how you declared the type initially. For example, if you converted to a List<List<String>>, you would technically be allowed to add a LinkedList<String> to it! And that’s not an ArrayList<String>. Allowing that cast would effectively break the generic type system.

In other words, you could convert to List<ArrayList<String>>. But not List<List<String>>.