/* * 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 java.util.Collection; import java.util.List; import java.util.Map; import java.util.Set; import javax.annotation.Nullable; import com.google.common.annotations.GwtCompatible; /** * A collection that maps keys to values, similar to {@link Map}, but in which * each key may be associated with multiple values. You can visualize the * contents of a multimap either as a map from keys to nonempty * collections of values: * * * * ... or as a single "flattened" collection of key-value pairs: * * * *

* Important: although the first interpretation resembles how most * multimaps are implemented, the design of the {@code Multimap} API is * based on the second 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>} in the first * place). * *

Example

* *

* The following code: * * 

 * {
 * 	@code
 *
 * 	ListMultimap multimap = ArrayListMultimap.create();
 * 	for (President pres : US_PRESIDENTS_IN_ORDER) {
 * 		multimap.put(pres.firstName(), pres.lastName());
 * 	}
 * 	for (String firstName : multimap.keySet()) {
 * 		List lastNames = multimap.get(firstName);
 * 		out.println(firstName + ": " + lastNames);
 * 	}
 * }
 *
* * ... produces output such as: * * 
 *    {@code
 *
 *   Zachary: [Taylor]
 *   John: [Adams, Adams, Tyler, Kennedy]  // Remember, Quincy!
 *   George: [Washington, Bush, Bush]
 *   Grover: [Cleveland, Cleveland]        // Two, non-consecutive terms, rep'ing NJ!
 *   ...}
 *
* *

Views

* *

* Much of the power of the multimap API comes from the view collections * it provides. These always reflect the latest state of the multimap itself. * When they support modification, the changes are write-through (they * automatically update the backing multimap). These view collections are: * *

* *

* 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 not views. * *

Subinterfaces

* *

* 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. * *

* 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. * *

* Warning: 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. * *

Comparison to a map of collections

* *

* Multimaps are commonly used in places where a {@code Map>} would otherwise have appeared. The differences include: * *

* *

Implementations

* *

* 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 custom 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. * *

Other Notes

* *

* As with {@code Map}, the behavior of a {@code Multimap} is not specified if * key objects already present in the multimap change in a manner that affects * {@code equals} comparisons. Use caution if mutable objects are used as keys * in a {@code Multimap}. * *

* 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}. * *

* See the Guava User Guide article on * {@code Multimap}. * * @author Jared Levy * @since 2.0 (imported from Google Collections Library) */ @GwtCompatible public interface Multimap { // Query Operations /** * Returns the number of key-value pairs in this multimap. * *

* Note: this method does not return the number of distinct keys * in the multimap, which is given by {@code keySet().size()} or * {@code asMap().size()}. See the opening section of the {@link Multimap} class * documentation for clarification. */ int size(); /** * Returns {@code true} if this multimap contains no key-value pairs. Equivalent * to {@code size() == 0}, but can in some cases be more efficient. */ boolean isEmpty(); /** * Returns {@code true} if this multimap contains at least one key-value pair * with the key {@code key}. */ boolean containsKey(@Nullable Object key); /** * Returns {@code true} if this multimap contains at least one key-value pair * with the value {@code value}. */ boolean containsValue(@Nullable Object value); /** * Returns {@code true} if this multimap contains at least one key-value pair * with the key {@code key} and the value {@code value}. */ boolean containsEntry(@Nullable Object key, @Nullable Object value); // Modification Operations /** * Stores a key-value pair in this multimap. * *

* 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. * * @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 with the key {@code key} and the value * {@code value} from this multimap, if such exists. If multiple key-value pairs * in the multimap fit this description, which one is removed is unspecified. * * @return {@code true} if the multimap changed */ boolean remove(@Nullable Object key, @Nullable Object value); // Bulk Operations /** * Stores a key-value pair in this multimap for each of {@code values}, all * using the same key, {@code key}. Equivalent to (but expected to be more * efficient than): * * 

	 *    {@code
	 * 
	 *   for (V value : values) {
	 *     put(key, value);
	 *   }}
	 *
* *

* In particular, this is a no-op if {@code values} is empty. * * @return {@code true} if the multimap changed */ boolean putAll(@Nullable K key, Iterable values); /** * Stores all key-value pairs of {@code multimap} in this multimap, in the order * returned by {@code multimap.entries()}. * * @return {@code true} if the multimap changed */ boolean putAll(Multimap multimap); /** * Stores a collection of values with the same key, replacing any existing * values for that key. * *

* If {@code values} is empty, this is equivalent to {@link #removeAll(Object) * removeAll(key)}. * * @return the collection of replaced values, or an empty collection if no * values were previously associated with the key. The collection * may be modifiable, but updating it will have no effect on the * multimap. */ Collection replaceValues(@Nullable K key, Iterable values); /** * Removes all values associated with the key {@code key}. * *

* Once this method returns, {@code key} will not be mapped to any values, so it * will not appear in {@link #keySet()}, {@link #asMap()}, or any other views. * * @return the values that were removed (possibly empty). The returned * collection may be modifiable, but updating it will have no * effect on the multimap. */ Collection removeAll(@Nullable Object key); /** * Removes all key-value pairs from the multimap, leaving it * {@linkplain #isEmpty empty}. */ void clear(); // Views /** * Returns a view collection of the values associated with {@code key} in this * multimap, if any. Note that when {@code containsKey(key)} is false, this * returns an empty collection, not {@code null}. * *

* Changes to the returned collection will update the underlying multimap, and * vice versa. */ Collection get(@Nullable K key); /** * Returns a view collection of all distinct keys contained in this * multimap. Note that the key set contains a key if and only if this multimap * maps that key to at least one value. * *

* Changes to the returned set will update the underlying multimap, and vice * versa. However, adding to the returned set is not possible. */ Set keySet(); /** * Returns a view collection containing the key from each key-value pair in this * multimap, without collapsing duplicates. This collection has the same * size as this multimap, and {@code keys().count(k) == * get(k).size()} for all {@code k}. * *

* Changes to the returned multiset will update the underlying multimap, and * vice versa. However, adding to the returned collection is not * possible. */ Multiset keys(); /** * Returns a view collection containing the value from each key-value * pair contained in this multimap, without collapsing duplicates (so {@code * values().size() == size()}). * *

* Changes to the returned collection will update the underlying multimap, and * vice versa. However, adding to the returned collection is not * possible. */ Collection values(); /** * Returns a view collection of all key-value pairs contained in this multimap, * as {@link Map.Entry} instances. * *

* Changes to the returned collection or the entries it contains will update the * underlying multimap, and vice versa. However, adding to the returned * collection is not possible. */ Collection> entries(); /** * Returns a view of this multimap as a {@code Map} from each distinct key to * the nonempty collection of that key's associated values. Note that * {@code this.asMap().get(k)} is equivalent to {@code this.get(k)} only when * {@code k} is a key contained in the multimap; otherwise it returns {@code * null} as opposed to an empty collection. * *

* Changes to the returned map or the collections that serve as its values will * update the underlying multimap, and vice versa. The map does not support * {@code put} or {@code putAll}, nor do its entries support * {@link Map.Entry#setValue setValue}. */ Map> 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. * *

* 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. * *

* 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. * *

* 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(); }