Multimap - Data Structures

A multimap (sometimes also multihash) 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 (see for example C++ Standard Template Library containers). Often the multimap is implemented as a map with lists or sets as the map values.


  • In a student enrollment system, where students may be enrolled in multiple classes simultaneously, there might be an association for each enrollment of a student in a course, where the key is the student ID and the value is the course ID. If a student is enrolled in three courses, there will be three associations containing the same key.
  • The index of a book may report any number of references for a given index term, and thus may be coded as a multimap from index terms to any number of reference locations.

Language support

C++'s Standard Template Library provides the multimap container for the sorted multimap using a self -balancing binary search tree,[1] and SGI's STL extension provides the hash _multimap container, which implements a multimap using a hash table. Apache Commons Collections provides a MultiMap interface for Java.[3] It also provides a MultiValueMap.implementing class that makes a MultiMap out of a Map object and a type of Collection. Google Collections also provides an interface Multimap and implementations.

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