com.google.common.collect.Multimap Maven / Gradle / Ivy
/*
* 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 multiple values. You can visualize the
* contents of a multimap either as a map from keys to nonempty
* collections of values:
*
*
* - a → 1, 2
*
- b → 3
*
*
* ... or as a single "flattened" collection of key-value pairs:
*
*
* - a → 1
*
- a → 2
*
- b → 3
*
*
* 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:
*
*
* - {@link #asMap}, mentioned above
* - {@link #keys}, {@link #keySet}, {@link #values}, {@link #entries}, which
* are similar to the corresponding view collections of {@link Map}
*
- and, notably, even the collection returned by {@link #get get(key)} is an
* active view of the values corresponding to {@code key}
*
*
* 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 and may or
* may not have the same {@code hashCode}. The recommended subinterfaces
* provide much stronger guarantees.
*
*
Comparison to a map of collections
*
* Multimaps are commonly used in places where a {@code Map>} would otherwise have appeared. The differences include:
*
*
* - There is no need to populate an empty collection before adding an entry
* with {@link #put put}.
*
- {@code get} never returns {@code null}, only an empty collection.
*
- 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 removing that key from the multimap.
*
- The total entry count is available as {@link #size}.
*
- Many complex operations become easier; for example, {@code
* Collections.min(multimap.values())} finds the smallest value across all
* keys.
*
*
* 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}.
*
*
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();
}