this code cant really get it to work tried combining two separate codes into one but I am coming into trouble

Question

need help with my code my switch statement doesn't work I've tried to change it from int to string but cant figure it out. The line I have trouble with is line 43 that one is the switch statement that I cant figure how to fix. can someone help me please this is very important thank you.

import java.util.Scanner; import java.util.Stack;

public class RPNCalculator
{
private static Stack<Integer> stack;
private static Object evaluator;
private static String again;
private static Scanner keyboard;

 public RPNCalculator()
    {
        stack = new Stack<Integer>(); //creates stack 
    }

 public static void main(String[] args)
    {
        String expression, again;
        int result;

        Scanner keyboard = new Scanner(System.in);

        do
        {  
            RPNCalculator evaluator = new RPNCalculator();
            System.out.println("Enter a valid post-fix expression one token " +
                               "at a time with a space between each token (e.g. 5 4 + 3 2 1 - + *)");
            System.out.println("Each token must be an integer or an operator (+,-,*,/)");
            System.out.println();
            System.out.println("Enter 'q' to quit, 'h' for help.");
            
            expression = keyboard.nextLine();

            result = evaluator.evaluate(expression);
            System.out.println();
            System.out.println("That expression equals " + result);
            
            while (true) {
                System.out.print("Enter command: ");
                Scanner scanner;
                Object input = scanner.nextLine();

                switch (  ) {
                    case "m":
                        if (!stack.isEmpty()) {
                            stack.push(-stack.pop());
                        } else {
                            System.out.println("Error: Stack is empty");
                        }
                        break;
                    case "r":
                        if (stack.size() < 2) {
                            System.out.println("Error: Stack does not have enough elements");
                        } else {
                            double a = stack.pop();
                            double b = stack.pop();
                            stack.push((int) a);
                            stack.push((int) b);
                        }
                        break;
                    case "d":
                        if (!stack.isEmpty()) {
                            double top = stack.peek();
                            stack.push((int) top);
                        } else {
                            System.out.println("Error: Stack is empty");
                        }
                        break;
                    case "p":
                        if (!stack.isEmpty()) {
                            System.out.println("Top item: " + stack.peek());
                        } else {
                            System.out.println("Error: Stack is empty");
                        }
                        break;
                    case "n":
                        if (!stack.isEmpty()) {
                            System.out.println("Top item removed: " + stack.pop());
                        } else {
                            System.out.println("Error: Stack is empty");
                        }
                        break;
                    case "f":
                        System.out.println("Stack contents: " + stack);
                        break;
                    case "c":
                        stack.clear();
                        System.out.println("Stack cleared");
                        break;
                    case "q":
                        System.out.println("Exiting program");
                        return;
                    case "h":
                    case "?":
                        System.out.println("Operations:");
                        System.out.println("m - unary minus");
                        System.out.println("r - exchange the top two items");
                        System.out.println("d - duplicate top item on stack");
                        System.out.println("p - print the top item");
                        System.out.println("n - print and remove the top item");
                        System.out.println("f - print all contents of the stack");
                        System.out.println("c - clear the stack");
                        System.out.println("q - quit");
                        System.out.println("h or ? - print help message");
                        break;
                    default:
                        
                        
                }
            
            }

            System.out.print("Evaluate another expression [Y/N]? ");
            again = keyboard.nextLine();
            System.out.println();
        }
        while (again.equalsIgnoreCase("y"));
   }

public int evaluate(String expr) 
    {
        int op1, op2, result = 0;
        String token;
        Scanner parser = new Scanner(expr);     

        while (parser.hasNext())        
        {
            token = parser.next();          

            if (isOperator(token))  //if operator pop 
            {
                op2 = (stack.pop()).intValue();
                op1 = (stack.pop()).intValue();
                result = evaluateSingleOperator(token.charAt(0), op1, op2);     //
                stack.push(new Integer(result));
            }
            else
                stack.push(new Integer(Integer.parseInt(token)));       
        }

        return result;
    }

 private boolean isOperator(String token)
    {
        return ( token.equals("+") || token.equals("-") ||
                 token.equals("*") || token.equals("/") || token.equals("%") );

    }

 private int evaluateSingleOperator(char operation, int op1, int op2)
    {
        int result = 0;

        switch (operation)
        {
            case '+':
                result = op1 + op2;
                break;
            case '-':
                result = op1 - op2;
                break;
            case '*':
                result = op1 * op2;
                break;
            case '/':
                result = op1 / op2;
                break;
            case '%':
                result = op1 % op2;
                break;
        }

        return result;
    }
 

}

Answer 1

@Justin, your switch statement is incomplete. switch needs to be provided an expression as a parameter e.g.

switch (input) {

I'm not a Java developer so I can only guide you so you can learn more about switch statement here.

You already seem to have got switch right in your code further below.

switch (operation)