You can either roll your own implementation of a:
Map<Key, List<Value>>
or use the Multimap from Guava, in their documentation they explain it as:
There are two ways to think of a Multimap conceptually: as a collection of mappings from single keys to single values:
a -> 1 a -> 2 a -> 4 b -> 3 c -> 5
or as a mapping from unique keys to collections of values:
a -> [1, 2, 4] b -> 3 c -> 5
The following blog post explains some of the advantages of using the Guava collection.
There is also the MultiValueMap from the Apache Commons Collections that solves the same problem should you have a preference when it comes to external libraries.