com.google.common.collect
Interface Multimap<K,V>

All Known Subinterfaces:

ListMultimap<K,V>, SetMultimap<K,V>, SortedSetMultimap<K,V>

All Known Implementing Classes:

ArrayListMultimap, ForwardingListMultimap, ForwardingMultimap, ForwardingSetMultimap, ForwardingSortedSetMultimap, HashMultimap, ImmutableListMultimap, ImmutableMultimap, ImmutableSetMultimap, LinkedHashMultimap, LinkedListMultimap, TreeMultimap

@GwtCompatible
public interface Multimap<K,V>

A collection that maps keys to values, similar to Map, but in which each key may be associated with multiple values. You can visualize the contents of a multimap either as a map from keys to collections of values:

• a → 1, 2
• b → 3

... or as a single "flattened" collection of key-value pairs:

• a → 1
• a → 2
• b → 3

Important: although the first interpretation resembles how most multimaps are implemented, the design of the Multimap API is based on the second form. So, using the multimap shown above as an example, the size() is 3, not 2, and the values() collection is [1, 2, 3], not [[1, 2], [3]]. For those times when the first style is more useful, use the multimap's asMap() view.

Example

The following code:

   ListMultimap<String, String> multimap = ArrayListMultimap.create();
   for (President pres : US_PRESIDENTS_IN_ORDER) {
     multimap.put(pres.firstName(), pres.lastName());
   }
   for (String firstName : multimap.keySet()) {
     List<String> lastNames = multimap.get(firstName);
     out.println(firstName + ": " + lastNames);
   }

... produces output such as:

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

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. When they support modification, the changes are write-through (they automatically update the backing multimap). These view collections are:

• asMap(), mentioned above
• keys(), keySet(), values(), entries(), which are similar to the corresponding view collections of Map
• and, notably, even the collection returned by get(key) is an active view of the values corresponding to key

The collections returned by the replaceValues and removeAll methods, which contain values that have just been removed from the multimap, are naturally not views.

Subinterfaces

Instead of using the Multimap interface directly, prefer the subinterfaces ListMultimap and SetMultimap. These take their names from the fact that the collections they return from get behave like (and, of course, implement) List and Set, respectively.

For example, the "presidents" code snippet above used a ListMultimap; if it had used a SetMultimap instead, two presidents would have vanished, and last names might or might not appear in chronological order.

Uses

Multimaps are commonly used anywhere a Map<K, Collection<V>> would otherwise have appeared. The advantages include:

• There is no need to populate an empty collection before adding an entry with put.
• get never returns null, only an empty collection.
• It will not retain empty collections after the last value for a key is removed. As a result, containsKey(java.lang.Object) behaves logically, and the multimap won't leak memory.
• The total entry count is available as size().
• Many complex operations become easier; for example, Collections.min(multimap.values()) finds the smallest value across all keys.

Implementations

As always, prefer the immutable implementations, ImmutableListMultimap and ImmutableSetMultimap. General-purpose mutable implementations are listed above under "All Known Implementing Classes". You can also create a custom multimap, backed by any Map and Collection types, using the Multimaps.newMultimap family of methods. Finally, another popular way to obtain a multimap is using Multimaps.index. See the Multimaps class for these and other static utilities related to multimaps.

Other Notes

All methods that modify the multimap are optional. The view collections returned by the multimap may or may not be modifiable. Any modification method that is not supported will throw UnsupportedOperationException.

See the Guava User Guide article on Multimap.

Since:

2.0 (imported from Google Collections Library)

Author:

Jared Levy

Method Summary

Map<K,Collection<V>>	asMap() Returns a map view that associates each key with the corresponding values in the multimap.
void	clear() Removes all key-value pairs from the multimap.
boolean	containsEntry(Object key, Object value) Returns true if the multimap contains the specified key-value pair.
boolean	containsKey(Object key) Returns true if the multimap contains any values for the specified key.
boolean	containsValue(Object value) Returns true if the multimap contains the specified value for any key.
Collection<Map.Entry<K,V>>	entries() Returns a collection of all key-value pairs.
boolean	equals(Object obj) Compares the specified object with this multimap for equality.
Collection<V>	get(K key) Returns a collection view of all values associated with a key.
int	hashCode() Returns the hash code for this multimap.
boolean	isEmpty() Returns true if the multimap contains no key-value pairs.
Multiset<K>	keys() Returns a collection, which may contain duplicates, of all keys.
Set<K>	keySet() Returns the set of all keys, each appearing once in the returned set.
boolean	put(K key, V value) Stores a key-value pair in the multimap.
boolean	putAll(K key, Iterable<? extends V> values) Stores a collection of values with the same key.
boolean	putAll(Multimap<? extends K,? extends V> multimap) Copies all of another multimap's key-value pairs into this multimap.
boolean	remove(Object key, Object value) Removes a single key-value pair from the multimap.
Collection<V>	removeAll(Object key) Removes all values associated with a given key.
Collection<V>	replaceValues(K key, Iterable<? extends V> values) Stores a collection of values with the same key, replacing any existing values for that key.
int	size() Returns the number of key-value pairs in the multimap.
Collection<V>	values() Returns a collection of all values in the multimap.

