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 *
42 * @since 2.0
43 * @see java.util.Collections#reverseOrder()
44 */
45 public class ComparableComparator<E extends Comparable<? super E>> implements Comparator<E>, Serializable {
46
47 /** Serialization version. */
48 private static final long serialVersionUID=-291439688585137865L;
49
50 /** The singleton instance. */
51 @SuppressWarnings("rawtypes")
52 public static final ComparableComparator INSTANCE = new ComparableComparator();
53
54 /**
55 * Gets the singleton instance of a ComparableComparator.
56 * <p>
57 * Developers are encouraged to use the comparator returned from this method
58 * instead of constructing a new instance to reduce allocation and GC overhead
59 * when multiple comparable comparators may be used in the same VM.
60 *
61 * @param <E> the element type
62 * @return the singleton ComparableComparator
63 * @since 4.0
64 */
65 public static <E extends Comparable<? super E>> ComparableComparator<E> comparableComparator() {
66 return INSTANCE;
67 }
68
69 /**
70 * Constructor whose use should be avoided.
71 * <p>
72 * Please use the {@link #comparableComparator()} method whenever possible.
73 */
74 public ComparableComparator() {
75 }
76
77 /**
78 * Compare the two {@link Comparable Comparable} arguments.
79 * This method is equivalent to:
80 * <pre>((Comparable)obj1).compareTo(obj2)</pre>
81 *
82 * @param obj1 the first object to compare
83 * @param obj2 the second object to compare
84 * @return negative if obj1 is less, positive if greater, zero if equal
85 * @throws NullPointerException if <i>obj1</i> is {@code null},
86 * or when {@code ((Comparable)obj1).compareTo(obj2)} does
87 * @throws ClassCastException if <i>obj1</i> is not a {@code Comparable},
88 * or when {@code ((Comparable)obj1).compareTo(obj2)} does
89 */
90 @Override
91 public int compare(final E obj1, final E obj2) {
92 return obj1.compareTo(obj2);
93 }
94
95 /**
96 * Returns {@code true} iff <i>that</i> Object is a {@link Comparator Comparator}
97 * whose ordering is known to be equivalent to mine.
98 * <p>
99 * 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 }