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 static com.google.common.base.Preconditions.checkNotNull;
import com.google.common.annotations.GwtCompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.CompatibleWith;
import com.google.errorprone.annotations.DoNotMock;
import java.util.Collection;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
import java.util.function.BiConsumer;
import org.checkerframework.checker.nullness.qual.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] // Remember, Quincy!
* George: [Washington, Bush, Bush]
* Grover: [Cleveland, Cleveland] // Two, non-consecutive terms, rep'ing NJ!
* ...
* }</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>{@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 and may or may not have the same {@code hashCode}. The recommended subinterfaces
* provide much stronger guarantees.
*
* <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 is contained in the multimap if and only if it 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>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}.
*
* <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=
* "https://github.com/google/guava/wiki/NewCollectionTypesExplained#multimap"> {@code
* Multimap}</a>.
*
* @author Jared Levy
* @since 2.0
*/
@DoNotMock("Use ImmutableMultimap, HashMultimap, or another implementation")
@GwtCompatible
public interface Multimap<K, V> {
// Query Operations
/**
* Returns the number of key-value pairs in this multimap.
*
* <p><b>Note:</b> this method does not return the number of <i>distinct keys</i> 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(@CompatibleWith("K") @Nullable Object key);
/**
* Returns {@code true} if this multimap contains at least one key-value pair with the value
* {@code value}.
*/
boolean containsValue(@CompatibleWith("V") @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(
@CompatibleWith("K") @Nullable Object key, @CompatibleWith("V") @Nullable Object value);
// Modification Operations
/**
* Stores a key-value pair in this 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.
*
* @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
*/
@CanIgnoreReturnValue
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
*/
@CanIgnoreReturnValue
boolean remove(
@CompatibleWith("K") @Nullable Object key, @CompatibleWith("V") @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):
*
* <pre>{@code
* for (V value : values) {
* put(key, value);
* }
* }</pre>
*
* <p>In particular, this is a no-op if {@code values} is empty.
*
* @return {@code true} if the multimap changed
*/
@CanIgnoreReturnValue
boolean putAll(@Nullable K key, Iterable<? extends V> 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
*/
@CanIgnoreReturnValue
boolean putAll(Multimap<? extends K, ? extends V> multimap);
/**
* Stores a collection of values with the same key, replacing any existing values for that key.
*
* <p>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 <i>may</i> be modifiable, but updating it will have
* no effect on the multimap.
*/
@CanIgnoreReturnValue
Collection<V> replaceValues(@Nullable K key, Iterable<? extends V> values);
/**
* Removes all values associated with the key {@code key}.
*
* <p>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 <i>may</i> be
* modifiable, but updating it will have no effect on the multimap.
*/
@CanIgnoreReturnValue
Collection<V> removeAll(@CompatibleWith("K") @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}.
*
* <p>Changes to the returned collection will update the underlying multimap, and vice versa.
*/
Collection<V> get(@Nullable K key);
/**
* Returns a view collection of all <i>distinct</i> 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.
*
* <p>Changes to the returned set will update the underlying multimap, and vice versa. However,
* <i>adding</i> to the returned set is not possible.
*/
Set<K> keySet();
/**
* Returns a view collection containing the key from each key-value pair in this multimap,
* <i>without</i> collapsing duplicates. This collection has the same size as this multimap, and
* {@code keys().count(k) == get(k).size()} for all {@code k}.
*
* <p>Changes to the returned multiset will update the underlying multimap, and vice versa.
* However, <i>adding</i> to the returned collection is not possible.
*/
Multiset<K> keys();
/**
* Returns a view collection containing the <i>value</i> from each key-value pair contained in
* this multimap, without collapsing duplicates (so {@code values().size() == size()}).
*
* <p>Changes to the returned collection will update the underlying multimap, and vice versa.
* However, <i>adding</i> to the returned collection is not possible.
*/
Collection<V> values();
/**
* Returns a view collection of all key-value pairs contained in this multimap, as {@link Entry}
* instances.
*
* <p>Changes to the returned collection or the entries it contains will update the underlying
* multimap, and vice versa. However, <i>adding</i> to the returned collection is not possible.
*/
Collection<Entry<K, V>> entries();
/**
* Performs the given action for all key-value pairs contained in this multimap. If an ordering is
* specified by the {@code Multimap} implementation, actions will be performed in the order of
* iteration of {@link #entries()}. Exceptions thrown by the action are relayed to the caller.
*
* <p>To loop over all keys and their associated value collections, write {@code
* Multimaps.asMap(multimap).forEach((key, valueCollection) -> action())}.
*
* @since 21.0
*/
default void forEach(BiConsumer<? super K, ? super V> action) {
checkNotNull(action);
entries().forEach(entry -> action.accept(entry.getKey(), entry.getValue()));
}
/**
* 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.
*
* <p>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 Entry#setValue setValue}.
*/
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}.
*
* <p>In general, two multimaps with identical key-value mappings may or may not have the same
* hash codes, depending on the implementation. For example, two {@link SetMultimap} instances
* with the same key-value mappings will have the same {@code hashCode}, but the {@code hashCode}
* of {@link ListMultimap} instances depends on the ordering of the values for each key.
*/
@Override
int hashCode();
}