Apache Commons Collections | MultiValuedMap Interface

Apache Commons’s MultiValuedMap Interface allows the mapping of a single key to multiple values in Java, unlike java.util.Map where a key can only be associated with a single value.


In the previous post, we have seen how to implement our own Multimap class in Java using a Map and a Collection. In this post, we will discuss various utility methods provided by Apache Commons’s MultiValuedMap Interface –


1. put(), get(), asMap() methods

1. We know that if we put two values into the java.util.Map using the same key, the first value will overriden by the second value. This behavior enforces the map’s key to associate with only one value, which is sometimes undesirable. To avoid it, Apache Commons provides MultiValuedMap that adds the new value to the collection stored against the key. So any put() operations on a key will not override the values set by the previous put() operations on that key.

2. The get() method of Apache Commons’s MultiValuedMap interface returns a collection holding all values associated with the specified key. When containsKey(key) is false, an empty collection is returned unlike Map.get() which returns a null value. Please note that the underlying MultiValuedMap gets updated on doing any changes to the returned collection, and vice versa.

3. We can also convert the MultiValuedMap<K,V> to Map<K,Collection<V>> using asMap() method provided by MultiValuedMap. Any changes made to the returned map or the collections that serve as its values will update the underlying multi-valued map, and vice versa.

Below is simple Java program to demonstrate Apache Commons’s MultiValuedMap class in Java.




John: [Adams, Tyler, Kennedy]
George: [Washington, Bush]

Above program uses ArrayListValuedHashMap implementation of MultiValuedMap interface that internally uses an ArrayList for storing the values for a given key and also allows the duplicate key-value pairs. Some implementations like HashSetValuedHashMap doesn’t allow duplicates. It all depends on the underlying data structure, for instance, an ArrayList supports duplicates whereas a HashSet doesn’t.


2. Iterate over the MultiValuedMap

MultiValuedMap interface provides keySet(), entries(), values(), keys() methods which are similar to the corresponding view collections of Map. To illustrate, consider below multi-valued map which is used in below functions as input to them.

Zachary –> Taylor
John –> Adams, Tyler, Kennedy
George –> Washington, Bush


2.1. keySet()

keySet() method returns a Set view of the keys contained in this multi-valued map.




George: [Washington, Bush]
Zachary: [Taylor]
John: [Adams, Tyler, Kennedy]


2.2. entries()

entries() method returns a Collection view of the mappings contained in this multi-valued map.




George: Washington
George: Bush
Zachary: Taylor
John: Adams
John: Tyler
John: Kennedy


2.3. keys() and values()

key() method returns a MultiSet view of the keys contained in this multi-valued map while the values() method returns a Collection view of all values contained in this multi-valued map.




[George:2, Zachary:1, John:3]
[Washington, Bush, Taylor, Adams, Tyler, Kennedy]


3. Remove and Search keys/values in the MultiValuedMap

Apache Commons’s MultiValuedMap provides remove() method that removes all values associated with the specified key. It also has removeMapping() method that removes the specified key-value pair from the multi-valued map. But note that no method is included for replacing the existing values for a particular key.

For searching key, values or key-value pairs, MultiValuedMap has three methods – containsKey(), containsValue() and containsEntry(), that checks if the multi-valued map contains at least one key-value pair with the specified key, specified value and the specified key-value pair respectively.




MultiValuedMap: {George=[Washington, Bush], John=[Adams, Kennedy]}

George key is deleted
John Adams is removed
John Kennedy is still there!

MultiValuedMap: {John=[Kennedy]}

We have seen that MultiValuedMap interface is implemented as Map<K,Collection<V>>. So one would expect the size() method to return the number of distinct keys. Instead, it returns the exact number of key-value pairs contained in it. We should use keySet().size() or asMap().size() to get the number of distinct keys.

Also See: Guava’s Multimap

References: MultiValuedMap (Apache Commons Collections 4.1 API)

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