Use diamond operator to simplyfiy generic collections creation. For example: ArrayList<ArrayList<Integer>> list = new ArrayList<>();
Use generalized types for declaration, unless you have good reason to use specific types. For example: List<List<Integer>> list = new ArrayList<>();
In most cases the "foreach" of Java is suffficient to iterate through collection. The C-like style with indexes is still applicable in some cases, but you will mostly use the "foreach": for (List<integer> subList : list) {...}
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Few suggestions/advises:
ArrayList<ArrayList<Integer>> list = new ArrayList<>();List<List<Integer>> list = new ArrayList<>();for (List<integer> subList : list) {...}