Java reverse polish stack problem

I have made a program that takes a mathematical expression and converts it to reverse polish notation format.

I then want to evaluate this expression to give the answer to the expression. This was fairly easy to implement on a basic level. Although I have now noticed that my program cannot deal with numbers over 9, negative numbers and decimal numbers. All of which I want to include.

Is there a way of making it work with the added features i want?

Thanks for any help :)