    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
    022/**
    023 * A {@link Comparator Comparator} that compares {@link Comparable Comparable}
    024 * objects.
    025 * <p>
    026 * This Comparator is useful, for example, for enforcing the natural order in
    027 * custom implementations of {@link java.util.SortedSet SortedSet} and
    028 * {@link java.util.SortedMap SortedMap}.
    029 * </p>
    030 * <p>
    031 * Note: In the 2.0 and 2.1 releases of Commons Collections, this class would
    032 * throw a {@link ClassCastException} if either of the arguments to
    033 * {@link #compare(Comparable, Comparable)} compare} were {@code null}, not
    034 * {@link Comparable Comparable}, or for which
    035 * {@link Comparable#compareTo(Object) compareTo} gave inconsistent results.
    036 * This is no longer the case. See {@link #compare(Comparable, Comparable)} compare} for
    037 * details.
    038 * </p>
    039 *
    040 * @param <E> the type of objects compared by this comparator
    041 *
    042 * @since 2.0
    043 * @see java.util.Collections#reverseOrder()
    044 */
    045public class ComparableComparator<E extends Comparable<? super E>> implements Comparator<E>, Serializable {
    046
    047    /** Serialization version. */
    048    private static final long serialVersionUID=-291439688585137865L;
    049
    050    /** The singleton instance. */
    051    @SuppressWarnings("rawtypes")
    052    public static final ComparableComparator INSTANCE = new ComparableComparator();
    053
    054    /**
    055     * Gets the singleton instance of a ComparableComparator.
    056     * <p>
    057     * Developers are encouraged to use the comparator returned from this method
    058     * instead of constructing a new instance to reduce allocation and GC overhead
    059     * when multiple comparable comparators may be used in the same VM.
    060     *
    061     * @param <E>  the element type
    062     * @return the singleton ComparableComparator
    063     * @since 4.0
    064     */
    065    public static <E extends Comparable<? super E>> ComparableComparator<E> comparableComparator() {
    066        return INSTANCE;
    067    }
    068
    069    /**
    070     * Constructor whose use should be avoided.
    071     * <p>
    072     * Please use the {@link #comparableComparator()} method whenever possible.
    073     */
    074    public ComparableComparator() {
    075    }
    076
    077    /**
    078     * Compare the two {@link Comparable Comparable} arguments.
    079     * This method is equivalent to:
    080     * <pre>((Comparable)obj1).compareTo(obj2)</pre>
    081     *
    082     * @param obj1  the first object to compare
    083     * @param obj2  the second object to compare
    084     * @return negative if obj1 is less, positive if greater, zero if equal
    085     * @throws NullPointerException if <i>obj1</i> is {@code null},
    086     *         or when {@code ((Comparable)obj1).compareTo(obj2)} does
    087     * @throws ClassCastException if <i>obj1</i> is not a {@code Comparable},
    088     *         or when {@code ((Comparable)obj1).compareTo(obj2)} does
    089     */
    090    @Override
    091    public int compare(final E obj1, final E obj2) {
    092        return obj1.compareTo(obj2);
    093    }
    094
    095    /**
    096     * Returns {@code true} iff <i>that</i> Object is a {@link Comparator Comparator}
    097     * whose ordering is known to be equivalent to mine.
    098     * <p>
    099     * This implementation returns {@code true} iff
    100     * {@code <i>object</i>.{@link Object#getClass() getClass()}} equals
    101     * {@code this.getClass()}. Subclasses may want to override this behavior to remain
    102     * consistent with the {@link Comparator#equals(Object)} contract.
    103     *
    104     * @param object  the object to compare with
    105     * @return {@code true} if equal
    106     * @since 3.0
    107     */
    108    @Override
    109    public boolean equals(final Object object) {
    110        return this == object ||
    111               null != object && object.getClass().equals(this.getClass());
    112    }
    113
    114    /**
    115     * Implement a hash code for this comparator that is consistent with
    116     * {@link #equals(Object) equals}.
    117     *
    118     * @return a hash code for this comparator.
    119     * @since 3.0
    120     */
    121    @Override
    122    public int hashCode() {
    123        return "ComparableComparator".hashCode();
    124    }
    125
    126}