How do I concatenate two lists to create a 3rd?

Question

I've been googling for this, and I found this answer. There is an Iterables.concat in guava. But this returns an Iterable and the next thing I want to do is sort the result. Collections.sort takes in a List, not an Iterable so I'd have to convert the Iterable into a List as my next step. Is there a more direct way to combine two Lists and then sort the result?

added as an answer (wasn't sure I understood the question, so I commented) — , Feb 04 '15 at 19:11
possible duplicate of [How do I join two lists in Java?](http://stackoverflow.com/questions/189559/how-do-i-join-two-lists-in-java) — Raedwald, Feb 05 '15 at 07:51
@Raedwald one minor difference is his question says, "no external libraries" — Daniel Kaplan, Feb 05 '15 at 21:18

score 6 · Answer 1 · answered Feb 04 '15 at 18:52

6

In Java 8:

List<E> sorted = Stream.concat(l1.stream(), l2.stream())
                       .sorted()
                       .collect(Collectors.toList());

answered Feb 04 '15 at 18:52

Jean Logeart

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score 4 · Accepted Answer · 2015-02-04T20:06:18.337

4

List<Something> list = Lists.newArrayList(Iterables.concat(...));
Collections.sort(list);

might be a solution here.

edited Feb 04 '15 at 20:06

answered Feb 04 '15 at 19:10

3

Shorter and likely faster: `Ordering.natural().sortedCopy(Iterables.concat(...))` – Louis Wasserman Feb 05 '15 at 00:48

score 3 · Answer 3 · edited Feb 04 '15 at 18:56

3

List has the addAll(Collection) method, which I think is useful for your purpose. You can copy the first list and make something like this:

copyList1.addAll(list2);

edited Feb 04 '15 at 18:56

yshavit

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answered Feb 04 '15 at 18:45

Daniel Facciabene

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score 2 · Answer 4 · answered Feb 04 '15 at 18:46

2

If a SortedSet is also good as return type (I find this is one you really want in most cases) you can do:

FluentIterable.from( list1 ).concat( list2 ).toSortedSet( Ordering.natural() );

answered Feb 04 '15 at 18:46

Wim Deblauwe

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FYI, there's also `toSortedList(Ordering.natural())`. – ColinD Feb 04 '15 at 19:50

RealSkeptic · Answer 5 · 2015-02-04T18:51:53.743

1

You can use

List<Something> list1;
List<Something> list2;

list1.addAll(list2);
Collections.sort( list1 );

(Assuming your list is a list of Comparables)

If you don't want to modify list1, you can use:

List<Something> list3 = new ArrayList<>();
Collections.copy( list3, list1 );
list3.addAll( list2 );
Collections.sort( list3 );

edited Feb 04 '15 at 18:51

answered Feb 04 '15 at 18:46

RealSkeptic

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score 1 · Answer 6 · edited May 23 '17 at 12:06

1

In Java 8 (replace String with whatever type your list contains):

List<String> newList = Stream.concat(listOne.stream(), listTwo.stream()).collect(Collectors.<String>toList());

This will concatenate your two lists to create a new, 3rd list. You can then sort it as you mentioned in your post by using Collections.sort:

Collections.sort(newList);

See also: How do I join two lists in Java?

edited May 23 '17 at 12:06

Community

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answered Feb 04 '15 at 18:49

Christian Wilkie

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score 1 · Answer 7 · answered Feb 04 '15 at 19:24

The following should work with JAVA 8 Stream API

class T {
     @Getter private int sortingProperty; 
    }
 List<T> list3 Stream.of(list1OfT,list2OfT)    
                                 .sorted(Comparing(T::getSortingProperty))
                                 .collect(toList());

How do I concatenate two lists to create a 3rd?

7 Answers7