/*
  This AspectJ aspect prevents different objects of the same class from
accessing eachother's private elements.  In other words, it imposes 
object-level privacy on top of the class-level privacy found in Java and
similar languages.  It does the check at RUNTIME, however.

   In a pointcut definition, "this" refers to the calling object, and
   "target" is the object being invoked on.
*/

aspect superprivate
{
    pointcut privateaccess() :
      get(private * *.*) || set(private * *.*) || call(private * *.*(..));

    before(Object a, Object b) :
	privateaccess() && this(a) && target(b)
	{
	    if (a!=b) throw 
              new Error("Attempt to access private element of "+a
			+" from "+b);
	}
} // aspect superprivate


/*
   Note that if I wanted different objects to read (get) eachother's 
private varibles without modifying them, I could have simply left out
the "set(private * *.*)" from the pointcut.  I can also control the specific
class that A or B can belong to, with (for example):
  
   if (B instanceof someclass) ... // "instanceof" is "is" in C#

This level of fine-tuning is not possible with conventional languages.

  Use this program in conjunction with any existing java program with:

  ajc yourprogram.java superprivate.java

*/