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

• Davis Broda: have you made an attempt at doing this on your own? a couple of foreach loops should do the job perfectly well.
• coderz: stackoverflow.com/questions/25147094/…

Answer 1

Try like this using flatMap:

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

• Marco Martins: I will leave this here in case it helps someone, if instead of List<List<Object>> you have List<Object[]>, use Arrays::stream instead of Collection::stream

Answer 2

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));

Answer 3

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

Now after some reasarch, I found this way.

• Manu Joy: Which is the best method for the question in terms of performance
• F. Müller: 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.

Answer 4

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);
}

Answer 5

This works for me!

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