Client-server app command pattern

Question

I'm writing a client-server app and I'm having a problem with using command pattern. In my program server recieves a String input command from a client, finds proper command for the input in the HashMap, executes it and sends back the return value. I'm having a problem with figuring out, how to write the commands that require more than 1 step(command has to ask ask a client for extra paramether/s and then should return final result).

command interface

public interface Command {
    public String execute();    
}

server communication with a client

 try  {
        ServerSocket serverSocket = new ServerSocket(port);
        Command c;

        while(true) {

            Socket clientSocket = serverSocket.accept();
            PrintWriter clientSocketWriter = new PrintWriter(clientSocket.getOutputStream(), true);

            BufferedReader clientSocketReader = new BufferedReader(new 
                  InputStreamReader(clientSocket.getInputStream()));

            String inputLine;

        //server recieves String command from client and sends back the result while semaphore is true
            while (Main.semaphore) {
                inputLine = clientSocketReader.readLine();
                c = CommandFactory.getCommandByName(inputLine);
                clientSocketWriter.println(c.execute());
            }

            serverSocket.close();
            clientSocketWriter.close();
            clientSocketReader.close();
            clientSocket.close();
        }

    } catch(Exception e) {
        e.printStackTrace();
    }

in one step commands there in no problem

public class CommandHelp implements Command {
    @Override
    public String execute() {
        //returns string of all commands
        return CommandFactory.getCommandByNames();
}

I don't know how to write a command, that needs an extra paramether for executing, it can't return the result right away without knowing it. Command should return number of permutations from x elements(client should choose).

public class CommandPermutationsCount implements Command {

    @Override
    public String execute() {
      //can't return anything yet
    }


    public long getPermutations(int elementsCount) {
         long result = 1;

         for (int factor = 2; factor <= elementsCount; factor++) {
            result *= factor;
         }
        return result;
    }

}

I've had an idea to make the Command void instead of String, but then I wouldn't be able to send comunicate with a client via clientSocketWriter. Is there any good way to make commands with more steps?

The whole point of command pattern is to encapsulate the logic of execution. What you are trying to do is expose that logic to server. — tsolakp

Trein Trein · Accepted Answer · 2017-12-26T21:22:19

I think you're in the right track. Why don't you allow the command identifier and its arguments to be passed in one line? I'm thinking of something like:

>> permute -a 12
>> shuffle -a [1, 2, 7, 6, 5, 3, 1]
>> add -a 4 -a 5

By allowing commands specified with arguments, your CommandFactory becomes something like:

class CommandFactory {
    public static Command parseCommand(String commandLine) {
        // Tokenize commandLine:
        // - First word is commandId
        // - Tokens following '-a' string are arguments
        // Return CommandHelp if no command with provided commandId is found
        return CommandHelp(commandLine);
    }
}

Now you can create specific command implementations inheriting from an abstract Command class that looks like this:

abstract class Command {
    private final String commandId;
    private final String commandArguments;

    Command(String commandId, List<String> commandArguments) {
        this.commandId = commandId;
        this.commandArguments = commandArguments;
    }

    abstract String execute();
}

Client-server app command pattern

1 Answers