DTRules/src/main/java/com/dtrules/interpreter/RNull.java

   1 /*
   2  * $Id$
   3  *
   4  * Copyright 2004-2007 MTBJ, Inc.
   5  *
   6  * Licensed under the Apache License, Version 2.0 (the "License");
   7  * you may not use this file except in compliance with the License.
   8  * You may obtain a copy of the License at
   9  *
  10  *      http://www.apache.org/licenses/LICENSE-2.0
  11  *
  12  * Unless required by applicable law or agreed to in writing, software
  13  * distributed under the License is distributed on an "AS IS" BASIS,
  14  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  15  * See the License for the specific language governing permissions and
  16  * limitations under the License.
  17  */
  18
  19 package com.dtrules.interpreter;
  20
  21 import com.dtrules.infrastructure.RulesException;
  22
  23 public class RNull extends ARObject {
  24
  25     /**
  26      * returns 0 if both are equal. -1 Otherwise.  A Null is considered
  27      * less than anything else.  We do this just so sorting arrays were
  28      * some values are null doesn't blow up.
  29      */
  30     public int compare(IRObject irObject) throws RulesException {
  31         if(irObject.equals(this))return 0;   // A Null is equal to other nulls.
  32         return -1;                           // A Null is less than anything else.
  33     }
  34
  35
  36
  37         static RNull theNullObject = new RNull();
  38         /**
  39          * Nobody needs the constructor.
  40          *
  41          */
  42     private RNull(){}
  43
  44     /**
  45      * Returns the RNull object.  There is no need for more than
  46      * one of these objects.
  47      * @return
  48      */
  49     public static RNull getRNull(){ return theNullObject; }
  50
  51         /**
  52      * A Null is only equal to the null object
  53      */
  54         public boolean equals(IRObject o) {
  55                 return o.type()==iNull;
  56         }
  57
  58         public String stringValue() {
  59                 return "null";
  60         }
  61
  62         /**
  63          * A Null is only equal to the null object, i.e. any
  64          * Null object.  They are all equal.
  65          */
  66         public boolean equals(Object arg0) {
  67                 return arg0.getClass().equals(this.getClass());
  68         }
  69
  70         /**
  71          * We override toString() to simply return "null".
  72          * Because we do this, we override hashcode() as well.
  73          */
  74         public String toString() {
  75                 return "null";
  76         }
  77
  78         public int hashCode() {
  79                 return 0;
  80         }
  81
  82         public int type() {
  83                 return iNull;
  84         }
  85 }