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A collection similar to a Map, but which may associate multiple values with a single key. If you call put(K, V) twice, with the same key but different values, the multimap contains mappings from the key to both values.

The methods get(K), keySet(), keys(), values(), entries(), and asMap() return collections that are views of the multimap. If the multimap is modifiable, updating it can change the contents of those collections, and updating the collections will change the multimap. In contrast, replaceValues(K, java.lang.Iterable) and removeAll(java.lang.Object) return collections that are independent of subsequent multimap changes.

Depending on the implementation, a multimap may or may not allow duplicate key-value pairs. In other words, the multimap contents after adding the same key and value twice varies between implementations. In multimaps allowing duplicates, the multimap will contain two mappings, and get will return a collection that includes the value twice. In multimaps not supporting duplicates, the multimap will contain a single mapping from the key to the value, and get will return a collection that includes the value once.

All methods that alter the multimap are optional, and the views returned by the multimap may or may not be modifiable. When modification isn't supported, those methods will throw an UnsupportedOperationException.

Since:

2 (imported from Google Collections Library)

Author:

Jared Levy

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 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.

The collections returned by asMap().get(Object) have the same behavior as those returned by get(K).

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