Mike Schaeffer's Blog

October 21, 2013

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. 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:

When you run the calculator, it works a lot like an interactive programming language with a read-eval-print-loop (REPL). The calculator presents a prompt, reads a command, executes it, prints the result, and then loops back around until the user types quit. A user session that computes the sum of two and three looks like this (user entry in bold):

Begin: class com.ksmpartners.rpncalc.basic.RpnCalc

> 2

1> 2.0
> 3

1> 2.0
2> 3.0
> +

1> 5.0
> quit
end run.

The implementation has to address several problems:

  • The stack has to be stored.
  • The current state of the calculator needs to be displayed to the user.
  • Commands need to be accepted from the user.
  • User commands need to be dispatched to code that performs the requested action.

Storage of the stack is simple. The basic version of the calculator uses a Java collection stored as an instance variable of the calculator class:

public class RpnCalc extends Calculator
    private Deque<Double> stack = new LinkedList<Double>();

Printing the stack is also simple, although it’s made more verbose by the fact that the Java foreach loop requires an Iterable, rather than an Iterator:

private void showStack()
    int ii = 0;
    for (Iterator<Double> it = stack.descendingIterator(); it.hasNext(); ) {
        Double val = it.next();
        System.out.println((ii + 1) + ">" + val);

Similarly, the read, eval, print loop looks almost exactly like you’d expect. The most interesting bits are these:

Command cmd = parseCommandString(cmdLine.trim());

The last two statements translate whatever the user entered into a Command, an object that encapsulates the user’s intent into something that can be stored and directly executed at a later point in time. It is a command in the sense of the GoF Command Pattern:

interface Command
    void execute();

The command themselves do their work via direct mutation of the state stored in the stack instance variable:

cmds.put("+", new Command() {
    public void execute() {
        Double x = stack.pop();
        Double y = stack.pop();
        stack.push(x + y);

One tricky detail of this design is hidden in the fact that every command string entered by the user ultimately goes through Command.execute(). There is no special case for entering numbers, which therefore must also be done via a command. What’s different about the numeric entry command is that it contains a bit of instance data representing the number to be entered. The command parser creates a separate instance of PushNumberCommand for each number that’s entered on the command line.

private class PushNumberCommand implements Command
        Double number;
        PushNumberCommand(Double number) { this.number = number; }
        public void execute() { stack.push(number); }

In this design, you can make a strong argument that parseCommandString serves as a rudimentary compiler. Rather than translating from Java into classfiles, it translates from text into command instances. While this specific implementation only translates from strings containing a single command into command objects that perform a single operation, there’s nothing inherent in the design that prevents it from parsing complex command strings into complex command objects:

private Command parseCommandString(String cmdStr)
    throws Exception
    Command cmd = cmds.get(cmdStr);
    if (cmd != null)
        return cmd;
        return new PushNumberCommand(Double.parseDouble(cmdStr));

Next time, I’ll talk a bit more about generalizing parseCommandString into a more powerful kind of compiler. This will enrich both the user command entry syntax, and give us a more powerful example of the command pattern within the calculator.