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 * <ul>
136 *   <li>{@link ImmutableListMultimap}
137 *   <li>{@link ImmutableSetMultimap}
138 *   <li>Configure your own mutable multimap with {@link MultimapBuilder}
139 *   <li>{@link LinkedListMultimap} (for one unusual kind of mutable {@code Multimap})
140 * </ul>
141 *
142 * Guava contains a number of other multimap implementations, such as {@link ArrayListMultimap}. In
143 * new code, we recommend using {@link MultimapBuilder} instead: It provides better control of how
144 * keys and values are stored.
145 *
146 * <h3>Other Notes</h3>
147 *
148 * <p>As with {@code Map}, the behavior of a {@code Multimap} is not specified if key objects
149 * already present in the multimap change in a manner that affects {@code equals} comparisons. Use
150 * caution if mutable objects are used as keys in a {@code Multimap}.
151 *
152 * <p>All methods that modify the multimap are optional. The view collections returned by the
153 * multimap may or may not be modifiable. Any modification method that is not supported will throw
154 * {@link UnsupportedOperationException}.
155 *
156 * <p>See the Guava User Guide article on <a href=
157 * "https://github.com/google/guava/wiki/NewCollectionTypesExplained#multimap">{@code Multimap}</a>.
158 *
159 * @author Jared Levy
160 * @since 2.0
161 */
162@DoNotMock("Use ImmutableMultimap, HashMultimap, or another implementation")
163@GwtCompatible
164public interface Multimap<K extends @Nullable Object, V extends @Nullable Object> {
165  // Query Operations
166
167  /**
168   * Returns the number of key-value pairs in this multimap.
169   *
170   * <p><b>Note:</b> this method does not return the number of <i>distinct keys</i> in the multimap,
171   * which is given by {@code keySet().size()} or {@code asMap().size()}. See the opening section of
172   * the {@link Multimap} class documentation for clarification.
173   */
174  int size();
175
176  /**
177   * Returns {@code true} if this multimap contains no key-value pairs. Equivalent to {@code size()
178   * == 0}, but can in some cases be more efficient.
179   */
180  boolean isEmpty();
181
182  /**
183   * Returns {@code true} if this multimap contains at least one key-value pair with the key {@code
184   * key}.
185   */
186  boolean containsKey(@CompatibleWith("K") @CheckForNull Object key);
187
188  /**
189   * Returns {@code true} if this multimap contains at least one key-value pair with the value
190   * {@code value}.
191   */
192  boolean containsValue(@CompatibleWith("V") @CheckForNull Object value);
193
194  /**
195   * Returns {@code true} if this multimap contains at least one key-value pair with the key {@code
196   * key} and the value {@code value}.
197   */
198  boolean containsEntry(
199      @CompatibleWith("K") @CheckForNull Object key,
200      @CompatibleWith("V") @CheckForNull Object value);
201
202  // Modification Operations
203
204  /**
205   * Stores a key-value pair in this multimap.
206   *
207   * <p>Some multimap implementations allow duplicate key-value pairs, in which case {@code put}
208   * always adds a new key-value pair and increases the multimap size by 1. Other implementations
209   * prohibit duplicates, and storing a key-value pair that's already in the multimap has no effect.
210   *
211   * @return {@code true} if the method increased the size of the multimap, or {@code false} if the
212   *     multimap already contained the key-value pair and doesn't allow duplicates
213   */
214  @CanIgnoreReturnValue
215  boolean put(@ParametricNullness K key, @ParametricNullness V value);
216
217  /**
218   * Removes a single key-value pair with the key {@code key} and the value {@code value} from this
219   * multimap, if such exists. If multiple key-value pairs in the multimap fit this description,
220   * which one is removed is unspecified.
221   *
222   * @return {@code true} if the multimap changed
223   */
224  @CanIgnoreReturnValue
225  boolean remove(
226      @CompatibleWith("K") @CheckForNull Object key,
227      @CompatibleWith("V") @CheckForNull Object value);
228
229  // Bulk Operations
230
231  /**
232   * Stores a key-value pair in this multimap for each of {@code values}, all using the same key,
233   * {@code key}. Equivalent to (but expected to be more efficient than):
234   *
235   * <pre>{@code
236   * for (V value : values) {
237   *   put(key, value);
238   * }
239   * }</pre>
240   *
241   * <p>In particular, this is a no-op if {@code values} is empty.
242   *
243   * @return {@code true} if the multimap changed
244   */
245  @CanIgnoreReturnValue
246  boolean putAll(@ParametricNullness K key, Iterable<? extends V> values);
247
248  /**
249   * Stores all key-value pairs of {@code multimap} in this multimap, in the order returned by
250   * {@code multimap.entries()}.
251   *
252   * @return {@code true} if the multimap changed
253   */
254  @CanIgnoreReturnValue
255  boolean putAll(Multimap<? extends K, ? extends V> multimap);
256
257  /**
258   * Stores a collection of values with the same key, replacing any existing values for that key.
