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

In Java 8:

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

Answer 2

This is simple and just one line, but will add the contents of listTwo to listOne. Do you really need to put the contents in a third list?

Collections.addAll(listOne, listTwo.toArray());

Answer 3

Slightly simpler:

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

Answer 4

The smartest in my opinion:

/**
 * @param smallLists
 * @return one big list containing all elements of the small ones, in the same order.
 */
public static <E> List<E> concatenate (final List<E> ... smallLists)
{
    final ArrayList<E> bigList = new ArrayList<E>();
    for (final List<E> list: smallLists)
    {
        bigList.addAll(list);
    }
    return bigList;
}

Answer 5

If you want to do this statically you can the following.

The examples uses 2 EnumSets in natural-order (==Enum-order) A, B and joins then in an ALL list.

public static final EnumSet<MyType> CATEGORY_A = EnumSet.of(A_1, A_2);
public static final EnumSet<MyType> CATEGORY_B = EnumSet.of(B_1, B_2, B_3);

public static final List<MyType> ALL = 
              Collections.unmodifiableList(
                  new ArrayList<MyType>(CATEGORY_A.size() + CATEGORY_B.size())
                  {{
                      addAll(CATEGORY_A);
                      addAll(CATEGORY_B);
                  }}
              );

Answer 6

No way near one-liner, but I think this is the simplest:

List<String> newList = new ArrayList<String>(l1);
newList.addAll(l2);

for(String w:newList)
        System.out.printf("%s ", w);