Visitable


1   package Jt.examples.patterns;
2   import java.util.*;
3   import java.lang.reflect.*;
4   import java.beans.*;
5   import java.io.*;
6   import Jt.*;
7   
8   
9   /**
10   * Demonstrates the use of the Visitor pattern.
11   */
12  
13  
14  public class Visitable extends JtObject {
15  
16  
17    public Visitable() {
18    }
19  
20  
21    /**
22      * Process object messages. 
23      * <ul>
24      * <li>JtACCEPT - Accept operation
25      * </ul>
26      * @param event Jt Message    
27      */
28  
29    public Object   processMessage (Object   event) {
30  
31     String   msgid = null;
32     JtMessage e = (JtMessage) event;
33     Object   content;
34     Object   data;
35     JtMessage aux;
36  
37       if (e == null)
38      return null;
39  
40       msgid = (String  ) e.getMsgId ();
41  
42       if (msgid == null)
43      return null;
44  
45       content = e.getMsgContent();
46  
47       if (msgid.equals ("JtREMOVE")) {
48         return (this);     
49       }
50  
51       if (msgid.equals ("JtACCEPT")) {
52  
53         if (content == null) {
54           handleError ("processMessage: invalid ACCEPT message; visitor reference is null");
55           return (null);
56         }
57  
58         // Send a JtVISIT message to the visitor.
59         // Include a reference to this object
60  
61         aux = new JtMessage ("JtVISIT");
62         aux.setMsgContent (this);
63         return (sendMessage (content, aux));   
64       }
65  
66  
67       handleError ("Visitable.processMessage: invalid message id:" + msgid);
68       return (null);
69  
70    }
71  
72  
73  }
74
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