001: /*
002: * Copyright 2001-2004 The Apache Software Foundation
003: *
004: * Licensed under the Apache License, Version 2.0 (the "License");
005: * you may not use this file except in compliance with the License.
006: * You may obtain a copy of the License at
007: *
008: * http://www.apache.org/licenses/LICENSE-2.0
009: *
010: * Unless required by applicable law or agreed to in writing, software
011: * distributed under the License is distributed on an "AS IS" BASIS,
012: * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013: * See the License for the specific language governing permissions and
014: * limitations under the License.
015: */
016: package org.apache.commons.collections.comparators;
017:
018: import java.io.Serializable;
019: import java.util.Comparator;
020:
021: /**
022: * Reverses the order of another comparator by reversing the arguments
023: * to its {@link #compare(Object, Object) compare} method.
024: *
025: * @since Commons Collections 2.0
026: * @version $Revision: 155406 $ $Date: 2005-02-26 12:55:26 +0000 (Sat, 26 Feb 2005) $
027: *
028: * @author Henri Yandell
029: * @author Michael A. Smith
030: *
031: * @see java.util.Collections#reverseOrder()
032: */
033: public class ReverseComparator implements Comparator, Serializable {
034:
035: /** Serialization version from Collections 2.0. */
036: private static final long serialVersionUID = 2858887242028539265L;
037:
038: /** The comparator being decorated. */
039: private Comparator comparator;
040:
041: //-----------------------------------------------------------------------
042: /**
043: * Creates a comparator that compares objects based on the inverse of their
044: * natural ordering. Using this Constructor will create a ReverseComparator
045: * that is functionally identical to the Comparator returned by
046: * java.util.Collections.<b>reverseOrder()</b>.
047: *
048: * @see java.util.Collections#reverseOrder()
049: */
050: public ReverseComparator() {
051: this (null);
052: }
053:
054: /**
055: * Creates a comparator that inverts the comparison
056: * of the given comparator. If you pass in <code>null</code>,
057: * the ReverseComparator defaults to reversing the
058: * natural order, as per
059: * {@link java.util.Collections#reverseOrder()}</b>.
060: *
061: * @param comparator Comparator to reverse
062: */
063: public ReverseComparator(Comparator comparator) {
064: if (comparator != null) {
065: this .comparator = comparator;
066: } else {
067: this .comparator = ComparableComparator.getInstance();
068: }
069: }
070:
071: //-----------------------------------------------------------------------
072: /**
073: * Compares two objects in reverse order.
074: *
075: * @param obj1 the first object to compare
076: * @param obj2 the second object to compare
077: * @return negative if obj1 is less, positive if greater, zero if equal
078: */
079: public int compare(Object obj1, Object obj2) {
080: return comparator.compare(obj2, obj1);
081: }
082:
083: //-----------------------------------------------------------------------
084: /**
085: * Implement a hash code for this comparator that is consistent with
086: * {@link #equals(Object) equals}.
087: *
088: * @return a suitable hash code
089: * @since Commons Collections 3.0
090: */
091: public int hashCode() {
092: return "ReverseComparator".hashCode() ^ comparator.hashCode();
093: }
094:
095: /**
096: * Returns <code>true</code> iff <i>that</i> Object is
097: * is a {@link Comparator} whose ordering is known to be
098: * equivalent to mine.
099: * <p>
100: * This implementation returns <code>true</code>
101: * iff <code><i>object</i>.{@link Object#getClass() getClass()}</code>
102: * equals <code>this.getClass()</code>, and the underlying
103: * comparators are equal.
104: * Subclasses may want to override this behavior to remain consistent
105: * with the {@link Comparator#equals(Object) equals} contract.
106: *
107: * @param object the object to compare to
108: * @return true if equal
109: * @since Commons Collections 3.0
110: */
111: public boolean equals(Object object) {
112: if (this == object) {
113: return true;
114: } else if (null == object) {
115: return false;
116: } else if (object.getClass().equals(this .getClass())) {
117: ReverseComparator thatrc = (ReverseComparator) object;
118: return comparator.equals(thatrc.comparator);
119: } else {
120: return false;
121: }
122: }
123:
124: }
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