Handling continuous user input in a CLI app in Java

Question

This question is in regards to handling continuous user input in a command-line application in Java.

Within Main, I have the following code:

    Scanner sc = new Scanner(System.in);

    while(sc.hasNextLine()) {

        String line = sc.nextLine().replaceAll("\n", "");

        // return pressed
        if (line.length() == 0) {
            continue;
        }

        // split line into arguments
        String[] args1 = line.split(" ");

        // process arguments
        if (args1.length > 0) {
            if (args1[0].equalsIgnoreCase("q")) {
                System.exit(0);
            } else if (args1[0].equalsIgnoreCase("someInput")) {
                // Put stuff here
            } else {
                System.out.println("exiting");
                System.exit(0);
            }
        }
    }

As you can tell, this is to handle continuous inputs from the command-line. The program quits if the input is q, and does other stuff when it's someInput. This program is essentially a state machine.

However, this program can get messy very very quick, since it will evolve into a bunch of if-statements and boolean flags to get into the if statements.

For example, as I programmed my current CLI application, it descended into 15+ if-statements, and it's a total MESS in terms of readability and maintainability.

My question is - what is the better way to handle continuous conditional user-input without using a mess of if-statements so it's more readable and maintainable?

Note: The inputs are non-discrete, meaning, it isn't simple string matching.

Maybe set up a "Handler" or "Command" class which you can add to a List. This way, you would ask each "Command" if it was able to process the "input" and if it can, pass it to that "Command" for further processing... — MadProgrammer
– MadProgrammer, Commented Jun 6, 2015 at 1:24
If you have a discrete set of ommands in place of "someInput" put the handlers in a map. — redge
– redge, Commented Jun 6, 2015 at 1:36
Exactly, redge. The inputs are non-discrete, meaning, they're not pre-defined commands. Imagine writing the vim application, there are not a discrete set of inputs, but a mess of different inputs. A state machine will get exponentially complicated if that were the way it was written. — theGreenCabbage
– theGreenCabbage, Commented Jun 6, 2015 at 1:37
Even vim has a discrete set of commands then each command has a different structure for its arguments. A map of handlers can be used to do the first level and then if a handler needs to have a different structure to handle its variants that is a separate problem. Unless you are trying to do an artificially intelligent natural language command interpreter I suggest you have a discrete set of commands. — redge
– redge, Commented Jun 6, 2015 at 1:41
I would suggest that your title is wrong. This is not so much about continuous input as it it about complex command structure — redge
– redge, Commented Jun 6, 2015 at 1:44

gerrytan · Accepted Answer · 2015-06-06 10:28:38Z

3

Assuming your input starts with a command token, you can create a Command interface and build a map lookup. For example:

interface Command {
  void invoke(String input);
}

Then convert your if statements into Command implementation, eg:

public class CookCommand impements Command {
  void invoke(String input) {
    ..
  }
}

public class ShowerCommand implements Command {
  void invoke(String input) {
    ..
  }
}

Then build a map with these commands objects:

Map<String, Command> commands = new HashMap<>();
commands.put("shower", new ShowerCommand());
commands.put("cook", new CookCommand());

Then on your eval loop just lookup this map to get associated Command object and invoke it

Command cmd = commands.get(commandName);
if(cmd != null) cmd.invoke(input);
else System.err.println("Unknown command " + cmd);

answered Jun 6, 2015 at 10:28

gerrytan

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3 Comments

theGreenCabbage Over a year ago

Hi gerry. What happens if the command isn't a discrete command?

redge Over a year ago

@theGreenCabbage you really need to explain what your command structure is and why it is not discrete as noone will be able to suggest a sensible solution otherwise.

theGreenCabbage Over a year ago

Hi @redge. Apologies. I'll post a sample snippet of my code tomorrow and give you guys a better example of what I am talking about, and you guys give me feedback on how it can be done better. Thanks for your participation in particular redge

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