path: root/guava/src/com/google/common/collect/Multimap.java

/*
 * 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 com.google.common.annotations.GwtCompatible;

import java.util.Collection;
import java.util.List;
import java.util.Map;
import java.util.Set;

import javax.annotation.Nullable;

/**
 * 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]
 *   George: [Washington, Bush, Bush]
 *   Grover: [Cleveland]
 *   ...}</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>
 * <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). The
 * recommended subinterfaces provide a much stronger guarantee.
 *
 * <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 contained in the multimap always 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>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=
 * "http://code.google.com/p/guava-libraries/wiki/NewCollectionTypesExplained#Multimap">
 * {@code Multimap}</a>.
 *
 * @author Jared Levy
 * @since 2.0 (imported from Google Collections Library)
 */
@GwtCompatible
public interface Multimap<K, V> {
  // Query Operations

  /** Returns the number of key-value pairs in the multimap. */
  int size();

  /** Returns {@code true} if the multimap contains no key-value pairs. */
  boolean isEmpty();

  /**
   * Returns {@code true} if the multimap contains any values for the specified
   * key.
   *
   * @param key key to search for in multimap
   */
  boolean containsKey(@Nullable Object key);

  /**
   * Returns {@code true} if the multimap contains the specified value for any
   * key.
   *
   * @param value value to search for in multimap
   */
  boolean containsValue(@Nullable Object value);

  /**
   * Returns {@code true} if the multimap contains the specified key-value pair.
   *
   * @param key key to search for in multimap
   * @param value value to search for in multimap
   */
  boolean containsEntry(@Nullable Object key, @Nullable Object value);

  // Modification Operations

  /**
   * Stores a key-value pair in the 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.
   *
   * @param key key to store in the multimap
   * @param value value to store in the multimap
   * @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
   */
  boolean put(@Nullable K key, @Nullable V value);

  /**
   * Removes a single key-value pair from the multimap.
   *
   * @param key key of entry to remove from the multimap
   * @param value value of entry to remove the multimap
   * @return {@code true} if the multimap changed
   */
  boolean remove(@Nullable Object key, @Nullable Object value);

  // Bulk Operations

  /**
   * Stores a collection of values with the same key.
   *
   * @param key key to store in the multimap
   * @param values values to store in the multimap
   * @return {@code true} if the multimap changed
   */
  boolean putAll(@Nullable K key, Iterable<? extends V> values);

  /**
   * Copies all of another multimap's key-value pairs into this multimap. The
   * order in which the mappings are added is determined by
   * {@code multimap.entries()}.
   *
   * @param multimap mappings to store in this multimap
   * @return {@code true} if the multimap changed
   */
  boolean putAll(Multimap<? extends K, ? extends V> multimap);

  /**
   * Stores a collection of values with the same key, replacing any existing
   * values for that key.
   *
   * @param key key to store in the multimap
   * @param values values to store in the multimap
   * @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.
   */
  Collection<V> replaceValues(@Nullable K key, Iterable<? extends V> values);

  /**
   * Removes all values associated with a given key.
   *
   * @param key key of entries to remove from the multimap
   * @return the collection of removed values, or an empty collection if no
   *     values were associated with the provided key. The collection
   *     <i>may</i> be modifiable, but updating it will have no effect on the
   *     multimap.
   */
  Collection<V> removeAll(@Nullable Object key);

  /**
   * Removes all key-value pairs from the multimap.
   */
  void clear();

  // Views

  /**
   * Returns a collection view containing the values associated with {@code key}
   * in this multimap, if any. Note that even when ({@code containsKey(key)} is
   * false, {@code get(key)} still returns an empty collection, not {@code
   * null}.
   *
   * <p>Changes to the returned collection will update the underlying multimap,
   * and vice versa.
   *
   * @param key key to search for in multimap
   * @return a view collection containing the zero or more values that the key
   *     maps to
   */
  Collection<V> get(@Nullable K key);

  /**
   * Returns the set of all keys, each appearing once in the returned set.
   * Changes to the returned set will update the underlying multimap, and vice
   * versa.
   *
   * @return the collection of distinct keys
   */
  Set<K> keySet();

  /**
   * Returns a collection, which may contain duplicates, of all keys. The number
   * of times of key appears in the returned multiset equals the number of
   * mappings the key has in the multimap. Changes to the returned multiset will
   * update the underlying multimap, and vice versa.
   *
   * @return a multiset with keys corresponding to the distinct keys of the
   *     multimap and frequencies corresponding to the number of values that
   *     each key maps to
   */
  Multiset<K> keys();

  /**
   * Returns a collection of all values in the multimap. Changes to the returned
   * collection will update the underlying multimap, and vice versa.
   *
   * @return collection of values, which may include the same value multiple
   *     times if it occurs in multiple mappings
   */
  Collection<V> values();

  /**
   * Returns a collection of all key-value pairs. Changes to the returned
   * collection will update the underlying multimap, and vice versa. The entries
   * collection does not support the {@code add} or {@code addAll} operations.
   *
   * @return collection of map entries consisting of key-value pairs
   */
  Collection<Map.Entry<K, V>> entries();

  /**
   * Returns a map view that associates each key with the corresponding values
   * in the multimap. Changes to the returned map, such as element removal, will
   * update the underlying multimap. The map does not support {@code setValue()}
   * on its entries, {@code put}, or {@code putAll}.
   *
   * <p>When passed a key that is present in the map, {@code
   * asMap().get(Object)} has the same behavior as {@link #get}, returning a
   * live collection. When passed a key that is not present, however, {@code
   * asMap().get(Object)} returns {@code null} instead of an empty collection.
   *
   * @return a map view from a key to its collection of values
   */
  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 {@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.
   *
   * <p>The hash code of a multimap is defined as the hash code of the map view,
   * as returned by {@link Multimap#asMap}.
   */
  @Override
  int hashCode();
}