CSE647: Testing and Verification
		     Homework 6 Solution (ver. 31 Oct)       Due 26 Oct 2000
			      Scott Stoller

1. In lecture on 16 October, we mentioned two ways of organizing 
the class hierarchy in the wrapper approach to intercepting method
invocations and method terminations:

 "one hierarchy"            "separate hierarchies"
       C                             C       CSS
      /	\		            /       /  
     C1	 CSS		           C1      C1SS  
    / \			          /       /    
   C2  C1SS		         C2      C2SS    
    \
     C2SS

Note: On the chalkboard, I wrote C' instead of CSS.

Compare the advantages and disadvantages of these two organizations.
Consider the effects on how method calls and field accesses are
intercepted.  Remember that the same piece of code (e.g., a method
invocation or field access) might sometimes be executed on an 
unshared object and sometimes on a shared object.

Answer: An advantage of "separate hierarchies" is that each stub class
CiSS needs to declare wrappers only for methods declared by Ci, not for
methods inherited by Ci.  A disadvantage of "separate hierarchies" is that
Ci and CiSS are not related (by inheritance), so the same code cannot be
used for both shared and unshared instances.  For example, consider
"invokevirtual Ci.m".  If we leave this unchanged, it can't be used for
shared objects.  If we change this to "invokevirtual CiSS.m", it can't be
used for unshared objects.

2. In lecture, we proposed to use the wrapper approach to intercept method
invocations and method terminations for call sites of the form
"invokevirtual C.m" (i.e., we introduce a wrapper class CSS and a wrapper
method CSS.m. and we do not modify the call site or the body of C.m).
This approach does not quite work for call sites of the form
"invokespecial C.m".  Describe how to transform such call sites using the
outside approach (mentioned in lecture).  Be reasonably detailed about
what the transformed bytecode looks like.  Don't forget about method
arguments and exceptions.

Answer: See section 9.5.1 of "Design of Java Checker", available from the
web page.  That section is relatively self-contained.  You don't need to
read that entire document.