Convert List of List into list in java

Question

List<List<String>> superlist = new ArrayList<List<String>>();

List<String> list1 = new ArrayList<String>();
list1.add("a1");
list1.add("a2");

List<String> list2 = new ArrayList<String>();
list2.add("b1");
list2.add("b2");

List<String> list3= new ArrayList<String>();
list3.add("c1");
list3.add("c2");

superlist.add(list1);
superlist.add(list2);
superlist.add(list3);

List<String> result= new ArrayList<>();

Now I want to create a new list which contains all the values in superList. Here result should contain a1,a2,b1,b2,c1,c2

have you made an attempt at doing this on your own? a couple of foreach loops should do the job perfectly well. — Davis Broda, Apr 14 '15 at 18:53
http://stackoverflow.com/questions/25147094/turn-a-list-of-lists-into-a-list-using-lambdas — coderz, Dec 20 '16 at 03:45

score 58 · Accepted Answer · edited Jul 11 '20 at 18:24

58

Try like this using flatMap:

List<List<Object>> list = 
List<Object> lst = list.stream()
        .flatMap(Collection::stream)
        .collect(Collectors.toList());

edited Jul 11 '20 at 18:24

Gautham

766
7
15

answered Apr 14 '15 at 18:56

Rahul Tripathi

168,305
31
280
331

Mick Mnemonic · Answer 2 · 2015-04-14T20:23:06.170

7

If you're on Java < 8 (and cannot use Streams), you can do this in a one-liner with Guava's Iterables.concat:

List<String> merged = Lists.newArrayList(Iterables.concat(superList));

edited Apr 14 '15 at 20:23

answered Apr 14 '15 at 19:37

Mick Mnemonic

7,808
2
26
30

score 3 · Answer 3 · answered Apr 14 '15 at 19:23

3

superlist.forEach(e -> result.addAll(e));

Now after some reasarch, I found this way.

answered Apr 14 '15 at 19:23

Manu Joy

1,739
6
18
30

Which is the best method for the question in terms of performance – Manu Joy Apr 14 '15 at 19:24
Actually, a plain for-loop would be even faster since iterators tend to consume quite a bit of memory if you use them all over the place. But most of the time performance is not even an issue, especially nowadays. I just noticed that this answer was posted almost 6 years ago. – F. Müller Jan 24 '21 at 05:46

score 2 · Answer 4 · answered Apr 14 '15 at 18:53

You would have to loop through every List in your superlist object in order to get all of the contents. You can use the addAll() method to copy each list's contents to your new List:

List<String> result = new ArrayList<String>();
for (List<String> list : superlist) {
    result.addAll(list);
}

score 0 · Answer 5 · answered Jan 24 '21 at 05:41

0

This works for me!

List<String> result= new ArrayList<>();
    
    superlist.stream().forEach(
            
            n->result.addAll(n)
            );
    
    System.out.println(result);

answered Jan 24 '21 at 05:41

nagendra babu

47
1

Convert List of List into list in java

5 Answers5

Linked

Related