    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 *      http://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 */
    017package org.apache.commons.collections4;
    018
    019import java.util.Set;
    020
    021/**
    022 * Defines a map that allows bidirectional lookup between key and values.
    023 * <p>
    024 * This extended {@code Map} represents a mapping where a key may
    025 * lookup a value and a value may lookup a key with equal ease.
    026 * This interface extends {@code Map} and so may be used anywhere a map
    027 * is required. The interface provides an inverse map view, enabling
    028 * full access to both directions of the {@code BidiMap}.
    029 * </p>
    030 * <p>
    031 * Implementations should allow a value to be looked up from a key and
    032 * a key to be looked up from a value with equal performance.
    033 * </p>
    034 * <p>
    035 * This map enforces the restriction that there is a 1:1 relation between
    036 * keys and values, meaning that multiple keys cannot map to the same value.
    037 * This is required so that "inverting" the map results in a map without
    038 * duplicate keys. See the {@link #put} method description for more information.
    039 * </p>
    040 *
    041 * @param <K> the type of the keys in the map
    042 * @param <V> the type of the values in the map
    043 * @since 3.0
    044 */
    045public interface BidiMap<K, V> extends IterableMap<K, V> {
    046
    047    /**
    048     * Gets the key that is currently mapped to the specified value.
    049     * <p>
    050     * If the value is not contained in the map, {@code null} is returned.
    051     * </p>
    052     * <p>
    053     * Implementations should seek to make this method perform equally as well
    054     * as {@code get(Object)}.
    055     * </p>
    056     *
    057     * @param value  the value to find the key for
    058     * @return the mapped key, or {@code null} if not found
    059     * @throws ClassCastException (optional) if the map limits the type of the
    060     *  value and the specified value is inappropriate
    061     * @throws NullPointerException (optional) if the map limits the values to
    062     *  non-null and null was specified
    063     */
    064    K getKey(Object value);
    065
    066    /**
    067     * Gets a view of this map where the keys and values are reversed.
    068     * <p>
    069     * Changes to one map will be visible in the other and vice versa.
    070     * This enables both directions of the map to be accessed as a {@code Map}.
    071     * </p>
    072     * <p>
    073     * Implementations should seek to avoid creating a new object every time this
    074     * method is called. See {@code AbstractMap.values()} etc. Calling this
    075     * method on the inverse map should return the original.
    076     * </p>
    077     *
    078     * @return an inverted bidirectional map
    079     */
    080    BidiMap<V, K> inverseBidiMap();
    081
    082    /**
    083     * Puts the key-value pair into the map, replacing any previous pair.
    084     * <p>
    085     * When adding a key-value pair, the value may already exist in the map
    086     * against a different key. That mapping is removed, to ensure that the
    087     * value only occurs once in the inverse map.
    088     * </p>
    089     * <pre>
    090     *  BidiMap map1 = new DualHashBidiMap();
    091     *  map.put("A","B");  // contains A mapped to B, as per Map
    092     *  map.put("A","C");  // contains A mapped to C, as per Map
    093     *
    094     *  BidiMap map2 = new DualHashBidiMap();
    095     *  map.put("A","B");  // contains A mapped to B, as per Map
    096     *  map.put("C","B");  // contains C mapped to B, key A is removed
    097     * </pre>
    098     *
    099     * @param key  the key to store
    100     * @param value  the value to store
    101     * @return the previous value mapped to this key
    102     * @throws UnsupportedOperationException if the {@code put} method is not supported
    103     * @throws ClassCastException (optional) if the map limits the type of the
    104     *  value and the specified value is inappropriate
    105     * @throws IllegalArgumentException (optional) if the map limits the values
    106     *  in some way and the value was invalid
    107     * @throws NullPointerException (optional) if the map limits the values to
    108     *  non-null and null was specified
    109     */
    110    @Override
    111    V put(K key, V value);
    112
    113    /**
    114     * Removes the key-value pair that is currently mapped to the specified
    115     * value (optional operation).
    116     * <p>
    117     * If the value is not contained in the map, {@code null} is returned.
    118     * </p>
    119     * <p>
    120     * Implementations should seek to make this method perform equally as well
    121     * as {@code remove(Object)}.
    122     * </p>
    123     *
    124     * @param value  the value to find the key-value pair for
    125     * @return the key that was removed, {@code null} if nothing removed
    126     * @throws ClassCastException (optional) if the map limits the type of the
    127     *  value and the specified value is inappropriate
    128     * @throws NullPointerException (optional) if the map limits the values to
    129     *  non-null and null was specified
    130     * @throws UnsupportedOperationException if this method is not supported
    131     *  by the implementation
    132     */
    133    K removeValue(Object value);
    134
    135    /**
    136     * Returns a {@link Set} view of the values contained in this map.
    137     * The set is backed by the map, so changes to the map are reflected
    138     * in the set, and vice-versa.  If the map is modified while an iteration
    139     * over the set is in progress (except through the iterator's own
    140     * {@code remove} operation), the results of the iteration are undefined.
    141     * The set supports element removal, which removes the corresponding
    142     * mapping from the map, via the {@code Iterator.remove},
    143     * {@code Collection.remove}, {@code removeAll},
    144     * {@code retainAll} and {@code clear} operations.  It does not
    145     * support the {@code add} or {@code addAll} operations.
    146     *
    147     * @return a set view of the values contained in this map
    148     */
    149    @Override
    150    Set<V> values();
    151}