    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(Object, Object) compare} were <code>null</code>, 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(Object, Object) 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    /**
    056     * Gets the singleton instance of a ComparableComparator.
    057     * <p>
    058     * Developers are encouraged to use the comparator returned from this method
    059     * instead of constructing a new instance to reduce allocation and GC overhead
    060     * when multiple comparable comparators may be used in the same VM.
    061     *
    062     * @param <E>  the element type
    063     * @return the singleton ComparableComparator
    064     * @since 4.0
    065     */
    066    public static <E extends Comparable<? super E>> ComparableComparator<E> comparableComparator() {
    067        return INSTANCE;
    068    }
    069
    070    //-----------------------------------------------------------------------
    071    /**
    072     * Constructor whose use should be avoided.
    073     * <p>
    074     * Please use the {@link #comparableComparator()} method whenever possible.
    075     */
    076    public ComparableComparator() {
    077        super();
    078    }
    079
    080    //-----------------------------------------------------------------------
    081    /**
    082     * Compare the two {@link Comparable Comparable} arguments.
    083     * This method is equivalent to:
    084     * <pre>((Comparable)obj1).compareTo(obj2)</pre>
    085     *
    086     * @param obj1  the first object to compare
    087     * @param obj2  the second object to compare
    088     * @return negative if obj1 is less, positive if greater, zero if equal
    089     * @throws NullPointerException if <i>obj1</i> is <code>null</code>,
    090     *         or when <code>((Comparable)obj1).compareTo(obj2)</code> does
    091     * @throws ClassCastException if <i>obj1</i> is not a <code>Comparable</code>,
    092     *         or when <code>((Comparable)obj1).compareTo(obj2)</code> does
    093     */
    094    @Override
    095    public int compare(final E obj1, final E obj2) {
    096        return obj1.compareTo(obj2);
    097    }
    098
    099    //-----------------------------------------------------------------------
    100    /**
    101     * Implement a hash code for this comparator that is consistent with
    102     * {@link #equals(Object) equals}.
    103     *
    104     * @return a hash code for this comparator.
    105     * @since 3.0
    106     */
    107    @Override
    108    public int hashCode() {
    109        return "ComparableComparator".hashCode();
    110    }
    111
    112    /**
    113     * Returns {@code true} iff <i>that</i> Object is is a {@link Comparator Comparator}
    114     * whose ordering is known to be equivalent to mine.
    115     * <p>
    116     * This implementation returns {@code true} iff
    117     * <code><i>object</i>.{@link Object#getClass() getClass()}</code> equals
    118     * <code>this.getClass()</code>. Subclasses may want to override this behavior to remain
    119     * consistent with the {@link Comparator#equals(Object)} contract.
    120     *
    121     * @param object  the object to compare with
    122     * @return {@code true} if equal
    123     * @since 3.0
    124     */
    125    @Override
    126    public boolean equals(final Object object) {
    127        return this == object ||
    128               null != object && object.getClass().equals(this.getClass());
    129    }
    130
    131}