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