com.google.guava/guava/25.1-jre : com/google/common/collect/Multimap.java

Multimap
https://github.com/google/guava/guava: Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more.
The Apache Software License, Version 2.0
Kevin Bourrillion (Google)
/*
 * 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 static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.GwtCompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.CompatibleWith;
import java.util.Collection;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
import java.util.function.BiConsumer;
import org.checkerframework.checker.nullness.qual.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 and may or may not have the same hashCode. The recommended subinterfaces provide much stronger guarantees. 
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: Jared Levy
Since: 2.0/**
 * A collection that maps keys to values, similar to {@link Map}, but in which each key may be
 * associated with <i>multiple</i> values. You can visualize the contents of a multimap either as a
 * map from keys to <i>nonempty</i> collections of values:
 *
 * <ul>
 *   <li>a → 1, 2
 *   <li>b → 3
 * </ul>
 *
 * ... or as a single "flattened" collection of key-value pairs:
 *
 * <ul>
 *   <li>a → 1
 *   <li>a → 2
 *   <li>b → 3
 * </ul>
 *
 * <p><b>Important:</b> although the first interpretation resembles how most multimaps are
 * <i>implemented</i>, the design of the {@code Multimap} API is based on the <i>second</i> form.
 * So, using the multimap shown above as an example, the {@link #size} is {@code 3}, not {@code 2},
 * and the {@link #values} collection is {@code [1, 2, 3]}, not {@code [[1, 2], [3]]}. For those
 * times when the first style is more useful, use the multimap's {@link #asMap} view (or create a
 * {@code Map<K, Collection<V>>} in the first place).
 *
 * <h3>Example</h3>
 *
 * <p>The following code:
 *
 * <pre>{@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);
 * }
 * }</pre>
 *
 * ... produces output such as:
 *
 * <pre>{@code
 * Zachary: [Taylor]
 * John: [Adams, Adams, Tyler, Kennedy]  // Remember, Quincy!
 * George: [Washington, Bush, Bush]
 * Grover: [Cleveland, Cleveland]        // Two, non-consecutive terms, rep'ing NJ!
 * ...
 * }</pre>
 *
 * <h3>Views</h3>
 *
 * <p>Much of the power of the multimap API comes from the <i>view collections</i> it provides.
 * These always reflect the latest state of the multimap itself. When they support modification, the
 * changes are <i>write-through</i> (they automatically update the backing multimap). These view
 * collections are:
 *
 * <ul>
 *   <li>{@link #asMap}, mentioned above
 *   <li>{@link #keys}, {@link #keySet}, {@link #values}, {@link #entries}, which are similar to the
 *       corresponding view collections of {@link Map}
 *   <li>and, notably, even the collection returned by {@link #get get(key)} is an active view of
 *       the values corresponding to {@code key}
 * </ul>
 *
 * <p>The collections returned by the {@link #replaceValues replaceValues} and {@link #removeAll
 * removeAll} methods, which contain values that have just been removed from the multimap, are
 * naturally <i>not</i> views.
 *
 * <h3>Subinterfaces</h3>
 *
 * <p>Instead of using the {@code Multimap} interface directly, prefer the subinterfaces {@link
 * ListMultimap} and {@link SetMultimap}. These take their names from the fact that the collections
 * they return from {@code get} behave like (and, of course, implement) {@link List} and {@link
 * Set}, respectively.
 *
 * <p>For example, the "presidents" code snippet above used a {@code ListMultimap}; if it had used a
 * {@code SetMultimap} instead, two presidents would have vanished, and last names might or might
 * not appear in chronological order.
 *
 * <p><b>Warning:</b> instances of type {@code Multimap} may not implement {@link 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 and may or may not have the same {@code hashCode}. The recommended subinterfaces
 * provide much stronger guarantees.
 *
 * <h3>Comparison to a map of collections</h3>
 *
 * <p>Multimaps are commonly used in places where a {@code Map<K, Collection<V>>} would otherwise
 * have appeared. The differences include:
 *
 * <ul>
 *   <li>There is no need to populate an empty collection before adding an entry with {@link #put
 *       put}.
 *   <li>{@code get} never returns {@code null}, only an empty collection.
 *   <li>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
 *       <i>removing</i> that key from the multimap.
 *   <li>The total entry count is available as {@link #size}.
 *   <li>Many complex operations become easier; for example, {@code
 *       Collections.min(multimap.values())} finds the smallest value across all keys.
 * </ul>
 *
 * <h3>Implementations</h3>
 *
 * <p>As always, prefer the immutable implementations, {@link ImmutableListMultimap} and {@link
 * ImmutableSetMultimap}. General-purpose mutable implementations are listed above under "All Known
 * Implementing Classes". You can also create a <i>custom</i> multimap, backed by any {@code Map}
 * and {@link Collection} types, using the {@link Multimaps#newMultimap Multimaps.newMultimap}
 * family of methods. Finally, another popular way to obtain a multimap is using {@link
 * Multimaps#index Multimaps.index}. See the {@link Multimaps} class for these and other static
 * utilities related to multimaps.
 *
 * <h3>Other Notes</h3>
 *
 * <p>As with {@code Map}, the behavior of a {@code Multimap} is not specified if key objects
 * already present in the multimap change in a manner that affects {@code equals} comparisons. Use
 * caution if mutable objects are used as keys in a {@code Multimap}.
 *
 * <p>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
 * {@link UnsupportedOperationException}.
 *
 * <p>See the Guava User Guide article on <a href=
 * "https://github.com/google/guava/wiki/NewCollectionTypesExplained#multimap"> {@code
 * Multimap}</a>.
 *
 * @author Jared Levy
 * @since 2.0
 */
@GwtCompatible
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. /**
   * Returns the number of key-value pairs in this multimap.
   *
   * <p><b>Note:</b> this method does not return the number of <i>distinct keys</i> in the multimap,
   * which is given by {@code keySet().size()} or {@code asMap().size()}. See the opening section of
   * the {@link 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. /**
   * Returns {@code true} if this multimap contains no key-value pairs. Equivalent to {@code 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. /**
   * Returns {@code true} if this multimap contains at least one key-value pair with the key {@code
   * key}.
   */
  boolean containsKey(@CompatibleWith("K") @Nullable Object key);

