001/*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.collect;
018
019import static com.google.common.base.Preconditions.checkNotNull;
020
021import com.google.common.annotations.GwtCompatible;
022import com.google.errorprone.annotations.CanIgnoreReturnValue;
023import com.google.errorprone.annotations.CompatibleWith;
024import java.util.Collection;
025import java.util.List;
026import java.util.Map;
027import java.util.Map.Entry;
028import java.util.Set;
029import java.util.function.BiConsumer;
030import org.checkerframework.checker.nullness.qual.Nullable;
031
032/**
033 * A collection that maps keys to values, similar to {@link Map}, but in which each key may be
034 * associated with <i>multiple</i> values. You can visualize the contents of a multimap either as a
035 * map from keys to <i>nonempty</i> collections of values:
036 *
037 * <ul>
038 *   <li>a → 1, 2
039 *   <li>b → 3
040 * </ul>
041 *
042 * ... or as a single "flattened" collection of key-value pairs:
043 *
044 * <ul>
045 *   <li>a → 1
046 *   <li>a → 2
047 *   <li>b → 3
048 * </ul>
049 *
050 * <p><b>Important:</b> although the first interpretation resembles how most multimaps are
051 * <i>implemented</i>, the design of the {@code Multimap} API is based on the <i>second</i> form.
052 * So, using the multimap shown above as an example, the {@link #size} is {@code 3}, not {@code 2},
053 * and the {@link #values} collection is {@code [1, 2, 3]}, not {@code [[1, 2], [3]]}. For those
054 * times when the first style is more useful, use the multimap's {@link #asMap} view (or create a
055 * {@code Map<K, Collection<V>>} in the first place).
056 *
057 * <h3>Example</h3>
058 *
059 * <p>The following code:
060 *
061 * <pre>{@code
062 * ListMultimap<String, String> multimap = ArrayListMultimap.create();
063 * for (President pres : US_PRESIDENTS_IN_ORDER) {
064 *   multimap.put(pres.firstName(), pres.lastName());
065 * }
066 * for (String firstName : multimap.keySet()) {
067 *   List<String> lastNames = multimap.get(firstName);
068 *   out.println(firstName + ": " + lastNames);
069 * }
070 * }</pre>
071 *
072 * ... produces output such as:
073 *
074 * <pre>{@code
075 * Zachary: [Taylor]
076 * John: [Adams, Adams, Tyler, Kennedy]  // Remember, Quincy!
077 * George: [Washington, Bush, Bush]
078 * Grover: [Cleveland, Cleveland]        // Two, non-consecutive terms, rep'ing NJ!
079 * ...
080 * }</pre>
081 *
082 * <h3>Views</h3>
083 *
084 * <p>Much of the power of the multimap API comes from the <i>view collections</i> it provides.
085 * These always reflect the latest state of the multimap itself. When they support modification, the
086 * changes are <i>write-through</i> (they automatically update the backing multimap). These view
087 * collections are:
088 *
089 * <ul>
090 *   <li>{@link #asMap}, mentioned above
091 *   <li>{@link #keys}, {@link #keySet}, {@link #values}, {@link #entries}, which are similar to the
092 *       corresponding view collections of {@link Map}
093 *   <li>and, notably, even the collection returned by {@link #get get(key)} is an active view of
094 *       the values corresponding to {@code key}
095 * </ul>
096 *
097 * <p>The collections returned by the {@link #replaceValues replaceValues} and {@link #removeAll
098 * removeAll} methods, which contain values that have just been removed from the multimap, are
099 * naturally <i>not</i> views.
100 *
101 * <h3>Subinterfaces</h3>
102 *
103 * <p>Instead of using the {@code Multimap} interface directly, prefer the subinterfaces {@link
104 * ListMultimap} and {@link SetMultimap}. These take their names from the fact that the collections
105 * they return from {@code get} behave like (and, of course, implement) {@link List} and {@link
106 * Set}, respectively.
