1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.apache.commons.collections.comparators;
18
19 import java.io.Serializable;
20 import java.util.Comparator;
21
22 /**
23 * A {@link Comparator Comparator} that compares {@link Comparable Comparable}
24 * objects.
25 * <p>
26 * This Comparator is useful, for example, for enforcing the natural order in
27 * custom implementations of {@link java.util.SortedSet SortedSet} and {@link java.util.SortedMap SortedMap}.
28 * <p>
29 * Note: In the 2.0 and 2.1 releases of Commons Collections, this class would
30 * throw a {@link ClassCastException} if either of the arguments to
31 * {@link #compare(Object, Object) compare} were <code>null</code>, not
32 * {@link Comparable Comparable}, or for which
33 * {@link Comparable#compareTo(Object) compareTo} gave inconsistent results.
34 * This is no longer the case. See {@link #compare(Object, Object) compare} for
35 * details.
36 *
37 * @since 2.0
38 * @version $Id: ComparableComparator.java 1436026 2013-01-21 00:55:20Z sebb $
39 *
40 * @see java.util.Collections#reverseOrder()
41 */
42 public class ComparableComparator<E extends Comparable<? super E>> implements Comparator<E>, Serializable {
43
44 /** Serialization version. */
45 private static final long serialVersionUID=-291439688585137865L;
46
47 /** The singleton instance. */
48 @SuppressWarnings("rawtypes")
49 public static final ComparableComparator<?> INSTANCE = new ComparableComparator();
50
51 //-----------------------------------------------------------------------
52 /**
53 * Gets the singleton instance of a ComparableComparator.
54 * <p>
55 * Developers are encouraged to use the comparator returned from this method
56 * instead of constructing a new instance to reduce allocation and GC overhead
57 * when multiple comparable comparators may be used in the same VM.
58 *
59 * @param <E> the element type
60 * @return the singleton ComparableComparator
61 */
62 @SuppressWarnings("unchecked")
63 public static <E extends Comparable<? super E>> ComparableComparator<E> comparableComparator() {
64 return (ComparableComparator<E>) INSTANCE;
65 }
66
67 //-----------------------------------------------------------------------
68 /**
69 * Constructor whose use should be avoided.
70 * <p>
71 * Please use the {@link #comparableComparator()} method whenever possible.
72 */
73 public ComparableComparator() {
74 super();
75 }
76
77 //-----------------------------------------------------------------------
78 /**
79 * Compare the two {@link Comparable Comparable} arguments.
80 * This method is equivalent to:
81 * <pre>((Comparable)obj1).compareTo(obj2)</pre>
82 *
83 * @param obj1 the first object to compare
84 * @param obj2 the second object to compare
85 * @return negative if obj1 is less, positive if greater, zero if equal
86 * @throws NullPointerException if <i>obj1</i> is <code>null</code>,
87 * or when <code>((Comparable)obj1).compareTo(obj2)</code> does
88 * @throws ClassCastException if <i>obj1</i> is not a <code>Comparable</code>,
89 * or when <code>((Comparable)obj1).compareTo(obj2)</code> does
90 */
91 public int compare(final E obj1, final E obj2) {
92 return obj1.compareTo(obj2);
93 }
94
95 //-----------------------------------------------------------------------
96 /**
97 * Implement a hash code for this comparator that is consistent with
98 * {@link #equals(Object) equals}.
99 *
100 * @return a hash code for this comparator.
101 * @since 3.0
102 */
103 @Override
104 public int hashCode() {
105 return "ComparableComparator".hashCode();
106 }
107
108 /**
109 * Returns {@code true} iff <i>that</i> Object is is a {@link Comparator Comparator}
110 * whose ordering is known to be equivalent to mine.
111 * <p>
112 * This implementation returns {@code true} iff
113 * <code><i>object</i>.{@link Object#getClass() getClass()}</code> equals
114 * <code>this.getClass()</code>. Subclasses may want to override this behavior to remain
115 * consistent with the {@link Comparator#equals(Object)} contract.
116 *
117 * @param object the object to compare with
118 * @return {@code true} if equal
119 * @since 3.0
120 */
121 @Override
122 public boolean equals(final Object object) {
123 return this == object ||
124 null != object && object.getClass().equals(this.getClass());
125 }
126
127 }