Iterating an Arraylist of Arraylists using forEach and lambda expression [duplicate]

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
– Holger, Commented Nov 13, 2019 at 13:31
@Holger or ... perhaps with better names: nestedLists.forEach(sub -> sub.forEach(i -> System.out.println(i))); ? — Naman
– Naman, Commented Nov 13, 2019 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
– Holger, Commented Nov 13, 2019 at 13:44

Eran · Accepted Answer · 2019-11-13 13:29:30Z

3

You can use Streams with flatMap:

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

answered Nov 13, 2019 at 13:29

Eran

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2 Comments

Somil Garg Over a year ago

Which one will have better performance using streams or the otherway

Naman Over a year ago

@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.

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