Why is insertion order not preserved in MultiMap?

Question

public class MultiMap_Test {
    public static void main(String[] args) {
        Multimap myMultimap = ArrayListMultimap.create();

          myMu         


        

Answer 1

Use LinkedListMultimap instead if you want to keep the insertion order:

Multimap<String, String> myMultimap = LinkedListMultimap.create();

Answer 2

Why is insertion order not preserved in MultiMap?

In fact, your problem is not with MultiMap but with the selected implementation. ArrayListMultimap uses a HashMap<K, Collection<V>> as implementation of the backing Map<K, Collection<V>>:

public static <K, V> ArrayListMultimap<K, V> create() {
    return new ArrayListMultimap<K, V>();
}

//...

private ArrayListMultimap() {
    super(new HashMap<K, Collection<V>>());
    expectedValuesPerKey = DEFAULT_VALUES_PER_KEY;
}

And HashMap doesn't preserve the order of the insertion of the elements.