I am building a calculator with Java and am having issues with it executing the operators

Question

I am fairly new to Java, never have done programming before. I am getting the following errors when trying to run the code for each of the operators. I am wanting it to take in the user input as a string but return a double as the result. I know there will probably be several errors, but any help is appreciated.

Exception in thread "main" java.lang.Error: Unresolved compilation problems: Type mismatch: cannot convert from String to double The operator - is undefined for the argument type(s) java.lang.String, java.lang.String The operator * is undefined for the argument type(s) java.lang.String, java.lang.String The operator / is undefined for the argument type(s) java.lang.String, java.lang.String The operator % is undefined for the argument type(s) java.lang.String, java.lang.String Cannot make a static reference to the non-static field Math Cannot make a static reference to the non-static field Math command cannot be resolved to a type The operator >= is undefined for the argument type(s) Class, int Syntax error on token ".", class expected after this token The operator <= is undefined for the argument type(s) String, int

at Calculator.main(Calculator.java:72)

import java.util.Scanner;
import java.lang.Math;

/*@public class for calculator*/
public class Calculator {
    String ADD = "+";
    String SUB = "-";
    String DIV = "/";
    String MUL = "*";
    String MOD = "%";
    String SQRT = "sqrt";
    String POW = "^";
    String Math.PI = "pi";

    public static void main(String[] args) {


        /*@Scanner to read user input */
        Scanner myScanner = new Scanner(System.in);
        boolean continueLoop = true;

        /*@try statement to begin program */
        try {



            String x,y,command;
            double result = 0;
            double myDouble = result;

            /*@print line welcoming user to calculator */
            System.out.print("Welcome to my Calculator! Lets Begin!" + "\n");
            /*@print line asking user to either type HELP or START to begin */
            System.out.print("Type HELP or START to begin: ");
            /*@string to read user input for HELP or START command */
            String firstCommand = myScanner.nextLine();
            if (firstCommand.equals("HELP")) {
                System.out.println("Welcome to my calculator. Make sure to enter only numbers into the first two fields. \nDo NOT divide by 0. \nBe sure to enter the number matching your desired operation.");
                System.out.print("Type HELP or START to begin: ");
                command = myScanner.nextLine();
            }




            Double firstNumber = Double.parseDouble(myScanner.nextLine());
            /*@print statement asking user for first number input */
            System.out.println("Please Enter your First number: ");
            /*@variable x to be read by scanner */

            x = myScanner.nextLine();

            Double secondNumber = Double.parseDouble(myScanner.nextLine());
            /*@print statement asking user for second number input *        
            System.out.println("Please Enter your Second Number: ");

            /*@variable y to be read by scanner */
            y = myScanner.nextLine();

            /*@param converts string into double*/
            Double converted = Double.parseDouble(myScanner.nextLine());    
            /*Print statements with instructions on what commands to put in and where to put in.*/
            System.out.print("\n1: Add. \n2: Sub. \n3: Mult. \n4: Divide. \n5: Modulo. \n6: Pi \n7: Pow.");
            System.out.print("\nEnter your operator: ");

            command = myScanner.nextLine();

            switch(command) {
            /*@returns addition of firstNumber and secondNumber */
            case "1":
                result = (x+y); break;
            /*@returns subtraction of firstNumber and secondNumber */
            case "2":
                result = (x-y); break;
            /*@returns multiplication of firstNumber and secondNumber */
            case "3":
                result = (x*y); break;
            /*@returns division of firstNumber and secondNumber */
            case "4":
                result = (x/y); break;
            /*@returns modulo of firstNumber and secondNumber */
            case "5":
                result = (x%y); break;
            /*@returns Math.PI */
            case "6":
                result = (Math.PI);
            /*@returns power of firstNumber to the second number secondNumber*/
            case "7":
                result = (Math.pow(x, y));
            }
            if(command. >= 1 && command <= 5)
                System.out.println("Your answer is: " + result);

            /*@if statements for all mathematical operators */
            // cannot get sqrt, pow, pie to work


        /*@throws exception if anything other than a number is typed in by user */  
        /*catch exception if anything other than number is keyed in*/
        } catch (InputMismatchException inputMismatchException) {
            /*@error statement printed*/
            System.err.printf("%nException: %s%n", inputMismatchException);
            /*@statement for scanner to read the next line*/
            myScanner.nextLine(); // discard input, so user can try again
            /*@print statement letting user know to try again*/
            System.out.printf("You must enter integers. Please try again %n%n");

        }
            /*@throws exception if zero is keyed in as denominator
            /*exception if zero is keyed in as the denominator in division since you cannot divide by zero*/
        catch (ArithmeticException arithmeticException) {
            /*error statement printed*/
            System.err.printf("%nException: %s%n", arithmeticException);
            /*print statement letting user that 0 is not valid and to please try again
            not able to get the loop to go back and restart*/
            System.out.printf("Zero is an invalid denominator.Please try again. %n %n");
        }
        /*@param finally statement closes scanner*/
        finally {
            myScanner.close();
        } while(continueLoop);

    } 



}

Answer 1

Below are few issues that I can see at first sight.

1. Missing catch or finally block when using try.
2. Comparing String with Integer (Need some typecasting before comparing)

3. myScanner.toString() doesn't read the user input. To read user input you might use myScanner.next().

4. Inside if condition if (command.equals("HELP"))... , here you haven't read the command from user input, It seems you wanted to use if (firstCommand.equals("HELP")) .

5.Variable named "firstNumber" and "secondNumber" never used in the code.

6. Using myScanner.nextLine() before prompting the user to provide input.
7. Variable used in case statement is of String type. However, treated like an Integer. correct way should be like case "1": // The datatype of command is String so it should be inside quotes "".