107 *
108 * <p>For example, the "presidents" code snippet above used a {@code ListMultimap}; if it had used a
109 * {@code SetMultimap} instead, two presidents would have vanished, and last names might or might
110 * not appear in chronological order.
111 *
112 * <p><b>Warning:</b> instances of type {@code Multimap} may not implement {@link Object#equals} in
113 * the way you expect. Multimaps containing the same key-value pairs, even in the same order, may or
114 * may not be equal and may or may not have the same {@code hashCode}. The recommended subinterfaces
115 * provide much stronger guarantees.
116 *
117 * <h3>Comparison to a map of collections</h3>
118 *
119 * <p>Multimaps are commonly used in places where a {@code Map<K, Collection<V>>} would otherwise
120 * have appeared. The differences include:
121 *
122 * <ul>
123 *   <li>There is no need to populate an empty collection before adding an entry with {@link #put
124 *       put}.
125 *   <li>{@code get} never returns {@code null}, only an empty collection.
126 *   <li>A key is contained in the multimap if and only if it maps to at least one value. Any
127 *       operation that causes a key to have zero associated values has the effect of
128 *       <i>removing</i> that key from the multimap.
129 *   <li>The total entry count is available as {@link #size}.
130 *   <li>Many complex operations become easier; for example, {@code
131 *       Collections.min(multimap.values())} finds the smallest value across all keys.
132 * </ul>
133 *
134 * <h3>Implementations</h3>
135 *
136 * <p>As always, prefer the immutable implementations, {@link ImmutableListMultimap} and {@link
137 * ImmutableSetMultimap}. General-purpose mutable implementations are listed above under "All Known
138 * Implementing Classes". You can also create a <i>custom</i> multimap, backed by any {@code Map}
139 * and {@link Collection} types, using the {@link Multimaps#newMultimap Multimaps.newMultimap}
140 * family of methods. Finally, another popular way to obtain a multimap is using {@link
141 * Multimaps#index Multimaps.index}. See the {@link Multimaps} class for these and other static
142 * utilities related to multimaps.
143 *
144 * <h3>Other Notes</h3>
145 *
146 * <p>As with {@code Map}, the behavior of a {@code Multimap} is not specified if key objects
147 * already present in the multimap change in a manner that affects {@code equals} comparisons. Use
148 * caution if mutable objects are used as keys in a {@code Multimap}.
149 *
150 * <p>All methods that modify the multimap are optional. The view collections returned by the
151 * multimap may or may not be modifiable. Any modification method that is not supported will throw
152 * {@link UnsupportedOperationException}.
153 *
154 * <p>See the Guava User Guide article on <a href=
155 * "https://github.com/google/guava/wiki/NewCollectionTypesExplained#multimap"> {@code
156 * Multimap}</a>.
157 *
158 * @author Jared Levy
159 * @since 2.0
160 */
161@GwtCompatible
162public interface Multimap<K, V> {
163  // Query Operations
164
165  /**
166   * Returns the number of key-value pairs in this multimap.
167   *
168   * <p><b>Note:</b> this method does not return the number of <i>distinct keys</i> in the multimap,
169   * which is given by {@code keySet().size()} or {@code asMap().size()}. See the opening section of
170   * the {@link Multimap} class documentation for clarification.
171   */
172  int size();
173
174  /**
175   * Returns {@code true} if this multimap contains no key-value pairs. Equivalent to {@code size()
176   * == 0}, but can in some cases be more efficient.
177   */
178  boolean isEmpty();
179
180  /**
181   * Returns {@code true} if this multimap contains at least one key-value pair with the key {@code
182   * key}.
183   */
184  boolean containsKey(@CompatibleWith("K") @Nullable Object key);
185
186  /**
187   * Returns {@code true} if this multimap contains at least one key-value pair with the value
188   * {@code value}.
189   */
190  boolean containsValue(@CompatibleWith("V") @Nullable Object value);
191
192  /**
193   * Returns {@code true} if this multimap contains at least one key-value pair with the key {@code
194   * key} and the value {@code value}.
