Merge 3 arraylist to one

Question

I want to merge down 3 arraylist in one in java. Does anyone know which is the best way to do such a thing?

I'm confused. The question says nothing about 2D lists, but the accepted answer and comments discuss 2D, and the other answer is exclusively for 2D lists. So, is this about taking some number of `List` and making a single `List` with all of the elements of the originals, or about making a `List>` containing each of the originals? — Erick G. Hagstrom, Feb 26 '16 at 14:44
Is it important that the output be an instance of `ArrayList` rather than just a `List`? — Erick G. Hagstrom, Feb 26 '16 at 14:57

Asaph · Accepted Answer · 2011-12-24T15:16:05.237

55

Use ArrayList.addAll(). Something like this should work (assuming lists contain String objects; you should change accordingly).

List<String> combined = new ArrayList<String>();
combined.addAll(firstArrayList);
combined.addAll(secondArrayList);
combined.addAll(thirdArrayList);

Update

I can see by your comments that you may actually be trying to create a 2D list. If so, code such as the following should work:

List<List<String>> combined2d = new ArrayList<List<String>>();
combined2d.add(firstArrayList);
combined2d.add(secondArrayList);
combined2d.add(thirdArrayList);

edited Dec 24 '11 at 15:16

answered Dec 24 '11 at 15:09

Asaph

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Xm I thik this solutions is about creating a 2d Arraylist. I want to put side by side 3 lists to one new. – snake plissken Dec 24 '11 at 15:13
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@snake plissken: You made no mention of 2D lists in the question. However, I've updated my answer to include a 2D solution too. – Asaph Dec 24 '11 at 15:14
2

@snakeplissken - This answer will do what you expect, no 2D involved. – ziesemer Dec 24 '11 at 15:15

score 10 · Answer 2 · edited Feb 26 '16 at 15:19

10

What about using java.util.Arrays.asList to simplify merging?

List<String> one = Arrays.asList("one","two","three");
List<String> two = Arrays.asList("four","five","six");
List<String> three = Arrays.asList("seven","eight","nine");

List<List<String>> merged = Arrays.asList(one, two, three);

edited Feb 26 '16 at 15:19

Ahmad

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answered Dec 24 '11 at 15:30

Edwin Dalorzo

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This is much better than the first answer. It is a much cleaner way of merging lists into a 2D list. – Michael Massey Jul 31 '15 at 07:47
Note: you would normally call this as `Arrays.asList()` – Tim Feb 11 '16 at 11:09
Note that this results in a fixed size `List>`, not an `ArrayList>`. OP specifically calls for `ArrayList`. I'm not sure whether they want 1D or 2D, but definitely `ArrayList`. – Erick G. Hagstrom Feb 26 '16 at 14:46

Dhruvan Ganesh · Answer 3 · 2017-12-14T13:09:05.373

Using Java 8 Streams:

List of List

List<List<String>> listOfList = Stream.of(list1, list2, list3).collect(Collectors.toList());

List of Strings

List<String> list = Stream.of(list1, list2, list3).flatMap(Collection::stream).collect(Collectors.toList());

Using Java 9 List.of static factory method (Warning: this list is immutable and disallows null)

List<List<String>> = List.of(list1, list2, list3);

Where list1, list2, list3 are of type List<String>

Merge 3 arraylist to one

3 Answers3

Update

Linked