    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     * Creates a comparator that compares objects based on the inverse of their
    043     * natural ordering.  Using this Constructor will create a ReverseComparator
    044     * that is functionally identical to the Comparator returned by
    045     * java.util.Collections.<b>reverseOrder()</b>.
    046     *
    047     * @see java.util.Collections#reverseOrder()
    048     */
    049    public ReverseComparator() {
    050        this(null);
    051    }
    052
    053    /**
    054     * Creates a comparator that inverts the comparison
    055     * of the given comparator.  If you pass in {@code null},
    056     * the ReverseComparator defaults to reversing the
    057     * natural order, as per {@link java.util.Collections#reverseOrder()}.
    058     *
    059     * @param comparator Comparator to reverse
    060     */
    061    public ReverseComparator(final Comparator<? super E> comparator) {
    062        this.comparator = comparator == null ? ComparatorUtils.NATURAL_COMPARATOR : comparator;
    063    }
    064
    065    /**
    066     * Compares two objects in reverse order.
    067     *
    068     * @param obj1  the first object to compare
    069     * @param obj2  the second object to compare
    070     * @return negative if obj1 is less, positive if greater, zero if equal
    071     */
    072    @Override
    073    public int compare(final E obj1, final E obj2) {
    074        return comparator.compare(obj2, obj1);
    075    }
    076
    077    /**
    078     * Returns {@code true} iff <i>that</i> Object is
    079     * a {@link Comparator} whose ordering is known to be
    080     * equivalent to mine.
    081     * <p>
    082     * This implementation returns {@code true}
    083     * iff {@code <i>object</i>.{@link Object#getClass() getClass()}}
    084     * equals {@code this.getClass()}, and the underlying
    085     * comparators are equal.
    086     * Subclasses may want to override this behavior to remain consistent
    087     * with the {@link Comparator#equals(Object) equals} contract.
    088     *
    089     * @param object  the object to compare to
    090     * @return true if equal
    091     * @since 3.0
    092     */
    093    @Override
    094    public boolean equals(final Object object) {
    095        if (this == object) {
    096            return true;
    097        }
    098        if (null == object) {
    099            return false;
    100        }
    101        if (object.getClass().equals(this.getClass())) {
    102            final ReverseComparator<?> thatrc = (ReverseComparator<?>) object;
    103            return comparator.equals(thatrc.comparator);
    104        }
    105        return false;
    106    }
    107
    108    /**
    109     * Implement a hash code for this comparator that is consistent with
    110     * {@link #equals(Object) equals}.
    111     *
    112     * @return a suitable hash code
    113     * @since 3.0
    114     */
    115    @Override
    116    public int hashCode() {
    117        return "ReverseComparator".hashCode() ^ comparator.hashCode();
    118    }
    119
    120}