Can I re-define class bytes in ClassLoader (not changing interface)?

[Robert Paris]

Let's say i have a class:

public class Test
{
   public blah()
   {
      //SOMETHING
   }
}

And it's been defined by the classloader and we're in runtime, when I decide i want to add some System.out.println( ... ) calls to it (for whatever reason, but I want to do it at RUNTIME). So I use BCEL (or whatever) and create the bytes for the statement, then get the bytes of the class and add the statement in to the method bytes (and alter for size, stack, etc) but keep the interface the same. Is this possible? What would the repercussions/possible problems be? (And I'm assuming I can make my own classloader)

[Robert Paris]

Well, it turns out that you can, BUT if you do then other classes will throw a ClassCastException when they try to use the new class. So whatever you want to do HAS to be done either at first loading/defining or you need to code ONLY to the interface AND load it ONLY explicitly (like Class.forName), thus only referencing the interface name. Then you can have something change the bytes, make a new class and return that and you'll still have the same interface. But of course, this means people might still be using the old version.