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Java > Open Source Codes > net > sf > jga > util > NullComparator


1 // ============================================================================
2 // $Id: NullComparator.java,v 1.1 2005/12/14 13:31:04 davidahall Exp $
3 // Copyright (c) 2005 David A. Hall
4 // ============================================================================
5 // The contents of this file are subject to the Common Development and
6 // Distribution License (CDDL), Version 1.0 (the License); you may not use this
7 // file except in compliance with the License. You should have received a copy
8 // of the the License along with this file: if not, a copy of the License is
9 // available from Sun Microsystems, Inc.
10 //
11 // http://www.sun.com/cddl/cddl.html
12 //
13 // From time to time, the license steward (initially Sun Microsystems, Inc.) may
14 // publish revised and/or new versions of the License. You may not use,
15 // distribute, or otherwise make this file available under subsequent versions
16 // of the License.
17 //
18 // Alternatively, the contents of this file may be used under the terms of the
19 // GNU Lesser General Public License Version 2.1 or later (the "LGPL"), in which
20 // case the provisions of the LGPL are applicable instead of those above. If you
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25 // If you do not delete the provisions above, a recipient may use your version
26 // of this file under the terms of either the CDDL or the LGPL.
27 //
28 // This library is distributed in the hope that it will be useful,
29 // but WITHOUT ANY WARRANTY; without even the implied warranty of
30 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
31 // ============================================================================
32
33 package net.sf.jga.util;
34
35 import java.io.Serializable JavaDoc;
36 import java.util.Comparator JavaDoc;
37
38 /**
39  * Comparator Decorator used to gracefully compare null and non-null values.
40  * In most cases, passing a null to a Comparator results in a NullPointerException,
41  * as (in the general case) null is not part of a class' strict ordering.
42  * However, in many applications, null objects must be included in an ordering.
43  * This wrapper can be used to shield an underlying Comparator from dealing
44  * with where in the ordering null values should fall.
45  * <p>
46  * A flag is provided that determines whether nulls are less than non-nulls,
47  * (the default case) or greater than non-nulls. When comparing two values,
48  * this comparator determines if either argument is null: if both are null,
49  * then 0 is returned (ie, all nulls are considered equal). If neither argument
50  * is null, then both arguments are passed to the wrapped comparator. If one
51  * argument is null, then a positive or negative integer is returned, depending
52  * on which argument was null and the setting of the flag.
53  */
54
55 // UNTESTED
56
57 public class NullComparator<T> implements Comparator JavaDoc<T>, Serializable JavaDoc {
58
59     // Flag determining if nulls appear before or after non-nulls in an ordering
60 private boolean _nullsAreLess = true;
61
62     // Comparator used to compare pairs of non-null values
63 private Comparator JavaDoc<T> _comp;
64
65     /**
66      * Builds a NullComparator that uses the given comparator to compare pairs
67      * of non-null values. Null values appear before non-null values in the
68      * resulting ordering.
69      */
70     public NullComparator(Comparator JavaDoc<T> comp) {
71         this(comp, true);
72     }
73
74     /**
75      * Builds a NullComparator that uses the given comparator to compare pairs
76      * of non-null values, and uses the flag to determine if non-null values
77      * appear before non-null values in the resulting ordering.
78      */
79     public NullComparator(Comparator JavaDoc<T> comp, boolean flag) {
80         if (comp == null)
81             throw new IllegalArgumentException JavaDoc("A Comparator is required");
82
83         _comp = comp;
84         _nullsAreLess = flag;
85     }
86
87
88     /**
89      * Returns the comparator used to compare pairs of non-null values.
90      */
91     public Comparator JavaDoc<T> getComparator() { return _comp; }
92
93
94     
95     public int compare(T x, T y) {
96         if (x == null)
97             if (y == null)
98                 return 0;
99             else
100                 return _nullsAreLess ? -1 : 1;
101         else
102             if (y == null)
103                 return _nullsAreLess ? 1 : -1;
104             else
105                 return _comp.compare(x, y);
106    }
107 }
108
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