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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;
 
A collection that maps keys to values, similar to java.util.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:

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 com.google.common.collect.ListMultimap and SetMultimap. These take their names from the fact that the collections they return from get behave like (and, of course, implement) java.util.List and java.util.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 java.lang.Object.equals(java.lang.Object) 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, com.google.common.collect.ImmutableListMultimap and com.google.common.collect.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 java.util.Collection types, using the Multimaps.newMultimap family of methods. Finally, another popular way to obtain a multimap is using Multimaps.index. See the com.google.common.collect.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 java.lang.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 this multimap.

Note: this method does not return the number of distinct keys in the multimap, which is given by keySet().size() or asMap().size(). See the opening section of the Multimap class documentation for clarification.

  int size();

  
Returns true if this multimap contains no key-value pairs. Equivalent to size() == 0, but can in some cases be more efficient.
  boolean isEmpty();

  
Returns true if this multimap contains at least one key-value pair with the key key.
  boolean containsKey(@Nullable Object key);

  
Returns true if this multimap contains at least one key-value pair with the value value.
  boolean containsValue(@Nullable Object value);

  
Returns true if this multimap contains at least one key-value pair with the key key and the value value.
  boolean containsEntry(@Nullable Object key, @Nullable Object value);
  // Modification Operations

  
Stores a key-value pair in this 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.

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 with the key key and the value value from this multimap, if such exists. If multiple key-value pairs in the multimap fit this description, which one is removed is unspecified.

Returns:
true if the multimap changed
  boolean remove(@Nullable Object key, @Nullable Object value);
  // Bulk Operations

  
Stores a key-value pair in this multimap for each of values, all using the same key, key. Equivalent to (but expected to be more efficient than):
   for (V value : values) {
     put(key, value);
   }

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

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

  
Stores all key-value pairs of multimap in this multimap, in the order returned by multimap.entries().

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).

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 the key 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.

Returns:
the values that were removed (possibly empty). The returned 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, leaving it isEmpty().
  void clear();
  // Views

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

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

  Collection<V> get(@Nullable K key);

  
Returns a view collection of all distinct keys contained in this multimap. Note that the key set contains a key if and only if this multimap maps that key to at least one value.

Changes to the returned set will update the underlying multimap, and vice versa. However, adding to the returned set is not possible.

  Set<K> keySet();

  
Returns a view collection containing the key from each key-value pair in this multimap, without collapsing duplicates. This collection has the same size as this multimap, and keys().count(k) == get(k).size() for all k.

Changes to the returned multiset will update the underlying multimap, and vice versa. However, adding to the returned collection is not possible.

  Multiset<K> keys();

  
Returns a view collection containing the value from each key-value pair contained in this multimap, without collapsing duplicates (so values().size() == size()).

Changes to the returned collection will update the underlying multimap, and vice versa. However, adding to the returned collection is not possible.

  Collection<V> values();

  
Returns a view collection of all key-value pairs contained in this multimap, as java.util.Map.Entry instances.

Changes to the returned collection or the entries it contains will update the underlying multimap, and vice versa. However, adding to the returned collection is not possible.

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

  
Returns a view of this multimap as a Map from each distinct key to the nonempty collection of that key's associated values. Note that this.asMap().get(k) is equivalent to this.get(k) only when k is a key contained in the multimap; otherwise it returns null as opposed to an empty collection.

Changes to the returned map or the collections that serve as its values will update the underlying multimap, and vice versa. The map does not support put or putAll, nor do its entries support java.util.Map.Entry.setValue(java.lang.Object).

  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 com.google.common.collect.ListMultimap instances depends on the ordering of the values for each key.

A non-empty SetMultimap cannot be equal to a non-empty com.google.common.collect.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 asMap().

  int hashCode();
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