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     */
017    package org.apache.commons.collections;
018    
019    /**
020     * Defines a map that allows bidirectional lookup between key and values.
021     * <p>
022     * This extended <code>Map</code> represents a mapping where a key may
023     * lookup a value and a value may lookup a key with equal ease.
024     * This interface extends <code>Map</code> and so may be used anywhere a map
025     * is required. The interface provides an inverse map view, enabling
026     * full access to both directions of the <code>BidiMap</code>.
027     * <p>
028     * Implementations should allow a value to be looked up from a key and
029     * a key to be looked up from a value with equal performance.
030     * <p>
031     * This map enforces the restriction that there is a 1:1 relation between
032     * keys and values, meaning that multiple keys cannot map to the same value. 
033     * This is required so that "inverting" the map results in a map without 
034     * duplicate keys. See the {@link #put} method description for more information.
035     * 
036     * @since Commons Collections 3.0
037     * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
038     *
039     * @author Stephen Colebourne
040     */
041    public interface BidiMap extends IterableMap {
042    
043        /**
044         * Obtains a <code>MapIterator</code> over the map.
045         * <p>
046         * A map iterator is an efficient way of iterating over maps.
047         * It does not require that the map is stored using Map Entry objects
048         * which can increase performance.
049         * <pre>
050         * BidiMap map = new DualHashBidiMap();
051         * MapIterator it = map.mapIterator();
052         * while (it.hasNext()) {
053         *   Object key = it.next();
054         *   Object value = it.getValue();
055         *   it.setValue("newValue");
056         * }
057         * </pre>
058         * 
059         * @return a map iterator
060         */
061        MapIterator mapIterator();
062        
063        /**
064         * Puts the key-value pair into the map, replacing any previous pair.
065         * <p>
066         * When adding a key-value pair, the value may already exist in the map
067         * against a different key. That mapping is removed, to ensure that the
068         * value only occurs once in the inverse map.
069         * <pre>
070         *  BidiMap map1 = new DualHashBidiMap();
071         *  map.put("A","B");  // contains A mapped to B, as per Map
072         *  map.put("A","C");  // contains A mapped to C, as per Map
073         * 
074         *  BidiMap map2 = new DualHashBidiMap();
075         *  map.put("A","B");  // contains A mapped to B, as per Map
076         *  map.put("C","B");  // contains C mapped to B, key A is removed
077         * </pre>
078         *
079         * @param key  the key to store
080         * @param value  the value to store
081         * @return the previous value mapped to this key
082         * 
083         * @throws UnsupportedOperationException if the <code>put</code> method is not supported
084         * @throws ClassCastException (optional) if the map limits the type of the 
085         *  value and the specified value is inappropriate
086         * @throws IllegalArgumentException (optional) if the map limits the values
087         *  in some way and the value was invalid
088         * @throws NullPointerException (optional) if the map limits the values to
089         *  non-null and null was specified
090         */
091        Object put(Object key, Object value);
092        
093        /**
094         * Gets the key that is currently mapped to the specified value.
095         * <p>
096         * If the value is not contained in the map, <code>null</code> is returned.
097         * <p>
098         * Implementations should seek to make this method perform equally as well
099         * as <code>get(Object)</code>.
100         *
101         * @param value  the value to find the key for
102         * @return the mapped key, or <code>null</code> if not found
103         * 
104         * @throws ClassCastException (optional) if the map limits the type of the 
105         *  value and the specified value is inappropriate
106         * @throws NullPointerException (optional) if the map limits the values to
107         *  non-null and null was specified
108         */
109        Object getKey(Object value);
110        
111        /**
112         * Removes the key-value pair that is currently mapped to the specified
113         * value (optional operation).
114         * <p>
115         * If the value is not contained in the map, <code>null</code> is returned.
116         * <p>
117         * Implementations should seek to make this method perform equally as well
118         * as <code>remove(Object)</code>.
119         *
120         * @param value  the value to find the key-value pair for
121         * @return the key that was removed, <code>null</code> if nothing removed
122         * 
123         * @throws ClassCastException (optional) if the map limits the type of the 
124         *  value and the specified value is inappropriate
125         * @throws NullPointerException (optional) if the map limits the values to
126         *  non-null and null was specified
127         * @throws UnsupportedOperationException if this method is not supported
128         *  by the implementation
129         */
130        Object removeValue(Object value);
131        
132        /**
133         * Gets a view of this map where the keys and values are reversed.
134         * <p>
135         * Changes to one map will be visible in the other and vice versa.
136         * This enables both directions of the map to be accessed as a <code>Map</code>.
137         * <p>
138         * Implementations should seek to avoid creating a new object every time this
139         * method is called. See <code>AbstractMap.values()</code> etc. Calling this
140         * method on the inverse map should return the original.
141         *
142         * @return an inverted bidirectional map
143         */
144        BidiMap inverseBidiMap();
145        
146    }