MultiValueMapAdapter.getFirst fails with IndexOutOfBoundsException in case of empty List

[vlavasa]

If You create MultiValueMap via MultiValueMapAdapter, and value list is empty, call of method getFirst fail on IndexOutOfBounds exception:

private static class MultiValueMapAdapter<K, V> implements MultiValueMap<K, V>, Serializable {

		private final Map<K, List<V>> map;

		public MultiValueMapAdapter(Map<K, List<V>> map) {
			Assert.notNull(map, "'map' must not be null");
			this.map = map;
		}

		@Override
		@Nullable
		public V getFirst(K key) {
			List<V> values = this.map.get(key);
			return (values != null ? values.get(0) : null);
		}

Good solution is:

return (values != null && !values.isEmpty() ? values.get(0) : null);

[jhoeller]

The common scenario is that such a list either contains one or more values or isn't set at all (in case of no value associated with a key). That said, point taken, we should align this with the existing LinkedMultiValueMap implementation where such a defensive emptiness check is present already.

[vlavasa]

The common scenario is that such a list either contains one or more values or isn't set at all (in case of no value associated with a key). That said, point taken, we should align this with the existing LinkedMultiValueMap implementation where such a defensive emptiness check is present already.

OK, but the adapter is used in method:
org.springframework.util.CollectionUtils#toMultiValueMap

and there is accepted any Map<K, List> map

[jhoeller]

Good point, it can be constructed with any target map content, including empty list values there. Also, an addAll call with an empty list argument will also lead to such empty list state, just like in LinkedMultiValueMap. I was just pointing out why it is uncommon to encounter empty lists there, otherwise it would be surprising for this bug to remain unnoticed so many years.

Thanks for raising this, in any case! I've marked it for a fix in the upcoming 5.2.7 and 5.1.16 releases.

[midumitrescu]

@jhoeller could I give it a go?

I have also prepared unit tests.