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The Apache Commons Collections package contains types that extend and augment the Java Collections Framework.

There is a newer version: 4.5.0-M1

/*
 * 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.Comparator;
import java.util.SortedMap;

/**
 * Defines a map that allows bidirectional lookup between key and values
 * and retains both keys and values in sorted order.
 * 
 * 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.
 * 
 *
 * @param  the type of the keys in the map
 * @param  the type of the values in the map
 * @since 3.0
 */
public interface SortedBidiMap extends OrderedBidiMap, SortedMap {

    /**
     * Gets a view of this map where the keys and values are reversed.
     * 
     * Changes to one map will be visible in the other and vice versa.
     * This enables both directions of the map to be accessed equally.
     * 

     * Implementations should seek to avoid creating a new object every time this
     * method is called. See AbstractMap.values() etc. Calling this
     * method on the inverse map should return the original.
     * 
     * Implementations must return a SortedBidiMap instance,
     * usually by forwarding to inverseSortedBidiMap().
     *
     * @return an inverted bidirectional map
     */
    @Override
    SortedBidiMap inverseBidiMap();

    /**
     * Get the comparator used for the values in the value-to-key map aspect.
     * @return Comparator<? super V>
     */
    Comparator valueComparator();
}