// Possible enhancements you can do:
// Check for zero before division.
// Additional operations: mod, square root, sign change, ...
// Make this work with doubles, BigInteger, or ...
// Format double results with DecimalFormat
// Add keyboard listener.
// Change to RPN (Reverse Polish Notation)
// How would you communicate an error to the caller? Ans: Exceptions.
// about the change (except perhaps to add a "." for floating-point input).
