How do I join two lists in Java?

Question

Conditions: do not modifiy the original lists; JDK only, no external libraries. Bonus points for a one-liner or a JDK 1.3 version.

Is there a simpler way than:

List<String> newList = new ArrayList<String>();
newList.addAll(listOne);
newList.addAll(listTwo);

Answer 1

Slightly simpler:

List newList = new ArrayList(listOne); newList.addAll(listTwo);

Answer 2

Off the top of my head, I can shorten it by one line:

List<String> newList = new ArrayList<String>(listOne);
newList.addAll(listTwo);

Answer 3

A little shorter would be:

List<String> newList = new ArrayList<String>(listOne);
newList.addAll(listTwo);

Answer 4

Perhaps I'm just too used to other languages... but, any chance this would work?

List newList = (new ArrayList(listOne)).addAll(listTwo);

---

Then, how about this? ;)

(List newList = new ArrayList(listOne)).addAll(listTwo);

Answer 5

Probably not simpler, but intriguing and ugly:

    List<String> newList = new ArrayList<String>() { { addAll(listOne); addAll(listTwo); } };

Don't use it in production code... ;)

Answer 6

You can do a oneliner if the target list is predeclared.

(newList = new ArrayList<String>(list1)).addAll(list2);

Answer 7

You could do it with a static import and a helper class

nb the generification of this class could probably be improved

public class Lists {

   private Lists() { } // can't be instantiated

   public static List join(List... lists) {
      List result = new ArrayList();
      for(List list : lists) {
         result.addAll(list);
      }
      return results;
   }

}

Then you can do things like

  import static Lists.join;
  List result = join(list1, list2, list3, list4);

Answer 8

I can't improve on the two-liner in the general case without introducing your own utility method, but if you do have lists of Strings and you're willing to assume those Strings don't contain commas, you can pull this long one-liner:

List<String> newList = new ArrayList<String>(Arrays.asList((listOne.toString().subString(1, listOne.length() - 1) + ", " + listTwo.toString().subString(1, listTwo.length() - 1)).split(", ")));

If you drop the generics, this should be JDK 1.4 compliant (though I haven't tested that). Also not recommended for production code ;-)

Answer 9

I'm not claiming that it's simple, but you mentioned bonus for one-liners ;-)

Collection mergedList = Collections.list(new sun.misc.CompoundEnumeration(new Enuleration[] {
    new Vector(list1).elements(),
    new Vector(list2).elements(),
    ...
}))

Answer 10

Use a Helper class.

I suggest:

    public static <E> Collection<E> addAll(Collection<E> dest, Collection<? extends E>... src) {
     for(Collection<? extends E> c : src) {
      dest.addAll(c);
     }

     return dest;
    }

    public static void main(String[] args) {
     System.out.println(addAll(new ArrayList<Object>(), Arrays.asList(1,2,3), Arrays.asList("a", "b", "c")));

     // does not compile
     // System.out.println(addAll(new ArrayList<Integer>(), Arrays.asList(1,2,3), Arrays.asList("a", "b", "c")));

     System.out.println(addAll(new ArrayList<Integer>(), Arrays.asList(1,2,3), Arrays.asList(4, 5, 6)));
    }

Answer 11

List<?> newList = ListUtils.combine(list1, list2);

Oh, you have to implement the combine method :-)

Answer 12

it is ok for you to try to make it one liner, but it will not be ok for production and other purposes or people to understand your code after it's off your hands. the clearer the better

Answer 13

hmmm it exists ListUtils.union(list1,list2); works cool!