Server.java

import java.io.IOException;
import java.io.BufferedReader;
import java.io.PrintWriter;
import java.io.InputStreamReader;
import java.net.ServerSocket;
import java.net.Socket;

public class Server {
    // Error codes
    private static int incorrectOperationCommand = -1;
    private static int numInputsLTTwo = -2;
    private static int numInputsGTFour = -3;
    private static int nan = -4;
    private static int exitCode = -5;

    // Acceptable operations
    public enum Operation {
        add, subtract, multiply
    }

    // Helper method to print in the "get: " format
    // parameters:
    //  message - String to print with "get: " appended
    public static void printGet(String message) {
        System.out.print("get: " + message);
    }

    // Helper method to print in the ", return: " format
    // parameters:
    //  message - String to print with ", return: " appended
    public static void printReturn(String message) {
        System.out.println(", return: " + message);
    }

    public static void main(String[] args) throws Exception {
        if(args.length != 1){
            System.out.println("Not enough arguments");
            System.exit(-1);
        }

        ServerSocket server = null;
        Socket client = null;
        BufferedReader inBuffer = null;
        PrintWriter outWriter = null;
        String message = null;

        // Get port number from args
        int serverPort = Integer.parseInt(args[0]);

        try {
            // Initiate ServerSocket
            server = new ServerSocket(serverPort);

            try {
                // Loop that waits for a client
                while(true) {
                    try {
                        // Establish connection with client
                        client = server.accept();
                        System.out.println("get connection from ... " + client.getInetAddress().getHostAddress());

                        try {
                            inBuffer = new BufferedReader(new InputStreamReader(client.getInputStream()));
                            outWriter = new PrintWriter(client.getOutputStream(), true);
                        } catch (IOException e) {
                            System.out.println("Could not create BufferedReader and/or PrintWriter");
                            client.close();
                            System.exit(-1);
                        }

                        // Send "Hello!" to client
                        outWriter.println("Hello!");

                        // Loop that constantly reads from client
                        while(true) {
                            message = inBuffer.readLine();
                            if(message != null) {
                                printGet(message);
                            } else {
                                System.out.println("Client closed unexpectedly");
                                client.close();
                            }

                            String[] messageArray = message.split("\\s+");

                            if (message.equalsIgnoreCase("bye")) {
                                outWriter.println(exitCode);
                                printReturn(Integer.toString(exitCode));

                                // Close current client socket connection
                                client.close();
                            }
                            else if(message.equalsIgnoreCase("terminate")) {
                                outWriter.println(exitCode);
                                printReturn(Integer.toString(exitCode));

                                // Close current client and server sockets and terminate
                                client.close();
                                server.close();
                                System.exit(0);
                            }
                            else {
                                boolean noErrors = true;
                                // Check if add, subtract, or multiply
                                try {
                                    Operation operation = Operation.valueOf(messageArray[0]);

                                    // Check number of inputs
                                    if(messageArray.length < 3) {
                                        // Send error code -2 for number of inputs is less than two
                                        outWriter.println(numInputsLTTwo);
                                        printReturn(Integer.toString(numInputsLTTwo));
                                    }
                                    else if(messageArray.length > 5) {
                                        // Send error code -3 for number of inputs is more than 4
                                        outWriter.println(numInputsGTFour);
                                        printReturn(Integer.toString(numInputsGTFour));
                                    }
                                    else {
                                        // Check everything is an int
                                        for(int i = 1; i < messageArray.length; i++) {
                                            if(!messageArray[i].matches("\\d+")) {
                                                // Send error code -4 for inputs contains non-numbers
                                                outWriter.println(nan);
                                                printReturn(Integer.toString(nan));

                                                noErrors = false;
                                                break;
                                            }
                                        }

                                        if(noErrors) {
                                            switch (operation) {
                                                case add:
                                                    int sum = 0;
                                                    for(int i = 1; i < messageArray.length; i++) {
                                                        sum += Integer.parseInt(messageArray[i]);
                                                    }

                                                    outWriter.println(sum);
                                                    printReturn(Integer.toString(sum));
                                                    break;
                                                case subtract:
                                                    int sub = Integer.parseInt(messageArray[1]);
                                                    for(int i = 2; i < messageArray.length; i++) {
                                                        sub -= Integer.parseInt(messageArray[i]);
                                                    }

                                                    outWriter.println(sub);
                                                    printReturn(Integer.toString(sub));
                                                    break;
                                                case multiply:
                                                    int mult = Integer.parseInt(messageArray[1]);
                                                    for(int i = 2; i < messageArray.length; i++) {
                                                        mult *= Integer.parseInt(messageArray[i]);
                                                    }

                                                    outWriter.println(mult);
                                                    printReturn(Integer.toString(mult));
                                                    break;
                                                default:
                                                    // Send error code -1 for incorrect operation command
                                                    outWriter.println(incorrectOperationCommand);
                                                    printReturn(Integer.toString(incorrectOperationCommand));
                                                    break;
                                            }
                                        }
                                    }
                                } catch(IllegalArgumentException e) {
                                    // Send error code -1 for incorrect operation command
                                    outWriter.println(incorrectOperationCommand);
                                    printReturn(Integer.toString(incorrectOperationCommand));
                                }
                            }
                        }
                    } catch(Exception e) {

                    } finally {
                        // Ensure client is closed
                        client.close();
                    }
                }
            } finally {
                // Ensure server is closed
                server.close();
            }
        }
        catch(IOException e) {
            System.out.println("ServerSocket could not be made");
            System.exit(-1);
        }
    }
}