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The Apache Commons Collections package contains 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.collections4;

import java.util.Comparator;

/**
 * Defines a type of Bag that maintains a sorted order among
 * its unique representative members.
 *
 * @param  the type of elements in this bag
 * @since 2.0
 */
public interface SortedBag extends Bag {

    /**
     * Returns the comparator associated with this sorted set, or null
     * if it uses its elements' natural ordering.
     *
     * @return the comparator in use, or null if natural ordering
     */
    Comparator comparator();

    /**
     * Returns the first (lowest) member.
     *
     * @return the first element in the sorted bag
     */
    E first();

    /**
     * Returns the last (highest) member.
     *
     * @return the last element in the sorted bag
     */
    E last();

}