将2d arraylist添加到3d arraylist

Question

I have got 3d arraylist and to each dimension of it I want to add a temporary 2d arraylists. For example

   List<List<Integer>> perm = new ArrayList<List<Integer>>();
   List<List<List<Integer>>> list1 = new ArrayList<List<List<Integer>>>();
   list1.get(0).add(perm);
   list1.get(1).add(perm);

but this one does not work. It tells me

   The method add(List<Integer>) in the type List<List<Integer>> is not applicable for the    
   arguments (List<List<Integer>>)

Answer 1

Look at the types more closely - you're trying to add a List<List<Integer>> to the first list inside list1 (because you've called get(0) .)

This first list (inside the topmost ArrayList ) however is of type List<Integer> , not List<List<Integer>> - thus the types don't match, and you get a compile error.