Implementing a try and catch block

Question

I am finally (almost) done with a little calculator project that I have been working on. Currently, it works with addition, subtraction, multiplication, division, mod, squaring, and negation of doubles and responds to some strings such as "exit", "clear", and "help".

I just need help implementing a try and catch block to return a message in the console such as "Invalid Input" whenever the user enters anything that is not a number after the console says "Enter first number" or "Enter second number".

I have messed around with a few different methods but nothing seems to be working. Advice would be greatly appreciated. Code:

import java.util.Scanner;


public class exit {

    static double num1;
    static double num2;
    static String operation;

    public static void main(String[] args) {

        boolean run = true;

        while (run) {

            System.out.println(" Enter: \n \b help for all usable operations \n continue to use the    calculator");
            Scanner input = new Scanner(System.in);
            String str1 = input.next();

            if (str1.equals("exit")) {

                System.exit(0);
            } else if (str1.equals("clear")) {

                System.out.println("0.0");
            } else if (str1.equals("help")) {
                System.out.println("please enter:\n + for addition \n - for subtraction \n * for     multiplication \n / for division \n -x for negation \n x^2 for squaring\n % for mod \n  remember to only input integer or double values\n");

            } else if (str1.equals("continue")) {

                System.out.println("\nEnter the first number:");
                num1 = input.nextInt();


                System.out.println
                        ("Display:" + num1);

                System.out.println("Please enter operation:");
                operation = input.next();


                System.out.println("Enter the second number:");
                num2 = input.nextInt();
                System.out.println("Display:" + num2);


                if ("+".equals(operation)) {

                    System.out.println((num1 + num2));
                } else if ("-".equals(operation)) {

                    System.out.println((num1 - num2));
                } else if ("/".equals(operation)) {

                    System.out.println((num1 / num2));
                } else if ("*".equals(operation)) {

                    System.out.println((num1 * num2));
                } else if ("-x".equals(operation)) {

                    System.out.println(-num1);
                } else if ("x^2".equals(operation)) {

                    System.out.println(num1 * num1);
                } else if ("sqrt".equals(operation)) {

                    System.out.println(Math.sqrt(num1));

                }


            }

        }
    }
}

why u need try-catch for that? you can check for numbers only in `if` or set input chars to integers only — user2377971, Oct 11 '13 at 10:13
http://stackoverflow.com/questions/3059333/validating-input-using-java-util-scanner — Suresh Atta, Oct 11 '13 at 10:14

score 3 · Answer 1 · answered Oct 11 '13 at 10:14

To check for the validity of number by implementing a try-catch, you could try to do something like this:

try{
    Double num = Double.parseDouble(input.nextLine()); // This is just a sample
    // int someInt = Integer.parseInt(someBasString); // Even this will throw NFE
}catch(NumberFormatException NFE){ // If its not a valid number, you'll get this exception
    // Do something on error
}

score 0 · Answer 2 · answered Oct 11 '13 at 10:23

You can use this

try{
num1 = input.nextInt();
..
num2 = input.nextInt();
}
catch(Exception ex)
{
  System.out.println("Invalid input")
}

to make it better before catching 'Exception' you can catch 'InputMismatchException' or any other exception that you are expecting .That way you can show a more relevant info from catch.

Implementing a try and catch block

2 Answers2