Stack implementation with StackInterface
There isn't really much to optimize here, but there are definitely improvements you can make regarding usability and coding style.


use generics instead of Object, otherwise the stack is difficult to use in practice.
in practice, you would probably want a stack to increase its size automatically instead of throwing an exception.
your indentation is not consistent, making your code difficult to read.
you need more vertical whitespace, for example before a new method
your variable names could use improvement. What's a sz? What's an a? What's an Astack?