src/main/java/org/apache/commons/collections4/BidiMap.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 org.apache.commons.collections4;

import java.util.Set;

/**
 * Defines a map that allows bidirectional lookup between key and values.
 * <p>
 * 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 {@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 {@code BidiMap}.
 * </p>
 * <p>
 * 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.
 * </p>
 * <p>
 * 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 required so that "inverting" the map results in a map without
 * duplicate keys. See the {@link #put} method description for more information.
 * </p>
 *
 * @param <K> the type of the keys in the map
 * @param <V> the type of the values in the map
 *
 * @since 3.0
 */
public interface BidiMap<K, V> extends IterableMap<K, V> {

    /**
     * Puts the key-value pair into the map, replacing any previous pair.
     * <p>
     * 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 occurs once in the inverse map.
     * </p>
     * <pre>
     *  BidiMap map1 = new DualHashBidiMap();
     *  map.put("A","B");  // contains A mapped to B, as per Map
     *  map.put("A","C");  // contains A mapped to C, as per Map
     *
     *  BidiMap map2 = new DualHashBidiMap();
     *  map.put("A","B");  // contains A mapped to B, as per Map
     *  map.put("C","B");  // contains C mapped to B, key A is removed
     * </pre>
     *
     * @param key  the key to store
     * @param value  the value to store
     * @return the previous value mapped to this key
     *
     * @throws UnsupportedOperationException if the {@code put} method is not supported
     * @throws ClassCastException (optional) if the map limits the type of the
     *  value and the specified value is inappropriate
     * @throws IllegalArgumentException (optional) if the map limits the values
     *  in some way and the value was invalid
     * @throws NullPointerException (optional) if the map limits the values to
     *  non-null and null was specified
     */
    @Override
    V put(K key, V value);

    /**
     * Gets the key that is currently mapped to the specified value.
     * <p>
     * If the value is not contained in the map, {@code null} is returned.
     * </p>
     * <p>
     * Implementations should seek to make this method perform equally as well
     * as {@code get(Object)}.
     * </p>
     *
     * @param value  the value to find the key for
     * @return the mapped key, or {@code null} if not found
     *
     * @throws ClassCastException (optional) if the map limits the type of the
     *  value and the specified value is inappropriate
     * @throws NullPointerException (optional) if the map limits the values to
     *  non-null and null was specified
     */
    K getKey(Object value);

    /**
     * Removes the key-value pair that is currently mapped to the specified
     * value (optional operation).
     * <p>
     * If the value is not contained in the map, {@code null} is returned.
     * </p>
     * <p>
     * Implementations should seek to make this method perform equally as well
     * as {@code remove(Object)}.
     * </p>
     *
     * @param value  the value to find the key-value pair for
     * @return the key that was removed, {@code null} if nothing removed
     *
     * @throws ClassCastException (optional) if the map limits the type of the
     *  value and the specified value is inappropriate
     * @throws NullPointerException (optional) if the map limits the values to
     *  non-null and null was specified
     * @throws UnsupportedOperationException if this method is not supported
     *  by the implementation
     */
    K removeValue(Object value);

    /**
     * Gets a view of this map where the keys and values are reversed.
     * <p>
     * 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}.
     * </p>
     * <p>
     * Implementations should seek to avoid creating a new object every time this
     * method is called. See {@code AbstractMap.values()} etc. Calling this
     * method on the inverse map should return the original.
     * </p>
     *
     * @return an inverted bidirectional map
     */
    BidiMap<V, K> inverseBidiMap();

    /**
     * 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 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 results of the iteration are undefined.
     * The set supports element removal, which removes the corresponding
     * mapping from the map, via the {@code Iterator.remove},
     * {@code Collection.remove}, {@code removeAll},
     * {@code retainAll} and {@code clear} operations.  It does not
     * support the {@code add} or {@code addAll} operations.
     *
     * @return a set view of the values contained in this map
     */
    @Override
    Set<V> values();
}