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

You can create your generic Java 8 utility method to concat any number of lists.

@SafeVarargs
public static <T> List<T> concat(List<T>... lists) {
    return Stream.of(lists).flatMap(List::stream).collect(Collectors.toList());
}

Answer 2

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<T> join(List<T>... lists) {
      List<T> result = new ArrayList<T>();
      for(List<T> list : lists) {
         result.addAll(list);
      }
      return results;
   }

}

Then you can do things like

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

Answer 3

In Java 8 (the other way):

List<?> newList = 
Stream.of(list1, list2).flatMap(List::stream).collect(Collectors.toList());

Answer 4

Found this question looking to concatenate arbitrary amount of lists, not minding external libraries. So, perhaps it will help someone else:

com.google.common.collect.Iterables#concat()

Useful if you want to apply the same logic to a number of different collections in one for().

Answer 5

Another Java 8 one-liner:

List<String> newList = Stream.of(listOne, listTwo)
                             .flatMap(x -> x.stream())
                             .collect(Collectors.toList());

As a bonus, since Stream.of() is variadic, you may concatenate as many lists as you like.

List<String> newList = Stream.of(listOne, listTwo, listThree)
                             .flatMap(x -> x.stream())
                             .collect(Collectors.toList());

Answer 6

Java 8 version with support for joining by object key:

public List<SomeClass> mergeLists(final List<SomeClass> left, final List<SomeClass> right, String primaryKey) {
    final Map<Object, SomeClass> mergedList = new LinkedHashMap<>();

    Stream.concat(left.stream(), right.stream())
        .map(someObject -> new Pair<Object, SomeClass>(someObject.getSomeKey(), someObject))
        .forEach(pair-> mergedList.put(pair.getKey(), pair.getValue()));

    return new ArrayList<>(mergedList.values());
}