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.Set;
028import java.util.function.BiConsumer;
029import javax.annotation.Nullable;
030
031/**
032 * A collection that maps keys to values, similar to {@link Map}, but in which
033 * each key may be associated with <i>multiple</i> values. You can visualize the
034 * contents of a multimap either as a map from keys to <i>nonempty</i>
035 * 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
051 * multimaps are <i>implemented</i>, the design of the {@code Multimap} API is
052 * based on the <i>second</i> form. So, using the multimap shown above as an
053 * example, the {@link #size} is {@code 3}, not {@code 2}, and the {@link
054 * #values} collection is {@code [1, 2, 3]}, not {@code [[1, 2], [3]]}. For
055 * those times when the first style is more useful, use the multimap's {@link
056 * #asMap} view (or create a {@code Map<K, Collection<V>>} in the first place).
057 *
058 * <h3>Example</h3>
059 *
060 * <p>The following code: <pre>   {@code
061 *
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 *   }}</pre>
070 *
071 * ... produces output such as: <pre>   {@code
072 *
073 *   Zachary: [Taylor]
074 *   John: [Adams, Adams, Tyler, Kennedy]  // Remember, Quincy!
075 *   George: [Washington, Bush, Bush]
076 *   Grover: [Cleveland, Cleveland]        // Two, non-consecutive terms, rep'ing NJ!
077 *   ...}</pre>
078 *
079 * <h3>Views</h3>
080 *
081 * <p>Much of the power of the multimap API comes from the <i>view
082 * collections</i> it provides. These always reflect the latest state of the
083 * multimap itself. When they support modification, the changes are
084 * <i>write-through</i> (they automatically update the backing multimap). These
085 * view collections are:
086 *
087 * <ul>
088 * <li>{@link #asMap}, mentioned above</li>
089 * <li>{@link #keys}, {@link #keySet}, {@link #values}, {@link #entries}, which
090 *     are similar to the corresponding view collections of {@link Map}
091 * <li>and, notably, even the collection returned by {@link #get get(key)} is an
092 *     active view of the values corresponding to {@code key}
093 * </ul>
094 *
095 * <p>The collections returned by the {@link #replaceValues replaceValues} and
096 * {@link #removeAll removeAll} methods, which contain values that have just
097 * been removed from the multimap, are naturally <i>not</i> views.
098 *
099 * <h3>Subinterfaces</h3>
100 *
101 * <p>Instead of using the {@code Multimap} interface directly, prefer the
102 * subinterfaces {@link ListMultimap} and {@link SetMultimap}. These take their
103 * names from the fact that the collections they return from {@code get} behave
104 * like (and, of course, implement) {@link List} and {@link Set}, respectively.
105 *
106 * <p>For example, the "presidents" code snippet above used a {@code
107 * ListMultimap}; if it had used a {@code SetMultimap} instead, two presidents
108 * would have vanished, and last names might or might not appear in
109 * chronological order.
110 *
111 * <p><b>Warning:</b> instances of type {@code Multimap} may not implement
112 * {@link Object#equals} in the way you expect.  Multimaps containing the same
113 * key-value pairs, even in the same order, may or may not be equal and may or
114 * 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,
120 * Collection<V>>} would otherwise have appeared. The differences include:
121 *
122 * <ul>
123 * <li>There is no need to populate an empty collection before adding an entry
124 *     with {@link #put 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
127 *     one value. Any operation that causes a key to have zero associated
128 *     values has the effect of <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
132 *     keys.
133 * </ul>
134 *
135 * <h3>Implementations</h3>
136 *
137 * <p>As always, prefer the immutable implementations, {@link
138 * ImmutableListMultimap} and {@link ImmutableSetMultimap}. General-purpose
139 * mutable implementations are listed above under "All Known Implementing
140 * Classes". You can also create a <i>custom</i> multimap, backed by any {@code
141 * Map} and {@link Collection} types, using the {@link Multimaps#newMultimap
142 * Multimaps.newMultimap} family of methods. Finally, another popular way to
143 * obtain a multimap is using {@link Multimaps#index Multimaps.index}. See
144 * the {@link Multimaps} class for these and other static utilities related
145 * to multimaps.
146 *
147 * <h3>Other Notes</h3>
148 *
149 * <p>As with {@code Map}, the behavior of a {@code Multimap} is not specified
150 * if key objects already present in the multimap change in a manner that
151 * affects {@code equals} comparisons.  Use caution if mutable objects are used
152 * as keys in a {@code Multimap}.
153 *
154 * <p>All methods that modify the multimap are optional. The view collections
155 * returned by the multimap may or may not be modifiable. Any modification
156 * method that is not supported will throw {@link
157 * UnsupportedOperationException}.
158 *
159 * <p>See the Guava User Guide article on <a href=
160 * "https://github.com/google/guava/wiki/NewCollectionTypesExplained#multimap">
161 * {@code Multimap}</a>.
