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## 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`

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