Why is insertion order not preserved in MultiMap?

Question

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

          myMultimap.put("classlabel", "tid");
          myMultimap.put("Y", "1");
          myMultimap.put("Y", "2");
          myMultimap.put("N", "4"); 

          // Iterating over entire MutliMap
          for(String value : myMultimap.values()) {           
              System.out.print(value);
          }
    }
}

The above code prints 1 2 tid 4.

I am not understanding why it is not printing tid 1 2 4.

This is the contract of `MultiMap`s... This is the price to pay for query-in performances. — Jean-Baptiste Yunès, Dec 04 '14 at 20:57
One option is to use `Map>` instead. That will give you the insert-in-order property. — merlin2011, Dec 04 '14 at 20:58

score 26 · Accepted Answer · answered Dec 04 '14 at 21:00

26

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

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

answered Dec 04 '14 at 21:00

Paul Lo

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Does LinkedListMultimap allows duplicate key ? – vikramvi Jan 05 '18 at 15:04
@vikramvi Yes LinkedListMultimap allows duplicates – raman rayat Aug 03 '18 at 18:49

score 3 · Answer 2 · answered Dec 04 '14 at 21:05

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.

Why is insertion order not preserved in MultiMap?

2 Answers2

Linked