Multimap

In computer science, a multimap (sometimes also multihash or multidict) is a generalization of a map or associative array abstract data type in which more than one value may be associated with and returned for a given key. Both map and multimap are particular cases of containers (for example, see C++ Standard Template Library containers). Often the multimap is implemented as a map with lists or sets as the map values.
Language support.
C++.
C++'s Standard Template Library provides the codice_1 container for the sorted multimap using a self-balancing binary search tree, and SGI's STL extension provides the codice_2 container, which implements a multimap using a hash table.
As of C++11, the Standard Template Library provides the codice_3 for the unordered multimap.
Dart.
Quiver provides a Multimap for Dart.
Java.
Apache Commons Collections provides a MultiMap interface for Java. It also provides a MultiValueMap implementing class that makes a MultiMap out of a Map object and a type of Collection.
Google Guava provides a Multimap interface and implementations of it.
Python.
Python provides a codice_4 class that can be used to create a multimap. The user can instantiate the class as codice_5.
OCaml.
OCaml's standard library module codice_6 implements a hash table where it's possible to store multiple values for a key.
Scala.
The Scala programming language's API also provides Multimap and implementations.