RPN Calc Part 2 – Composite Commands

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 immediate failure is because the parser considers the entire input string as a single command, without regard to whitespace. The parser looks for a command named "1 2", fails to find it, and then attempts to parse the entire string as a number. The number parse then fails and throws an exception that kills the process. This part of the problem can be fixed by altering the parser to split the input string into tokens, and parsing each token separately as its own command. Unfortunately, this shows the second more structural problem. The way the REPL is currently written, it’s built to read and execute a single Command for each prompt:

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

    cmd.execute();

To correct the second problem, the data flow between parseCommandString and execute needs to be widened so that the parser can return multiple commands for a single input string. The easiest way to achieve this is to have the parser return a list of commands:

    List<Command> cmds = parseCommandString(cmdLine.trim());

    for(Command cmd : cmds)
        cmd.execute();

This approach works well because it’s simple. It doesn’t add any additional class definitions, there’s still a single call site for execute(), and java.util.List is well-understood by developers that use Java. However, if you’re willing to trade some of that simplicity away, there’s an alternative approach to the problem. The alternative approach allows the original REPL to go unchanged, and still process groups of commands. It does this by introducing a new and more powerful kind of command object that can be used to compose lists of commands into single Command instances. The parser then uses these CompositeCommands to signal to the REPL that there are multiple commands to be executed:

    private Command parseCommandString(String cmdStr)
        throws Exception
    {
        List<Command> subCmds = new LinkedList<Command>();

        for (String subCmdStr : cmdStr.split("\\s+"))
            subCmds.add(parseSingleCommand(subCmdStr));

        return new CompositeCommand(subCmds);
    }

There are a few things to notice about this code. The first is that the type signature of parseCommandString is unchanged: it’s still a function from String to Command. This is why the REPL doesn’t need to be updated. The second aspect of the code is that there’s still a list of command objects: The list of commands we introduced when we modified the REPL example is still present, even with the CompositeCommand. What the composite command object does is hide the list from the REPL; It wraps the list up, abstracts it away, and lets the REPL pretend that there aren’t any lists.

The internal machinery that lets composite command achieve this isn’t too involved. All CompositeCommand has to do is remember the Command objects it’s composing, and execute each of those commands as if they were entered individually:

    private class CompositeCommand implements Command
    {
        private List<Command> subCmds = new LinkedList<Command>();

        CompositeCommand(Collection<Command> subCmds)
        {
            this.subCmds.addAll(subCmds);
        }

        public void execute()
        {
            for(Command subCmd : subCmds)
                subCmd.execute();
        }
    }

And that’s all there is to it. While this approach doesn’t have the surface simplicity of our first attempt, what it has done for us is give us a new abstraction for sequences of commands. This is fully a third of the ‘sequence’, ‘selection’, and ‘iteration’ that are necessary to express any computable program. That’s not a bad result for a single class definition.


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One Response to RPN Calc Part 2 – Composite Commands

  1. Pingback: RPN Calc Part 9 – State and Commands in Clojure |

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