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}