Reverse Polish Notation expressions parser in Java

• Parser has to be a Java application that can be run on a JVM
• Parser has to consume RPN expression given in a string form (ie. as java main method parameter) but should be implemented in a way that allows to easily extend this (ie. to read RPN expression from file or database)
• Parser has to print the result of RPN expression evaluation on a screen (ie. System.out) but should be implemented in a way that allows to easily extend this (ie. to write RPN expression evaluation result to file or database)
• Parser has to handle 2 kinds of arithmetical operations (addition and subtraction) but should be implemented in a way that allows to easily extend the number of supported operations (multiplication, division, etc.)
• Parser has to be buildable and runnable using maven. The solution should be of “production quality”.