Declare a list with list of lists

Question

List<List<Integer>> l = new LinkedList<LinkedList<Integer>>();
List<List<Integer>> l2 = new List<LinkedList<Integer>>();
List<List<Integer>> l3 = new LinkedList<List<Integer>();
List<List<Integer>> l4 = new LinkedList<>();

I cannot understand why only the third and the fourth statements work.

Same reason that you can't do `LinkedList list = new LinkedList();`. — Oliver Charlesworth, Jan 02 '16 at 20:18
The second one doesn't work because `List` is an interface. You can't make a `new` interface without an implementation. — OneCricketeer, Jan 02 '16 at 20:19
Also, http://stackoverflow.com/questions/2745265/is-listdog-a-subclass-of-listanimal-why-arent-javas-generics-implicitly-p — Sotirios Delimanolis, Jan 02 '16 at 20:31

score 3 · Accepted Answer · answered Jan 02 '16 at 20:22

The first declaration does not work because a LinkedList<LinkedList<Integer>> is not a List<List<Integer>>. For example, by the declaration, you could add an ArrayList<Integer> to a List<List<Integer>> (a list of lists of integers). However, an ArrayList<Integer> could not be added to a LinkedList<LinkedList<Integer>> (a linked list of linked lists of integers).

The second initialization, starting with a new List could not work, as List is an interface, not a class, so it cannot be initialized with the new operator.

@cricket_007 That's an example of an [anonymous class](https://docs.oracle.com/javase/tutorial/java/javaOO/anonymousclasses.html) (and not an interface). — Elliott Frisch, Jan 02 '16 at 20:25

Declare a list with list of lists

1 Answers1