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 */
017 package org.apache.commons.collections.comparators;
018
019 import java.io.Serializable;
020 import java.util.Comparator;
021
022 /**
023 * A {@link Comparator Comparator} that compares
024 * {@link Comparable Comparable} objects.
025 * <p />
026 * This Comparator is useful, for example,
027 * for enforcing the natural order in custom implementations
028 * of SortedSet and SortedMap.
029 * <p />
030 * Note: In the 2.0 and 2.1 releases of Commons Collections,
031 * this class would throw a {@link ClassCastException} if
032 * either of the arguments to {@link #compare(Object, Object) compare}
033 * were <code>null</code>, not {@link Comparable Comparable},
034 * or for which {@link Comparable#compareTo(Object) compareTo} gave
035 * inconsistent results. This is no longer the case. See
036 * {@link #compare(Object, Object) compare} for details.
037 *
038 * @since Commons Collections 2.0
039 * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
040 *
041 * @author Henri Yandell
042 *
043 * @see java.util.Collections#reverseOrder()
044 */
045 public class ComparableComparator implements Comparator, Serializable {
046
047 /** Serialization version. */
048 private static final long serialVersionUID=-291439688585137865L;
049
050 /** The singleton instance. */
051 private static final ComparableComparator instance = new ComparableComparator();
052
053 //-----------------------------------------------------------------------
054 /**
055 * Gets the singleton instance of a ComparableComparator.
056 * <p>
057 * Developers are encouraged to use the comparator returned from this method
058 * instead of constructing a new instance to reduce allocation and GC overhead
059 * when multiple comparable comparators may be used in the same VM.
060 *
061 * @return the singleton ComparableComparator
062 */
063 public static ComparableComparator getInstance() {
064 return instance;
065 }
066
067 //-----------------------------------------------------------------------
068 /**
069 * Constructor whose use should be avoided.
070 * <p>
071 * Please use the {@link #getInstance()} method whenever possible.
072 */
073 public ComparableComparator() {
074 super();
075 }
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 when <i>obj1</i> is <code>null</code>,
087 * or when <code>((Comparable)obj1).compareTo(obj2)</code> does
088 * @throws ClassCastException when <i>obj1</i> is not a <code>Comparable</code>,
089 * or when <code>((Comparable)obj1).compareTo(obj2)</code> does
090 */
091 public int compare(Object obj1, Object obj2) {
092 return ((Comparable)obj1).compareTo(obj2);
093 }
094
095 //-----------------------------------------------------------------------
096 /**
097 * Implement a hash code for this comparator that is consistent with
098 * {@link #equals(Object) equals}.
099 *
100 * @return a hash code for this comparator.
101 * @since Commons Collections 3.0
102 */
103 public int hashCode() {
104 return "ComparableComparator".hashCode();
105 }
106
107 /**
108 * Returns <code>true</code> iff <i>that</i> Object is
109 * is a {@link Comparator Comparator} whose ordering is
110 * known to be equivalent to mine.
111 * <p>
112 * This implementation returns <code>true</code>
113 * iff <code><i>object</i>.{@link Object#getClass() getClass()}</code>
114 * equals <code>this.getClass()</code>.
115 * Subclasses may want to override this behavior to remain consistent
116 * with the {@link Comparator#equals(Object)} contract.
117 *
118 * @param object the object to compare with
119 * @return true if equal
120 * @since Commons Collections 3.0
121 */
122 public boolean equals(Object object) {
123 return (this == object) ||
124 ((null != object) && (object.getClass().equals(this.getClass())));
125 }
126
127 }