  Returns true if this multimap contains at least one key-value pair with the value value. /**
   * Returns {@code true} if this multimap contains at least one key-value pair with the value
   * {@code value}.
   */
  boolean containsValue(@CompatibleWith("V") @Nullable Object value);

  Returns true if this multimap contains at least one key-value pair with the key 
key and the value value. /**
   * Returns {@code true} if this multimap contains at least one key-value pair with the key {@code
   * key} and the value {@code value}.
   */
  boolean containsEntry(
      @CompatibleWith("K") @Nullable Object key, @CompatibleWith("V") @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/**
   * Stores a key-value pair in this multimap.
   *
   * <p>Some multimap implementations allow duplicate key-value pairs, in which case {@code 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.
   *
   * @return {@code true} if the method increased the size of the multimap, or {@code false} if the
   *     multimap already contained the key-value pair and doesn't allow duplicates
   */
  @CanIgnoreReturnValue
  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/**
   * Removes a single key-value pair with the key {@code key} and the value {@code value} from this
   * multimap, if such exists. If multiple key-value pairs in the multimap fit this description,
   * which one is removed is unspecified.
   *
   * @return {@code true} if the multimap changed
   */
  @CanIgnoreReturnValue
  boolean remove(
      @CompatibleWith("K") @Nullable Object key, @CompatibleWith("V") @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/**
   * Stores a key-value pair in this multimap for each of {@code values}, all using the same key,
   * {@code key}. Equivalent to (but expected to be more efficient than):
   *
   * <pre>{@code
   * for (V value : values) {
   *   put(key, value);
   * }
   * }</pre>
   *
   * <p>In particular, this is a no-op if {@code values} is empty.
   *
   * @return {@code true} if the multimap changed
   */
  @CanIgnoreReturnValue
  boolean putAll(@Nullable K key, Iterable<? 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/**
   * Stores all key-value pairs of {@code multimap} in this multimap, in the order returned by
   * {@code multimap.entries()}.
   *
   * @return {@code true} if the multimap changed
   */
  @CanIgnoreReturnValue
  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./**
   * Stores a collection of values with the same key, replacing any existing values for that key.
   *
   * <p>If {@code values} is empty, this is equivalent to {@link #removeAll(Object) removeAll(key)}.
   *
   * @return the collection of replaced values, or an empty collection if no values were previously
   *     associated with the key. The collection <i>may</i> be modifiable, but updating it will have
   *     no effect on the multimap.
   */
  @CanIgnoreReturnValue
  Collection<V> replaceValues(@Nullable K key, Iterable<? 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./**
   * Removes all values associated with the key {@code key}.
   *
   * <p>Once this method returns, {@code key} will not be mapped to any values, so it will not
   * appear in {@link #keySet()}, {@link #asMap()}, or any other views.
   *
   * @return the values that were removed (possibly empty). The returned collection <i>may</i> be
   *     modifiable, but updating it will have no effect on the multimap.
   */
  @CanIgnoreReturnValue
  Collection<V> removeAll(@CompatibleWith("K") @Nullable Object key);

