NullType


1   /*
2    * Bytecode Analysis Framework
3    * Copyright (C) 2003,2004 University of Maryland
4    * 
5    * This library is free software; you can redistribute it and/or
6    * modify it under the terms of the GNU Lesser General Public
7    * License as published by the Free Software Foundation; either
8    * version 2.1 of the License, or (at your option) any later version.
9    * 
10   * This library is distributed in the hope that it will be useful,
11   * but WITHOUT ANY WARRANTY; without even the implied warranty of
12   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
13   * Lesser General Public License for more details.
14   * 
15   * You should have received a copy of the GNU Lesser General Public
16   * License along with this library; if not, write to the Free Software
17   * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
18   */
19  
20  package edu.umd.cs.findbugs.ba.type;
21  
22  import org.apache.bcel.generic.ReferenceType;
23  import org.apache.bcel.generic.Type;
24  
25  /**
26   * Special type representing the null value.
27   * This is a type which is higher in the lattice than any object type,
28   * but lower than the overall Top type.  It represents the type
29   * of the null value, which may logically be merged with any
30   * object type without loss of information.
31   *
32   * @author David Hovemeyer
33   * @see TypeAnalysis
34   * @see TypeFrame
35   * @see TypeMerger
36   */
37  public class NullType extends Type implements ExtendedTypes {
38  
39      private static final long serialVersionUID = 1L;
40      private static final Type theInstance = new NullType();
41  
42      private NullType() {
43          super(T_NULL, "<null type>");
44      }
45  
46      @Override  
47           public int hashCode() {
48          return System.identityHashCode(this);
49      }
50  
51      @Override  
52           public boolean equals(Object   o) {
53          return o == this;
54      }
55  
56      public static Type instance() {
57          return theInstance;
58      }
59  }
60  
61  // vim:ts=4
62
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