    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    @SuppressWarnings("unchecked")
    063    public ReverseComparator(final Comparator<? super E> comparator) {
    064        this.comparator = comparator == null ? ComparatorUtils.NATURAL_COMPARATOR : comparator;
    065    }
    066
    067    //-----------------------------------------------------------------------
    068    /**
    069     * Compares two objects in reverse order.
    070     *
    071     * @param obj1  the first object to compare
    072     * @param obj2  the second object to compare
    073     * @return negative if obj1 is less, positive if greater, zero if equal
    074     */
    075    @Override
    076    public int compare(final E obj1, final E obj2) {
    077        return comparator.compare(obj2, obj1);
    078    }
    079
    080    //-----------------------------------------------------------------------
    081    /**
    082     * Implement a hash code for this comparator that is consistent with
    083     * {@link #equals(Object) equals}.
    084     *
    085     * @return a suitable hash code
    086     * @since 3.0
    087     */
    088    @Override
    089    public int hashCode() {
    090        return "ReverseComparator".hashCode() ^ comparator.hashCode();
    091    }
    092
    093    /**
    094     * Returns <code>true</code> iff <i>that</i> Object is
    095     * is a {@link Comparator} whose ordering is known to be
    096     * equivalent to mine.
    097     * <p>
    098     * This implementation returns <code>true</code>
    099     * iff <code><i>object</i>.{@link Object#getClass() getClass()}</code>
    100     * equals <code>this.getClass()</code>, and the underlying
    101     * comparators are equal.
    102     * Subclasses may want to override this behavior to remain consistent
    103     * with the {@link Comparator#equals(Object) equals} contract.
    104     *
    105     * @param object  the object to compare to
    106     * @return true if equal
    107     * @since 3.0
    108     */
    109    @Override
    110    public boolean equals(final Object object) {
    111        if (this == object) {
    112            return true;
    113        }
    114        if (null == object) {
    115            return false;
    116        }
    117        if (object.getClass().equals(this.getClass())) {
    118            final ReverseComparator<?> thatrc = (ReverseComparator<?>) object;
    119            return comparator.equals(thatrc.comparator);
    120        }
    121        return false;
    122    }
    123
    124}