  Removes all key-value pairs from the multimap, leaving it empty. /** Removes all key-value pairs from the multimap, leaving it {@linkplain #isEmpty empty}. */
  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.
/**
   * Returns a view collection of the values associated with {@code key} in this multimap, if any.
   * Note that when {@code containsKey(key)} is false, this returns an empty collection, not {@code
   * null}.
   *
   * <p>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.
/**
   * Returns a view collection of all <i>distinct</i> 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.
   *
   * <p>Changes to the returned set will update the underlying multimap, and vice versa. However,
   * <i>adding</i> 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.
/**
   * Returns a view collection containing the key from each key-value pair in this multimap,
   * <i>without</i> collapsing duplicates. This collection has the same size as this multimap, and
   * {@code keys().count(k) == get(k).size()} for all {@code k}.
   *
   * <p>Changes to the returned multiset will update the underlying multimap, and vice versa.
   * However, <i>adding</i> 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.
/**
   * Returns a view collection containing the <i>value</i> from each key-value pair contained in
   * this multimap, without collapsing duplicates (so {@code values().size() == size()}).
   *
   * <p>Changes to the returned collection will update the underlying multimap, and vice versa.
   * However, <i>adding</i> to the returned collection is not possible.
   */
  Collection<V> values();

  Returns a view collection of all key-value pairs contained in this multimap, as 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.
/**
   * Returns a view collection of all key-value pairs contained in this multimap, as {@link Entry}
   * instances.
   *
   * <p>Changes to the returned collection or the entries it contains will update the underlying
   * multimap, and vice versa. However, <i>adding</i> to the returned collection is not possible.
   */
  Collection<Entry<K, V>> entries();

  Performs the given action for all key-value pairs contained in this multimap. If an ordering is specified by the Multimap implementation, actions will be performed in the order of iteration of entries(). Exceptions thrown by the action are relayed to the caller. To loop over all keys and their associated value collections, write 
Multimaps.asMap(multimap).forEach((key, valueCollection) -> action()). 
Since: 21.0/**
   * Performs the given action for all key-value pairs contained in this multimap. If an ordering is
   * specified by the {@code Multimap} implementation, actions will be performed in the order of
   * iteration of {@link #entries()}. Exceptions thrown by the action are relayed to the caller.
   *
   * <p>To loop over all keys and their associated value collections, write {@code
   * Multimaps.asMap(multimap).forEach((key, valueCollection) -> action())}.
   *
   * @since 21.0
   */
  default void forEach(BiConsumer<? super K, ? super V> action) {
    checkNotNull(action);
    entries().forEach(entry -> action.accept(entry.getKey(), entry.getValue()));
  }

  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 setValue. /**
   * Returns a view of this multimap as a {@code Map} from each distinct key to the nonempty
   * collection of that key's associated values. Note that {@code this.asMap().get(k)} is equivalent
   * to {@code this.get(k)} only when {@code k} is a key contained in the multimap; otherwise it
   * returns {@code null} as opposed to an empty collection.
   *
   * <p>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 {@code put} or {@code putAll},
   * nor do its entries support {@link Entry#setValue setValue}.
   */
  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. /**
   * Compares the specified object with this multimap for equality. Two multimaps are equal when
   * their map views, as returned by {@link #asMap}, are also equal.
   *
   * <p>In general, two multimaps with identical key-value mappings may or may not be equal,
   * depending on the implementation. For example, two {@link SetMultimap} instances with the same
   * key-value mappings are equal, but equality of two {@link ListMultimap} instances depends on the
   * ordering of the values for each key.
   *
   * <p>A non-empty {@link SetMultimap} cannot be equal to a non-empty {@link ListMultimap}, since
   * their {@link #asMap} views contain unequal collections as values. However, any two empty
   * multimaps are equal, because they both have empty {@link #asMap} views.
   */
  @Override
  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. 
In general, two multimaps with identical key-value mappings may or may not have the same hash codes, depending on the implementation. For example, two SetMultimap instances with the same key-value mappings will have the same hashCode, but the hashCode of ListMultimap instances depends on the ordering of the values for each key. /**
   * Returns the hash code for this multimap.
   *
   * <p>The hash code of a multimap is defined as the hash code of the map view, as returned by
   * {@link Multimap#asMap}.
   *
   * <p>In general, two multimaps with identical key-value mappings may or may not have the same
   * hash codes, depending on the implementation. For example, two {@link SetMultimap} instances
   * with the same key-value mappings will have the same {@code hashCode}, but the {@code hashCode}
   * of {@link ListMultimap} instances depends on the ordering of the values for each key.
   */
  @Override
  int hashCode();
}
/

com.google.guava/ guava/ 25.1-jre/ com/google/common/collect/Multimap.java

Example

Views

Subinterfaces

Comparison to a map of collections

Implementations

Other Notes
