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Types that extend and augment the Java Collections Framework.
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/*
 *  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.bidimap;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.HashMap;
import java.util.Map;

import org.apache.commons.collections.BidiMap;

/**
 * Implementation of BidiMap that uses two HashMap instances.
 * 
 * Two HashMap instances are used in this class.
 * This provides fast lookups at the expense of storing two sets of map entries.
 * Commons Collections would welcome the addition of a direct hash-based
 * implementation of the BidiMap interface.
 * 
 * NOTE: From Commons Collections 3.1, all subclasses will use HashMap
 * and the flawed createMap method is ignored.
 * 
 * @since Commons Collections 3.0
 * @version $Id: DualHashBidiMap.java 646777 2008-04-10 12:33:15Z niallp $
 * 
 * @author Matthew Hawthorne
 * @author Stephen Colebourne
 */
public class DualHashBidiMap
        extends AbstractDualBidiMap implements Serializable {

    /** Ensure serialization compatibility */
    private static final long serialVersionUID = 721969328361808L;

    /**
     * Creates an empty HashBidiMap.
     */
    public DualHashBidiMap() {
        super(new HashMap(), new HashMap());
    }

    /** 
     * Constructs a HashBidiMap and copies the mappings from
     * specified Map.  
     *
     * @param map  the map whose mappings are to be placed in this map
     */
    public DualHashBidiMap(Map map) {
        super(new HashMap(), new HashMap());
        putAll(map);
    }
    
    /** 
     * Constructs a HashBidiMap that decorates the specified maps.
     *
     * @param normalMap  the normal direction map
     * @param reverseMap  the reverse direction map
     * @param inverseBidiMap  the inverse BidiMap
     */
    protected DualHashBidiMap(Map normalMap, Map reverseMap, BidiMap inverseBidiMap) {
        super(normalMap, reverseMap, inverseBidiMap);
    }

    /**
     * Creates a new instance of this object.
     * 
     * @param normalMap  the normal direction map
     * @param reverseMap  the reverse direction map
     * @param inverseBidiMap  the inverse BidiMap
     * @return new bidi map
     */
    protected BidiMap createBidiMap(Map normalMap, Map reverseMap, BidiMap inverseBidiMap) {
        return new DualHashBidiMap(normalMap, reverseMap, inverseBidiMap);
    }

    // Serialization
    //-----------------------------------------------------------------------
    private void writeObject(ObjectOutputStream out) throws IOException {
        out.defaultWriteObject();
        out.writeObject(maps[0]);
    }

    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
        in.defaultReadObject();
        maps[0] = new HashMap();
        maps[1] = new HashMap();
        Map map = (Map) in.readObject();
        putAll(map);
    }

}