259   *
260   * <p>If {@code values} is empty, this is equivalent to {@link #removeAll(Object) removeAll(key)}.
261   *
262   * @return the collection of replaced values, or an empty collection if no values were previously
263   *     associated with the key. The collection <i>may</i> be modifiable, but updating it will have
264   *     no effect on the multimap.
265   */
266  @CanIgnoreReturnValue
267  Collection<V> replaceValues(@ParametricNullness K key, Iterable<? extends V> values);
268
269  /**
270   * Removes all values associated with the key {@code key}.
271   *
272   * <p>Once this method returns, {@code key} will not be mapped to any values, so it will not
273   * appear in {@link #keySet()}, {@link #asMap()}, or any other views.
274   *
275   * @return the values that were removed (possibly empty). The returned collection <i>may</i> be
276   *     modifiable, but updating it will have no effect on the multimap.
277   */
278  @CanIgnoreReturnValue
279  Collection<V> removeAll(@CompatibleWith("K") @CheckForNull Object key);
280
281  /** Removes all key-value pairs from the multimap, leaving it {@linkplain #isEmpty empty}. */
282  void clear();
283
284  // Views
285
286  /**
287   * Returns a view collection of the values associated with {@code key} in this multimap, if any.
288   * Note that when {@code containsKey(key)} is false, this returns an empty collection, not {@code
289   * null}.
290   *
291   * <p>Changes to the returned collection will update the underlying multimap, and vice versa.
292   */
293  Collection<V> get(@ParametricNullness K key);
294
295  /**
296   * Returns a view collection of all <i>distinct</i> keys contained in this multimap. Note that the
297   * key set contains a key if and only if this multimap maps that key to at least one value.
298   *
299   * <p>Changes to the returned set will update the underlying multimap, and vice versa. However,
300   * <i>adding</i> to the returned set is not possible.
301   */
302  Set<K> keySet();
303
304  /**
305   * Returns a view collection containing the key from each key-value pair in this multimap,
306   * <i>without</i> collapsing duplicates. This collection has the same size as this multimap, and
307   * {@code keys().count(k) == get(k).size()} for all {@code k}.
308   *
309   * <p>Changes to the returned multiset will update the underlying multimap, and vice versa.
310   * However, <i>adding</i> to the returned collection is not possible.
311   */
312  Multiset<K> keys();
313
314  /**
315   * Returns a view collection containing the <i>value</i> from each key-value pair contained in
316   * this multimap, without collapsing duplicates (so {@code values().size() == size()}).
317   *
318   * <p>Changes to the returned collection will update the underlying multimap, and vice versa.
319   * However, <i>adding</i> to the returned collection is not possible.
320   */
321  Collection<V> values();
322
323  /**
324   * Returns a view collection of all key-value pairs contained in this multimap, as {@link Entry}
325   * instances.
326   *
327   * <p>Changes to the returned collection or the entries it contains will update the underlying
328   * multimap, and vice versa. However, <i>adding</i> to the returned collection is not possible.
329   */
330  Collection<Entry<K, V>> entries();
331
332  /**
333   * Returns a view of this multimap as a {@code Map} from each distinct key to the nonempty
334   * collection of that key's associated values. Note that {@code this.asMap().get(k)} is equivalent
335   * to {@code this.get(k)} only when {@code k} is a key contained in the multimap; otherwise it
336   * returns {@code null} as opposed to an empty collection.
337   *
338   * <p>Changes to the returned map or the collections that serve as its values will update the
339   * underlying multimap, and vice versa. The map does not support {@code put} or {@code putAll},
340   * nor do its entries support {@link Entry#setValue setValue}.
341   */
342  Map<K, Collection<V>> asMap();
343
344  // Comparison and hashing
345
346  /**
347   * Compares the specified object with this multimap for equality. Two multimaps are equal when
348   * their map views, as returned by {@link #asMap}, are also equal.
349   *
350   * <p>In general, two multimaps with identical key-value mappings may or may not be equal,
351   * depending on the implementation. For example, two {@link SetMultimap} instances with the same
352   * key-value mappings are equal, but equality of two {@link ListMultimap} instances depends on the
353   * ordering of the values for each key.
354   *
355   * <p>A non-empty {@link SetMultimap} cannot be equal to a non-empty {@link ListMultimap}, since
356   * their {@link #asMap} views contain unequal collections as values. However, any two empty
357   * multimaps are equal, because they both have empty {@link #asMap} views.
358   */
359  @Override
360  boolean equals(@CheckForNull Object obj);
361
362  /**
363   * Returns the hash code for this multimap.
364   *
365   * <p>The hash code of a multimap is defined as the hash code of the map view, as returned by
366   * {@link Multimap#asMap}.
367   *
368   * <p>In general, two multimaps with identical key-value mappings may or may not have the same
369   * hash codes, depending on the implementation. For example, two {@link SetMultimap} instances
370   * with the same key-value mappings will have the same {@code hashCode}, but the {@code hashCode}
371   * of {@link ListMultimap} instances depends on the ordering of the values for each key.
372   */
373  @Override
374  int hashCode();
375}