The key-value data model allows efficient storage and retrieval of data but is limited to associating a single value against a given key. In this paper, we present the implementation of a Multimap data structure in OpenJDK that allows the association of multiple values with a single key. The implementation consists of three layers, an interface MultiMap, an abstract class AbstractMultiMap which provides common functionality, and three concrete implementations: ArrayListMultiMap, SetMultiMap, and TreeMultiMap. Each implementation provides unique properties for the keys and the associated values. Experiments compare the performance of inserting key-value pairs in OpenJDK multimap implementation, multimap in the Google Guava library, and multimap in C++.