162 *
163 * @author Jared Levy
164 * @since 2.0
165 */
166@GwtCompatible
167public interface Multimap<K, V> {
168  // Query Operations
169
170  /**
171   * Returns the number of key-value pairs in this multimap.
172   *
173   * <p><b>Note:</b> this method does not return the number of <i>distinct
174   * keys</i> in the multimap, which is given by {@code keySet().size()} or
175   * {@code asMap().size()}. See the opening section of the {@link Multimap}
176   * class documentation for clarification.
177   */
178  int size();
179
180  /**
181   * Returns {@code true} if this multimap contains no key-value pairs.
182   * Equivalent to {@code size() == 0}, but can in some cases be more efficient.
183   */
184  boolean isEmpty();
185
186  /**
187   * Returns {@code true} if this multimap contains at least one key-value pair
188   * with the key {@code key}.
189   */
190  boolean containsKey(@CompatibleWith("K") @Nullable Object key);
191
192  /**
193   * Returns {@code true} if this multimap contains at least one key-value pair
194   * with the value {@code value}.
195   */
196  boolean containsValue(@CompatibleWith("V") @Nullable Object value);
197
198  /**
199   * Returns {@code true} if this multimap contains at least one key-value pair
200   * with the key {@code key} and the value {@code value}.
201   */
202  boolean containsEntry(
203      @CompatibleWith("K") @Nullable Object key,
204      @CompatibleWith("V") @Nullable Object value);
205
206  // Modification Operations
207
208  /**
209   * Stores a key-value pair in this multimap.
210   *
211   * <p>Some multimap implementations allow duplicate key-value pairs, in which
212   * case {@code put} always adds a new key-value pair and increases the
213   * multimap size by 1. Other implementations prohibit duplicates, and storing
214   * a key-value pair that's already in the multimap has no effect.
215   *
216   * @return {@code true} if the method increased the size of the multimap, or
217   *     {@code false} if the multimap already contained the key-value pair and
218   *     doesn't allow duplicates
219   */
220  @CanIgnoreReturnValue
221  boolean put(@Nullable K key, @Nullable V value);
222
223  /**
224   * Removes a single key-value pair with the key {@code key} and the value
225   * {@code value} from this multimap, if such exists. If multiple key-value
226   * pairs in the multimap fit this description, which one is removed is
227   * unspecified.
228   *
229   * @return {@code true} if the multimap changed
230   */
231  @CanIgnoreReturnValue
232  boolean remove(
233      @CompatibleWith("K") @Nullable Object key,
234      @CompatibleWith("V") @Nullable Object value);
235
236  // Bulk Operations
237
238  /**
239   * Stores a key-value pair in this multimap for each of {@code values}, all
240   * using the same key, {@code key}. Equivalent to (but expected to be more
241   * efficient than): <pre>   {@code
242   *
243   *   for (V value : values) {
244   *     put(key, value);
245   *   }}</pre>
246   *
247   * <p>In particular, this is a no-op if {@code values} is empty.
248   *
249   * @return {@code true} if the multimap changed
250   */
251  @CanIgnoreReturnValue
252  boolean putAll(@Nullable K key, Iterable<? extends V> values);
253
254  /**
255   * Stores all key-value pairs of {@code multimap} in this multimap, in the
256   * order returned by {@code multimap.entries()}.
257   *
258   * @return {@code true} if the multimap changed
259   */
260  @CanIgnoreReturnValue
261  boolean putAll(Multimap<? extends K, ? extends V> multimap);
262
263  /**
264   * Stores a collection of values with the same key, replacing any existing
265   * values for that key.
266   *
267   * <p>If {@code values} is empty, this is equivalent to
268   * {@link #removeAll(Object) removeAll(key)}.
269   *
270   * @return the collection of replaced values, or an empty collection if no
271   *     values were previously associated with the key. The collection
272   *     <i>may</i> be modifiable, but updating it will have no effect on the
273   *     multimap.
274   */
275  @CanIgnoreReturnValue
276  Collection<V> replaceValues(@Nullable K key, Iterable<? extends V> values);
277
278  /**
279   * Removes all values associated with the key {@code key}.
280   *
281   * <p>Once this method returns, {@code key} will not be mapped to any values,
282   * so it will not appear in {@link #keySet()}, {@link #asMap()}, or any other
283   * views.
284   *
285   * @return the values that were removed (possibly empty). The returned
286   *     collection <i>may</i> be modifiable, but updating it will have no
287   *     effect on the multimap.
288   */
289  @CanIgnoreReturnValue
290  Collection<V> removeAll(@CompatibleWith("K") @Nullable Object key);
291
292  /**
293   * Removes all key-value pairs from the multimap, leaving it {@linkplain
294   * #isEmpty empty}.
