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}