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.collections4.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
28 * {@link java.util.SortedMap SortedMap}.
29 * </p>
30 * <p>
31 * Note: In the 2.0 and 2.1 releases of Commons Collections, this class would
32 * throw a {@link ClassCastException} if either of the arguments to
33 * {@link #compare(Comparable, Comparable)} compare} were {@code null}, not
34 * {@link Comparable Comparable}, or for which
35 * {@link Comparable#compareTo(Object) compareTo} gave inconsistent results.
36 * This is no longer the case. See {@link #compare(Comparable, Comparable)} compare} for
37 * details.
38 * </p>
39 *
40 * @param <E> the type of objects compared by this comparator
41 * @since 2.0
42 * @see java.util.Collections#reverseOrder()
43 */
44 public class ComparableComparator<E extends Comparable<? super E>> implements Comparator<E>, Serializable {
45
46 /** Serialization version. */
47 private static final long serialVersionUID = -291439688585137865L;
48
49 /** The singleton instance. */
50 @SuppressWarnings("rawtypes")
51 public static final ComparableComparator INSTANCE = new ComparableComparator();
52
53 /**
54 * Gets the singleton instance of a ComparableComparator.
55 * <p>
56 * Developers are encouraged to use the comparator returned from this method
57 * instead of constructing a new instance to reduce allocation and GC overhead
58 * when multiple comparable comparators may be used in the same VM.
59 *
60 * @param <E> the element type
61 * @return the singleton ComparableComparator
62 * @since 4.0
63 */
64 public static <E extends Comparable<? super E>> ComparableComparator<E> comparableComparator() {
65 return INSTANCE;
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 }
75
76 /**
77 * Compare the two {@link Comparable Comparable} arguments.
78 * This method is equivalent to:
79 * <pre>((Comparable)obj1).compareTo(obj2)</pre>
80 *
81 * @param obj1 the first object to compare
82 * @param obj2 the second object to compare
83 * @return negative if obj1 is less, positive if greater, zero if equal
84 * @throws NullPointerException if <em>obj1</em> is {@code null},
85 * or when {@code ((Comparable)obj1).compareTo(obj2)} does
86 * @throws ClassCastException if <em>obj1</em> is not a {@code Comparable},
87 * or when {@code ((Comparable)obj1).compareTo(obj2)} does
88 */
89 @Override
90 public int compare(final E obj1, final E obj2) {
91 return obj1.compareTo(obj2);
92 }
93
94 /**
95 * Returns {@code true} iff <em>that</em> Object is a {@link Comparator Comparator}
96 * whose ordering is known to be equivalent to mine.
97 * <p>
98 * This implementation returns {@code true} iff
99 * {@code <em>object</em>.{@link Object#getClass() getClass()}} equals
100 * {@code this.getClass()}. Subclasses may want to override this behavior to remain
101 * consistent with the {@link Comparator#equals(Object)} contract.
102 *
103 * @param object the object to compare with
104 * @return {@code true} if equal
105 * @since 3.0
106 */
107 @Override
108 public boolean equals(final Object object) {
109 return this == object ||
110 null != object && object.getClass().equals(this.getClass());
111 }
112
113 /**
114 * Implement a hash code for this comparator that is consistent with
115 * {@link #equals(Object) equals}.
116 *
117 * @return a hash code for this comparator.
118 * @since 3.0
119 */
120 @Override
121 public int hashCode() {
122 return "ComparableComparator".hashCode();
123 }
124
125 }