295   */
296  void clear();
297
298  // Views
299
300  /**
301   * Returns a view collection of the values associated with {@code key} in this
302   * multimap, if any. Note that when {@code containsKey(key)} is false, this
303   * returns an empty collection, not {@code null}.
304   *
305   * <p>Changes to the returned collection will update the underlying multimap,
306   * and vice versa.
307   */
308  Collection<V> get(@Nullable K key);
309
310  /**
311   * Returns a view collection of all <i>distinct</i> keys contained in this
312   * multimap. Note that the key set contains a key if and only if this multimap
313   * maps that key to at least one value.
314   *
315   * <p>Changes to the returned set will update the underlying multimap, and
316   * vice versa. However, <i>adding</i> to the returned set is not possible.
317   */
318  Set<K> keySet();
319
320  /**
321   * Returns a view collection containing the key from each key-value pair in
322   * this multimap, <i>without</i> collapsing duplicates. This collection has
323   * the same size as this multimap, and {@code keys().count(k) ==
324   * get(k).size()} for all {@code k}.
325   *
326   * <p>Changes to the returned multiset will update the underlying multimap,
327   * and vice versa. However, <i>adding</i> to the returned collection is not
328   * possible.
329   */
330  Multiset<K> keys();
331
332  /**
333   * Returns a view collection containing the <i>value</i> from each key-value
334   * pair contained in this multimap, without collapsing duplicates (so {@code
335   * values().size() == size()}).
336   *
337   * <p>Changes to the returned collection will update the underlying multimap,
338   * and vice versa. However, <i>adding</i> to the returned collection is not
339   * possible.
340   */
341  Collection<V> values();
342
343  /**
344   * Returns a view collection of all key-value pairs contained in this
345   * multimap, as {@link Map.Entry} instances.
346   *
347   * <p>Changes to the returned collection or the entries it contains will
348   * update the underlying multimap, and vice versa. However, <i>adding</i> to
349   * the returned collection is not possible.
350   */
351  Collection<Map.Entry<K, V>> entries();
352
353  /**
354   * Performs the given action for all key-value pairs contained in this multimap. If an ordering is
355   * specified by the {@code Multimap} implementation, actions will be performed in the order of
356   * iteration of {@link #entries()}. Exceptions thrown by the action are relayed to the caller.
357   *
358   * <p>To loop over all keys and their associated value collections, write
359   * {@code Multimaps.asMap(multimap).forEach((key, valueCollection) -> action())}.
360   *
361   * @since 21.0
362   */
363  default void forEach(BiConsumer<? super K, ? super V> action) {
364    checkNotNull(action);
365    entries().forEach(entry -> action.accept(entry.getKey(), entry.getValue()));
366  }
367
368  /**
369   * Returns a view of this multimap as a {@code Map} from each distinct key
370   * to the nonempty collection of that key's associated values. Note that
371   * {@code this.asMap().get(k)} is equivalent to {@code this.get(k)} only when
372   * {@code k} is a key contained in the multimap; otherwise it returns {@code
373   * null} as opposed to an empty collection.
374   *
375   * <p>Changes to the returned map or the collections that serve as its values
376   * will update the underlying multimap, and vice versa. The map does not
377   * support {@code put} or {@code putAll}, nor do its entries support {@link
378   * Map.Entry#setValue setValue}.
379   */
380  Map<K, Collection<V>> asMap();
381
382  // Comparison and hashing
383
384  /**
385   * Compares the specified object with this multimap for equality. Two
386   * multimaps are equal when their map views, as returned by {@link #asMap},
387   * are also equal.
388   *
389   * <p>In general, two multimaps with identical key-value mappings may or may
390   * not be equal, depending on the implementation. For example, two
391   * {@link SetMultimap} instances with the same key-value mappings are equal,
392   * but equality of two {@link ListMultimap} instances depends on the ordering
393   * of the values for each key.
394   *
395   * <p>A non-empty {@link SetMultimap} cannot be equal to a non-empty
396   * {@link ListMultimap}, since their {@link #asMap} views contain unequal
397   * collections as values. However, any two empty multimaps are equal, because
398   * they both have empty {@link #asMap} views.
399   */
400  @Override
401  boolean equals(@Nullable Object obj);
402
403  /**
404   * Returns the hash code for this multimap.
405   *
406   * <p>The hash code of a multimap is defined as the hash code of the map view,
407   * as returned by {@link Multimap#asMap}.
408   *
409   * <p>In general, two multimaps with identical key-value mappings may or may
410   * not have the same hash codes, depending on the implementation. For
411   * example, two {@link SetMultimap} instances with the same key-value
412   * mappings will have the same {@code hashCode}, but the {@code hashCode}
413   * of {@link ListMultimap} instances depends on the ordering of the values
414   * for each key.
415   */
416  @Override
417  int hashCode();
418}