src/java/org/apache/commons/collections/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.collections;

/**
 * Defines a map that allows bidirectional lookup between key and values.
 * <p>
 * This extended <code>Map</code> 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</code> 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</code>.
 * <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>
 * 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.
 *
 * @param <K> the type of the keys in the map
 * @param <V> the type of the values in the map
 * @since Commons Collections 3.0
 * @version $Revision$ $Date$
 *
 * @author Stephen Colebourne
 */
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.
     * <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</code> 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
     */
    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</code> is returned.
     * <p>
     * Implementations should seek to make this method perform equally as well
     * as <code>get(Object)</code>.
     *
     * @param value  the value to find the key for
     * @return the mapped key, or <code>null</code> 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</code> is returned.
     * <p>
     * Implementations should seek to make this method perform equally as well
     * as <code>remove(Object)</code>.
     *
     * @param value  the value to find the key-value pair for
     * @return the key that was removed, <code>null</code> 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</code>.
     * <p>
     * Implementations should seek to avoid creating a new object every time this
     * method is called. See <code>AbstractMap.values()</code> etc. Calling this
     * method on the inverse map should return the original.
     *
     * @return an inverted bidirectional map
     */
    BidiMap<V, K> inverseBidiMap();

}