Method Detail

size

int size()

Returns the number of key-value pairs in the multimap.

isEmpty

boolean isEmpty()

Returns true if the multimap contains no key-value pairs.

containsKey

boolean containsKey(@Nullable
                    Object key)

Returns true if the multimap contains any values for the specified key.

Parameters:

key - key to search for in multimap

containsValue

boolean containsValue(@Nullable
                      Object value)

Returns true if the multimap contains the specified value for any key.

Parameters:

value - value to search for in multimap

containsEntry

boolean containsEntry(@Nullable
                      Object key,
                      @Nullable
                      Object value)

Returns true if the multimap contains the specified key-value pair.

Parameters:

key - key to search for in multimap

value - value to search for in multimap

put

boolean put(@Nullable
            K key,
            @Nullable
            V value)

Stores a key-value pair in the multimap.

Some multimap implementations allow duplicate key-value pairs, in which case put always adds a new key-value pair and increases the multimap size by 1. Other implementations prohibit duplicates, and storing a key-value pair that's already in the multimap has no effect.

Parameters:

key - key to store in the multimap

value - value to store in the multimap

Returns:

true if the method increased the size of the multimap, or false if the multimap already contained the key-value pair and doesn't allow duplicates

remove

boolean remove(@Nullable
               Object key,
               @Nullable
               Object value)

Removes a single key-value pair from the multimap.

Parameters:

key - key of entry to remove from the multimap

value - value of entry to remove the multimap

Returns:

true if the multimap changed

putAll

boolean putAll(@Nullable
               K key,
               Iterable<? extends V> values)

Stores a collection of values with the same key.

Parameters:

key - key to store in the multimap

values - values to store in the multimap

Returns:

true if the multimap changed

putAll

boolean putAll(Multimap<? extends K,? extends V> multimap)

Copies all of another multimap's key-value pairs into this multimap. The order in which the mappings are added is determined by multimap.entries().

Parameters:

multimap - mappings to store in this multimap

Returns:

true if the multimap changed

replaceValues

Collection<V> replaceValues(@Nullable
                            K key,
                            Iterable<? extends V> values)

Stores a collection of values with the same key, replacing any existing values for that key.

Parameters:

key - key to store in the multimap

values - values to store in the multimap

Returns:

the collection of replaced values, or an empty collection if no values were previously associated with the key. The collection may be modifiable, but updating it will have no effect on the multimap.

removeAll

Collection<V> removeAll(@Nullable
                        Object key)

Removes all values associated with a given key.

Parameters:

key - key of entries to remove from the multimap

Returns:

the collection of removed values, or an empty collection if no values were associated with the provided key. The collection may be modifiable, but updating it will have no effect on the multimap.

clear

void clear()

Removes all key-value pairs from the multimap.

get

Collection<V> get(@Nullable
                  K key)

Returns a collection view of all values associated with a key. If no mappings in the multimap have the provided key, an empty collection is returned.

Changes to the returned collection will update the underlying multimap, and vice versa.

Parameters:

key - key to search for in multimap

Returns:

the collection of values that the key maps to

keySet

Set<K> keySet()

Returns the set of all keys, each appearing once in the returned set. Changes to the returned set will update the underlying multimap, and vice versa.

Returns:

the collection of distinct keys

keys

Multiset<K> keys()

Returns a collection, which may contain duplicates, of all keys. The number of times of key appears in the returned multiset equals the number of mappings the key has in the multimap. Changes to the returned multiset will update the underlying multimap, and vice versa.

Returns:

a multiset with keys corresponding to the distinct keys of the multimap and frequencies corresponding to the number of values that each key maps to

values

Collection<V> values()

Returns a collection of all values in the multimap. Changes to the returned collection will update the underlying multimap, and vice versa.

Returns:

collection of values, which may include the same value multiple times if it occurs in multiple mappings

entries

Collection<Map.Entry<K,V>> entries()

Returns a collection of all key-value pairs. Changes to the returned collection will update the underlying multimap, and vice versa. The entries collection does not support the add or addAll operations.

Returns:

collection of map entries consisting of key-value pairs

asMap

Map<K,Collection<V>> asMap()

Returns a map view that associates each key with the corresponding values in the multimap. Changes to the returned map, such as element removal, will update the underlying multimap. The map does not support setValue() on its entries, put, or putAll.

When passed a key that is present in the map, asMap().get(Object) has the same behavior as get(K), returning a live collection. When passed a key that is not present, however, asMap().get(Object) returns null instead of an empty collection.

Returns:

a map view from a key to its collection of values

equals

boolean equals(@Nullable
               Object obj)

Compares the specified object with this multimap for equality. Two multimaps are equal when their map views, as returned by asMap(), are also equal.

In general, two multimaps with identical key-value mappings may or may not be equal, depending on the implementation. For example, two SetMultimap instances with the same key-value mappings are equal, but equality of two ListMultimap instances depends on the ordering of the values for each key.

A non-empty SetMultimap cannot be equal to a non-empty ListMultimap, since their asMap() views contain unequal collections as values. However, any two empty multimaps are equal, because they both have empty asMap() views.

Overrides:

equals in class Object

hashCode

int hashCode()

Returns the hash code for this multimap.

The hash code of a multimap is defined as the hash code of the map view, as returned by asMap().

Overrides:

hashCode in class Object