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  /*
   * Copyright (C) 2007 The Guava Authors
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   * http://www.apache.org/licenses/LICENSE-2.0
   *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
 
 package com.google.common.collect;
 
 
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
 
 import  javax.annotation.Nullable;

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 nonempty 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 (or create a Map<K, Collection<V>> in the first place).

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]  // Remember, Quincy!
   George: [Washington, Bush, Bush]
   Grover: [Cleveland, Cleveland]        // Two, non-consecutive terms, rep'ing NJ!
   ...

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.

Warning: instances of type Multimap may not implement Object.equals in the way you expect (multimaps containing the same key-value pairs, even in the same order, may or may not be equal). The recommended subinterfaces provide a much stronger guarantee.

Comparison to a map of collections

Multimaps are commonly used in places where a Map<K, Collection<V>> would otherwise have appeared. The differences include:

  • There is no need to populate an empty collection before adding an entry with put.
  • get never returns null, only an empty collection.
  • 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 have zero associated values has the effect of removing that key from the multimap.
  • 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

As with Map, the behavior of a Multimap is not specified if key objects already present in the multimap change in a manner that affects equals comparisons. Use caution if mutable objects are used as keys in a Multimap.

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.

Author(s):
Jared Levy
Since:
2.0 (imported from Google Collections Library)
public interface Multimap<K, V> {
  // Query Operations

  
Returns the number of key-value pairs in the multimap.
  int size();

  
Returns true if the multimap contains no key-value pairs.
  boolean isEmpty();

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

Parameters:
key key to search for in multimap
  boolean containsKey(@Nullable Object key);

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

Parameters:
value value to search for in multimap
  boolean containsValue(@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
  boolean containsEntry(@Nullable Object key, @Nullable Object value);
  // Modification Operations

  
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
  boolean put(@Nullable K key, @Nullable V 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
  boolean remove(@Nullable Object key, @Nullable Object value);
  // Bulk Operations

  
Stores key-value pairs in this multimap with one key and multiple values.

This is equivalent to

   for (V value : values) {
     put(key, value);
    }

In particular, this is a no-op if values is empty.

Parameters:
key key to store in the multimap
values values to store in the multimap
Returns:
true if the multimap changed
  boolean putAll(@Nullable K keyIterable<? extends V> values);

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

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

If values is empty, this is equivalent to removeAll(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.
  Collection<V> replaceValues(@Nullable K keyIterable<? extends V> values);

  
Removes all values associated with a given key.

Once this method returns, key will not be mapped to any values, so it will not appear in keySet(), asMap(), or any other views.

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.
  Collection<V> removeAll(@Nullable Object key);

  
Removes all key-value pairs from the multimap.
  void clear();
  // Views

  
Returns a collection view containing the values associated with key in this multimap, if any. Note that even when (containsKey(key) is false, get(key) still returns an empty collection, not null.

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

Parameters:
key key to search for in multimap
Returns:
a view collection containing the zero or more values that the key maps to
  Collection<V> get(@Nullable K key);

  
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.

Note that the key set contains a key if and only if this multimap maps that key to at least one value.

Returns:
the collection of distinct keys
  Set<K> keySet();

  
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
  Multiset<K> keys();

  
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
  Collection<V> values();

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

  
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, 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
  Map<K, Collection<V>> asMap();
  // Comparison and hashing

  
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.

  boolean equals(@Nullable Object obj);

  
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 Multimap.asMap.

  int hashCode();
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