Initialize List<List<Integer>> in Java

Question

How can I initialize List<List<Integer>> in Java?

I know List is an interface and I can use ArrayList or LinkedList to implement List<Integer> list = new ArrayList<Integer>(), but when I initialize List<List<Integer>> list = new ArrayList<ArrayList<Integer>>(); I get error incompatible types:

ArrayList<ArrayList<Integer>> cannot be converted to List<List<Integer>>.

So how can I proceed?

Answer 1

Use

List<List<Integer>> list = new ArrayList<List<Integer>>();

or since Java 1.7

List<List<Integer>> list = new ArrayList<>();

Answer 2

You can define it as List<List<Integer>> list = new ArrayList<List<Integer>>();.

Then while defining the inner List you can take care of initialising it as ArrayList<Integer>.