195   */
196  boolean containsEntry(
197      @CompatibleWith("K") @Nullable Object key, @CompatibleWith("V") @Nullable Object value);
198
199  // Modification Operations
200
201  /**
202   * Stores a key-value pair in this multimap.
203   *
204   * <p>Some multimap implementations allow duplicate key-value pairs, in which case {@code put}
205   * always adds a new key-value pair and increases the multimap size by 1. Other implementations
206   * prohibit duplicates, and storing a key-value pair that's already in the multimap has no effect.
207   *
208   * @return {@code true} if the method increased the size of the multimap, or {@code false} if the
209   *     multimap already contained the key-value pair and doesn't allow duplicates
210   */
211  @CanIgnoreReturnValue
212  boolean put(@Nullable K key, @Nullable V value);
213
214  /**
215   * Removes a single key-value pair with the key {@code key} and the value {@code value} from this
216   * multimap, if such exists. If multiple key-value pairs in the multimap fit this description,
217   * which one is removed is unspecified.
218   *
219   * @return {@code true} if the multimap changed
220   */
221  @CanIgnoreReturnValue
222  boolean remove(
223      @CompatibleWith("K") @Nullable Object key, @CompatibleWith("V") @Nullable Object value);
224
225  // Bulk Operations
226
227  /**
228   * Stores a key-value pair in this multimap for each of {@code values}, all using the same key,
229   * {@code key}. Equivalent to (but expected to be more efficient than):
230   *
231   * <pre>{@code
232   * for (V value : values) {
233   *   put(key, value);
234   * }
235   * }</pre>
236   *
237   * <p>In particular, this is a no-op if {@code values} is empty.
238   *
239   * @return {@code true} if the multimap changed
240   */
241  @CanIgnoreReturnValue
242  boolean putAll(@Nullable K key, Iterable<? extends V> values);
243
244  /**
245   * Stores all key-value pairs of {@code multimap} in this multimap, in the order returned by
246   * {@code multimap.entries()}.
247   *
248   * @return {@code true} if the multimap changed
249   */
250  @CanIgnoreReturnValue
251  boolean putAll(Multimap<? extends K, ? extends V> multimap);
252
253  /**
254   * Stores a collection of values with the same key, replacing any existing values for that key.
255   *
256   * <p>If {@code values} is empty, this is equivalent to {@link #removeAll(Object) removeAll(key)}.
257   *
258   * @return the collection of replaced values, or an empty collection if no values were previously
259   *     associated with the key. The collection <i>may</i> be modifiable, but updating it will have
260   *     no effect on the multimap.
261   */
262  @CanIgnoreReturnValue
263  Collection<V> replaceValues(@Nullable K key, Iterable<? extends V> values);
264
265  /**
266   * Removes all values associated with the key {@code key}.
267   *
268   * <p>Once this method returns, {@code key} will not be mapped to any values, so it will not
269   * appear in {@link #keySet()}, {@link #asMap()}, or any other views.
270   *
271   * @return the values that were removed (possibly empty). The returned collection <i>may</i> be
272   *     modifiable, but updating it will have no effect on the multimap.
273   */
274  @CanIgnoreReturnValue
275  Collection<V> removeAll(@CompatibleWith("K") @Nullable Object key);
276
277  /** Removes all key-value pairs from the multimap, leaving it {@linkplain #isEmpty empty}. */
278  void clear();
279
280  // Views
281
282  /**
283   * Returns a view collection of the values associated with {@code key} in this multimap, if any.
284   * Note that when {@code containsKey(key)} is false, this returns an empty collection, not {@code
285   * null}.
286   *
287   * <p>Changes to the returned collection will update the underlying multimap, and vice versa.
288   */
289  Collection<V> get(@Nullable K key);
290
291  /**
292   * Returns a view collection of all <i>distinct</i> keys contained in this multimap. Note that the
293   * key set contains a key if and only if this multimap maps that key to at least one value.
294   *
295   * <p>Changes to the returned set will update the underlying multimap, and vice versa. However,
296   * <i>adding</i> to the returned set is not possible.
