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Java > Open Source Codes > com > presumo > jms > selector > SelectorFalseException


1 /**
2  * This file is part of Presumo.
3  *
4  * Presumo is free software; you can redistribute it and/or modify
5  * it under the terms of the GNU General Public License as published by
6  * the Free Software Foundation; either version 2 of the License, or
7  * (at your option) any later version.
8  *
9  * Presumo is distributed in the hope that it will be useful,
10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
12  * GNU General Public License for more details.
13  *
14  * You should have received a copy of the GNU General Public License
15  * along with Presumo; if not, write to the Free Software
16  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
17  *
18  *
19  * Copyright 2001 Dan Greff
20  */
21 package com.presumo.jms.selector;
22
23 /**
24  * JMS defines some conditions that will always render a selector
25  * false. One example is trying to add two strings. While the
26  * parser can detect errors in literals at parse time, it can not
27  * tell what type an identifier is until evaluation time. If an
28  * identifier type is incorrectly used JMS says that the selector
29  * is invalid. Another example would be referencing an object
30  * property from anywhere in the sql expression.
31  *
32  * The evaluate methods will throw this exception if they encounter
33  * a situation like the ones above. This will effectively short-circuit
34  * the evaluation. If you call evaluate on the node in the expression
35  * tree you must catch this exception. If it occurs you can assume
36  * the selector is false. If the selector does not throw an
37  * exception you still have to check the return type to see if
38  * the evaluation was true or false or unknown. (unknown == false).
39  *
40  * @author Dan Greff
41  */
42 public final class SelectorFalseException extends Exception JavaDoc
43 {
44   //
45 // No reason to have multiple instance of this class so it
46 // is a singleton.
47 //
48
49   private static SelectorFalseException onlyInstance;
50   
51   static SelectorFalseException getInstance()
52   {
53     if (onlyInstance == null)
54       onlyInstance = new SelectorFalseException();
55     return onlyInstance;
56   }
57
58   private SelectorFalseException() {}
59
60 }
61
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