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:

Answer 1

Java 8 version with support for joining by object key:

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

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

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