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