297   */
298  Set<K> keySet();
299
300  /**
301   * Returns a view collection containing the key from each key-value pair in this multimap,
302   * <i>without</i> collapsing duplicates. This collection has the same size as this multimap, and
303   * {@code keys().count(k) == get(k).size()} for all {@code k}.
304   *
305   * <p>Changes to the returned multiset will update the underlying multimap, and vice versa.
306   * However, <i>adding</i> to the returned collection is not possible.
307   */
308  Multiset<K> keys();
309
310  /**
311   * Returns a view collection containing the <i>value</i> from each key-value pair contained in
312   * this multimap, without collapsing duplicates (so {@code values().size() == size()}).
313   *
314   * <p>Changes to the returned collection will update the underlying multimap, and vice versa.
315   * However, <i>adding</i> to the returned collection is not possible.
316   */
317  Collection<V> values();
318
319  /**
320   * Returns a view collection of all key-value pairs contained in this multimap, as {@link Entry}
321   * instances.
322   *
323   * <p>Changes to the returned collection or the entries it contains will update the underlying
324   * multimap, and vice versa. However, <i>adding</i> to the returned collection is not possible.
325   */
326  Collection<Entry<K, V>> entries();
327
328  /**
329   * Performs the given action for all key-value pairs contained in this multimap. If an ordering is
330   * specified by the {@code Multimap} implementation, actions will be performed in the order of
331   * iteration of {@link #entries()}. Exceptions thrown by the action are relayed to the caller.
332   *
333   * <p>To loop over all keys and their associated value collections, write {@code
334   * Multimaps.asMap(multimap).forEach((key, valueCollection) -> action())}.
335   *
336   * @since 21.0
337   */
338  default void forEach(BiConsumer<? super K, ? super V> action) {
339    checkNotNull(action);
340    entries().forEach(entry -> action.accept(entry.getKey(), entry.getValue()));
341  }
342
343  /**
344   * Returns a view of this multimap as a {@code Map} from each distinct key to the nonempty
345   * collection of that key's associated values. Note that {@code this.asMap().get(k)} is equivalent
346   * to {@code this.get(k)} only when {@code k} is a key contained in the multimap; otherwise it
347   * returns {@code null} as opposed to an empty collection.
348   *
349   * <p>Changes to the returned map or the collections that serve as its values will update the
350   * underlying multimap, and vice versa. The map does not support {@code put} or {@code putAll},
351   * nor do its entries support {@link Entry#setValue setValue}.
352   */
353  Map<K, Collection<V>> asMap();
354
355  // Comparison and hashing
356
357  /**
358   * Compares the specified object with this multimap for equality. Two multimaps are equal when
359   * their map views, as returned by {@link #asMap}, are also equal.
360   *
361   * <p>In general, two multimaps with identical key-value mappings may or may not be equal,
362   * depending on the implementation. For example, two {@link SetMultimap} instances with the same
363   * key-value mappings are equal, but equality of two {@link ListMultimap} instances depends on the
364   * ordering of the values for each key.
365   *
366   * <p>A non-empty {@link SetMultimap} cannot be equal to a non-empty {@link ListMultimap}, since
367   * their {@link #asMap} views contain unequal collections as values. However, any two empty
368   * multimaps are equal, because they both have empty {@link #asMap} views.
369   */
370  @Override
371  boolean equals(@Nullable Object obj);
372
373  /**
374   * Returns the hash code for this multimap.
375   *
376   * <p>The hash code of a multimap is defined as the hash code of the map view, as returned by
377   * {@link Multimap#asMap}.
378   *
379   * <p>In general, two multimaps with identical key-value mappings may or may not have the same
380   * hash codes, depending on the implementation. For example, two {@link SetMultimap} instances
381   * with the same key-value mappings will have the same {@code hashCode}, but the {@code hashCode}
382   * of {@link ListMultimap} instances depends on the ordering of the values for each key.
383   */
384  @Override
385  int hashCode();
386}