001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      https://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017
018package org.apache.commons.collections4;
019
020import java.util.Set;
021
022/**
023 * Defines a map that allows bidirectional lookup between key and values.
024 * <p>
025 * This extended {@code Map} represents a mapping where a key may lookup a value and a value may lookup a key with equal ease. This interface extends
026 * {@code Map} and so may be used anywhere a map is required. The interface provides an inverse map view, enabling full access to both directions of the
027 * {@code BidiMap}.
028 * </p>
029 * <p>
030 * Implementations should allow a value to be looked up from a key and a key to be looked up from a value with equal performance.
031 * </p>
032 * <p>
033 * This map enforces the restriction that there is a 1:1 relation between keys and values, meaning that multiple keys cannot map to the same value. This is
034 * required so that "inverting" the map results in a map without duplicate keys. See the {@link #put} method description for more information.
035 * </p>
036 *
037 * @param <K> The type of the keys in the map
038 * @param <V> The type of the values in the map
039 * @since 3.0
040 */
041public interface BidiMap<K, V> extends IterableMap<K, V> {
042
043    /**
044     * Gets the key that is currently mapped to the specified value.
045     * <p>
046     * If the value is not contained in the map, {@code null} is returned.
047     * </p>
048     * <p>
049     * Implementations should seek to make this method perform equally as well as {@code get(Object)}.
050     * </p>
051     *
052     * @param value The value to find the key for.
053     * @return The mapped key, or {@code null} if not found.
054     * @throws ClassCastException   (optional) if the map limits the type of the value and the specified value is inappropriate.
055     * @throws NullPointerException (optional) if the map limits the values to non-null and null was specified.
056     */
057    K getKey(Object value);
058
059    /**
060     * Gets a view of this map where the keys and values are reversed.
061     * <p>
062     * Changes to one map will be visible in the other and vice versa. This enables both directions of the map to be accessed as a {@code Map}.
063     * </p>
064     * <p>
065     * Implementations should seek to avoid creating a new object every time this method is called. See {@code AbstractMap.values()} etc. Calling this method on
066     * the inverse map should return the original.
067     * </p>
068     *
069     * @return An inverted bidirectional map
070     */
071    BidiMap<V, K> inverseBidiMap();
072
073    /**
074     * Puts the key-value pair into the map, replacing any previous pair.
075     * <p>
076     * When adding a key-value pair, the value may already exist in the map against a different key. That mapping is removed, to ensure that the value only
077     * occurs once in the inverse map.
078     * </p>
079     *
080     * <pre>
081     * BidiMap map1 = new DualHashBidiMap();
082     * map.put("A", "B"); // contains A mapped to B, as per Map
083     * map.put("A", "C"); // contains A mapped to C, as per Map
084     * BidiMap map2 = new DualHashBidiMap();
085     * map.put("A", "B"); // contains A mapped to B, as per Map
086     * map.put("C", "B"); // contains C mapped to B, key A is removed
087     * </pre>
088     *
089     * @param key   The key to store.
090     * @param value The value to store.
091     * @return The previous value mapped to this key.
092     * @throws UnsupportedOperationException if the {@code put} method is not supported.
093     * @throws ClassCastException            (optional) if the map limits the type of the value and the specified value is inappropriate.
094     * @throws IllegalArgumentException      (optional) if the map limits the values in some way and the value was invalid.
095     * @throws NullPointerException          (optional) if the map limits the values to non-null and null was specified.
096     */
097    @Override
098    V put(K key, V value);
099
100    /**
101     * Removes the key-value pair that is currently mapped to the specified value (optional operation).
102     * <p>
103     * If the value is not contained in the map, {@code null} is returned.
104     * </p>
105     * <p>
106     * Implementations should seek to make this method perform equally as well as {@code remove(Object)}.
107     * </p>
108     *
109     * @param value The value to find the key-value pair for.
110     * @return The key that was removed, {@code null} if nothing removed.
111     * @throws ClassCastException            (optional) if the map limits the type of the value and the specified value is inappropriate.
112     * @throws NullPointerException          (optional) if the map limits the values to non-null and null was specified.
113     * @throws UnsupportedOperationException if this method is not supported by the implementation.
114     */
115    K removeValue(Object value);
116
117    /**
118     * Returns a {@link Set} view of the values contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and
119     * vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's own {@code remove} operation), the
120     * results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the
121     * {@code Iterator.remove}, {@code Collection.remove}, {@code removeAll}, {@code retainAll} and {@code clear} operations. It does not support the
122     * {@code add} or {@code addAll} operations.
123     *
124     * @return A set view of the values contained in this map.
125     */
126    @Override
127    Set<V> values();
128}