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 }