Iterating an Arraylist of Arraylists using forEach and lambda expression

Question

I have an ArrayList

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

How to iterate this list using forEach and lambda Expression.

for example an Arraylist can be iterated as:-

List<Integer> singlelist = new ArrayList<>();
singlelist.forEach((ele)->System.out.println(ele));

How to do the same for ArrayList containing Arraylists.

Just the same way, `list.forEach(x -> x.forEach(y -> System.out.println(y)));` or perhaps with better names: `list.forEach(sub -> sub.forEach(i -> System.out.println(i)));` — Holger, Nov 13 '19 at 13:31
@Holger or ... *perhaps with better names: `nestedLists.forEach(sub -> sub.forEach(i -> System.out.println(i)));`* ? — Naman, Nov 13 '19 at 13:41
@Naman well, I didn’t want to change what’s perhaps given by the task. However, if we change the name, it would be even better to have a name which reflects the purpose of the list, rather than the fact that it is a nested list, which we can already derive from its type. — Holger, Nov 13 '19 at 13:44

score 3 · Accepted Answer · answered Nov 13 '19 at 13:29

3

You can use Streams with flatMap:

list.stream()
    .flatMap(List::stream)
    .forEach(System.out::println);

answered Nov 13 '19 at 13:29

Eran

Which one will have better performance using streams or the otherway – Somil Garg Nov 13 '19 at 13:42
2

@SomilGarg Are you really concerned about the performance of a `System.out.print`? The difference between the two would anyway be trivial, given the task executed. Even if you want, you can rather benchmark them and test them. – Naman Nov 13 '19 at 13:44

1 Answers1