Sort a stack using Java
The first thing you should do is assess the visibility of your methods.  Some should be public, and others private.  I don't see any that should stay package-protected as you have them now.

Next, instead of having a printStack() method, you should consider overriding the toString() method.  Just a Java standard.

Now, I don't know if you're as far as multi-threading in your modules, but I can see some cases where your Stack is not thread-safe (particularly around the sorting).  I'll leave the problem of how to solve this as an exercise to the questioner.