MultiMap in Java Guava

10 Jul, 2020
Introduction : A Multimap is a general way to associate keys with arbitrarily many values. Guava’s Multimap framework makes it easy to handle a mapping from keys to multiple values. There are two ways to think of a Multimap conceptually :1) As a collection of mappings from single keys to single values.
a -> 1
a -> 2
a -> 4
b -> 3
c -> 5

2) As a mapping from unique keys to collections of values.
a -> [1, 2, 4]
b -> [3]
c -> [5] 

Declaration : The declaration of com.google.common.collect.Multimap<K, V> interface is as :
@GwtCompatible
public interface Multimap

Below given is the list of some methods provided by Guava’s Multimap Interface :Views : Multimap also supports a number of powerful views. Much of the power of the multimap API comes from the view collections it provides. These always reflect the latest state of the multimap itself.
asMap views any Multimap<K, V> as a Map<K, Collection<V>>. The returned map supports remove, and changes to the returned collections write through, but the map does not support put or putAll.
entries views the Collection<Map.Entry<K, V>> of all entries in the Multimap. (For a SetMultimap, this is a Set.)
keySet views the distinct keys in the Multimap as a Set.
keys views the keys of the Multimap as a Multiset, with multiplicity equal to the number of values associated to that key. Elements can be removed from the Multiset, but not added, changes will write through.
values() views all the values in the Multimap as a “flattened” Collection<V>, all as one collection. This is similar to Iterables.concat(multimap.asMap().values()), but returns a full Collection instead.
Some other methods provided by Guava’s Multimap Interface are :
Multimap V/s Map : A Multimap<K, V> is not a Map<K, Collection<V>>, though such a map might be used in a Multimap implementation. Below given are the differences :
Multimap.get(key) always returns a non-null, possibly empty collection. This doesn’t imply that the multimap spends any memory associated with the key, but instead, the returned collection is a view that allows you to add associations with the key if you like.
If you prefer the more Map-like behavior of returning null for keys that aren’t in the multimap, use the asMap() view to get a Map<K, Collection<V>>.
Multimap.containsKey(key) is true if and only if there are any elements associated with the specified key. In particular, if a key k was previously associated with one or more values which have since been removed from the multimap, Multimap.containsKey(k) will return false.
Multimap.entries() returns all entries for all keys in the Multimap. If you want all key-collection entries, use asMap().entrySet().
Multimap.size() returns the number of entries in the entire multimap, not the number of distinct keys. Use Multimap.keySet().size() instead to get the number of distinct keys.
Implementations : Multimap provides a wide variety of implementations. You can use it in most places you would have used a Map<K, Collection<V>>.
Advantages : Multimaps are commonly used in places where a Map<K, Collection<V>> would otherwise have appeared.
There is no need to populate an empty collection before adding an entry with put().
The get() method never returns null, only an empty collection (we do not need to check against null like in Map<String, Collection<V>> test case).
A key is contained in the Multimap if and only if it maps to at least one value. Any operation that causes a key to has zero associated values, has the effect of removing that key from the Multimap.
The total entry values count is available as size().
References :Google Guava
java-guava
java-map
Java
Java
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