    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 */
    017package org.apache.commons.collections4.comparators;
    018
    019import java.io.Serializable;
    020import java.util.Comparator;
    021
    022import org.apache.commons.collections4.ComparatorUtils;
    023
    024/**
    025 * Reverses the order of another comparator by reversing the arguments
    026 * to its {@link #compare(Object, Object) compare} method.
    027 *
    028 * @param <E> the type of objects compared by this comparator
    029 *
    030 * @since 2.0
    031 * @see java.util.Collections#reverseOrder()
    032 */
    033public class ReverseComparator<E> implements Comparator<E>, Serializable {
    034
    035    /** Serialization version from Collections 2.0. */
    036    private static final long serialVersionUID = 2858887242028539265L;
    037
    038    /** The comparator being decorated. */
    039    private final Comparator<? super E> comparator;
    040
    041    //-----------------------------------------------------------------------
    042    /**
    043     * Creates a comparator that compares objects based on the inverse of their
    044     * natural ordering.  Using this Constructor will create a ReverseComparator
    045     * that is functionally identical to the Comparator returned by
    046     * java.util.Collections.<b>reverseOrder()</b>.
    047     *
    048     * @see java.util.Collections#reverseOrder()
    049     */
    050    public ReverseComparator() {
    051        this(null);
    052    }
    053
    054    /**
    055     * Creates a comparator that inverts the comparison
    056     * of the given comparator.  If you pass in <code>null</code>,
    057     * the ReverseComparator defaults to reversing the
    058     * natural order, as per {@link java.util.Collections#reverseOrder()}.
    059     *
    060     * @param comparator Comparator to reverse
    061     */
    062    public ReverseComparator(final Comparator<? super E> comparator) {
    063        this.comparator = comparator == null ? ComparatorUtils.NATURAL_COMPARATOR : comparator;
    064    }
    065
    066    //-----------------------------------------------------------------------
    067    /**
    068     * Compares two objects in reverse order.
    069     *
    070     * @param obj1  the first object to compare
    071     * @param obj2  the second object to compare
    072     * @return negative if obj1 is less, positive if greater, zero if equal
    073     */
    074    @Override
    075    public int compare(final E obj1, final E obj2) {
    076        return comparator.compare(obj2, obj1);
    077    }
    078
    079    //-----------------------------------------------------------------------
    080    /**
    081     * Implement a hash code for this comparator that is consistent with
    082     * {@link #equals(Object) equals}.
    083     *
    084     * @return a suitable hash code
    085     * @since 3.0
    086     */
    087    @Override
    088    public int hashCode() {
    089        return "ReverseComparator".hashCode() ^ comparator.hashCode();
    090    }
    091
    092    /**
    093     * Returns <code>true</code> iff <i>that</i> Object is
    094     * is a {@link Comparator} whose ordering is known to be
    095     * equivalent to mine.
    096     * <p>
    097     * This implementation returns <code>true</code>
    098     * iff <code><i>object</i>.{@link Object#getClass() getClass()}</code>
    099     * equals <code>this.getClass()</code>, and the underlying
    100     * comparators are equal.
    101     * Subclasses may want to override this behavior to remain consistent
    102     * with the {@link Comparator#equals(Object) equals} contract.
    103     *
    104     * @param object  the object to compare to
    105     * @return true if equal
    106     * @since 3.0
    107     */
    108    @Override
    109    public boolean equals(final Object object) {
    110        if (this == object) {
    111            return true;
    112        }
    113        if (null == object) {
    114            return false;
    115        }
    116        if (object.getClass().equals(this.getClass())) {
    117            final ReverseComparator<?> thatrc = (ReverseComparator<?>) object;
    118            return comparator.equals(thatrc.comparator);
    119        }
    120        return false;
    121    }
    122
    123}