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 *      https://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;
021import java.util.SortedMap;
022import java.util.SortedSet;
023
024/**
025 * A {@link Comparator Comparator} that compares {@link Comparable Comparable}
026 * objects.
027 * <p>
028 * This Comparator is useful, for example, for enforcing the natural order in
029 * custom implementations of {@link SortedSet SortedSet} and
030 * {@link SortedMap SortedMap}.
031 * </p>
032 * <p>
033 * Note: In the 2.0 and 2.1 releases of Commons Collections, this class would
034 * throw a {@link ClassCastException} if either of the arguments to
035 * {@link #compare(Comparable, Comparable)} compare} were {@code null}, not
036 * {@link Comparable Comparable}, or for which
037 * {@link Comparable#compareTo(Object) compareTo} gave inconsistent results.
038 * This is no longer the case. See {@link #compare(Comparable, Comparable)} compare} for
039 * details.
040 * </p>
041 *
042 * @param <E> The type of objects compared by this comparator
043 * @since 2.0
044 * @see java.util.Collections#reverseOrder()
045 */
046public class ComparableComparator<E extends Comparable<? super E>> implements Comparator<E>, Serializable {
047
048    /** Serialization version. */
049    private static final long serialVersionUID = -291439688585137865L;
050
051    /** The singleton instance. */
052    @SuppressWarnings("rawtypes")
053    public static final ComparableComparator INSTANCE = new ComparableComparator();
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     * Constructor whose use should be avoided.
072     * <p>
073     * Please use the {@link #comparableComparator()} method whenever possible.
074     */
075    public ComparableComparator() {
076    }
077
078    /**
079     * Compare the two {@link Comparable Comparable} arguments.
080     * This method is equivalent to:
081     * <pre>((Comparable)obj1).compareTo(obj2)</pre>
082     *
083     * @param obj1  The first object to compare
084     * @param obj2  The second object to compare
085     * @return negative if obj1 is less, positive if greater, zero if equal
086     * @throws NullPointerException if <em>obj1</em> is {@code null},
087     *         or when {@code ((Comparable)obj1).compareTo(obj2)} does
088     * @throws ClassCastException if <em>obj1</em> is not a {@code Comparable},
089     *         or when {@code ((Comparable)obj1).compareTo(obj2)} does
090     */
091    @Override
092    public int compare(final E obj1, final E obj2) {
093        return obj1.compareTo(obj2);
094    }
095
096    /**
097     * Returns {@code true} iff <em>that</em> Object is a {@link Comparator Comparator}
098     * whose ordering is known to be equivalent to mine.
099     * <p>
100     * This implementation returns {@code true} iff
101     * {@code <em>object</em>.{@link Object#getClass() getClass()}} equals
102     * {@code this.getClass()}. Subclasses may want to override this behavior to remain
103     * consistent with the {@link Comparator#equals(Object)} contract.
104     *
105     * @param object  The object to compare with
106     * @return {@code true} if equal
107     * @since 3.0
108     */
109    @Override
110    public boolean equals(final Object object) {
111        return this == object ||
112               object != null && object.getClass().equals(this.getClass());
113    }
114
115    /**
116     * Implement a hash code for this comparator that is consistent with
117     * {@link #equals(Object) equals}.
118     *
119     * @return A hash code for this comparator.
120     * @since 3.0
121     */
122    @Override
123    public int hashCode() {
124        return "ComparableComparator".hashCode();
125    }
126
127}