If you’ve played around with the basic version of the Java calculator, you may have tried to enter multiple commands on the same prompt line:
Begin: class com.ksmpartners.rpncalc.basic.RpnCalc
> 1 2
Uncaught Exception: For input string: "1 2"
java.lang.NumberFormatException: For input string: "1 2"
This is a convenient way to enter calculator commands, but it doesn’t work for two reasons. This post discusses why it fails, and how to fix it:
The first implementation of the RPN calculator I’m going to look at is the basic Java implementation. The link takes you directly a a view of the source file for this version, but if you’d like to play with the code, you can also clone the whole project with git: https://github.com/ksmpartners/blog-rpncalc. Cloning the git project will bring down all the versions of the calculator, and also let you compile and run the software locally. I’ll discuss more about how the implementation works, after the jump:
One of the things I remember most about middle school math class is that I went through it in a perpetual state of disorganization. During one particularly bad spell, I lost two calculators within a week. The loss, and the reaction of my parents, drove me to try to fix the problem once and for all. My plan was simple: buy an expensive calculator with the hope that it’d serve as an incentive to keep track of my stuff. The next weekend, I took weeks of allowance money to the local Service Merchandise and bought a new HP-11C pocket calculator. Almost 30 years later, I still have both the calculator and a fascination for its unusual Reverse Polish Notation (RPN) user interface. Over those decades, I’ve also found out that RPN provides a good way to explore a number of fundamental ideas in the field of software design.