How to check the data type of a user Input? - Java

Question

Im trying to make sure the user is inputing a number and not a word. How would I do this? I tried this, but when I type in numbers it still says that it is a string.

import java.util.Scanner;
class Main {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        while(true) {
            System.out.println("What is the diameter of the sphere (cm): ");
            var diameter = input.next();
            if (diameter instanceof String) {
                System.out.println("Please enter a number");
            } else {
                break;
            }
        }
    }
}

Because it is--you can either try to scan an integer (nextInt IIRC) or do a conversion after-the-fact, which gives you a bit more flexibility. — Dave Newton
– Dave Newton, Commented Mar 10, 2021 at 17:50

Casey · Accepted Answer · 2021-03-10 17:56:41Z

1

Incorrect input will throw an exception of the type InputMismatchException when called with the input.next* family of functions (Boolean, Byte, Double, Float, Int, Line, Long, Short).

If you are expecting a specific type, use that version of the function and wrap it in a try-catch block with:

import java.util.Scanner;
class Main {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        while(true) {
            System.out.println("What is the diameter of the sphere (cm): ");
            try {
                var diameter = input.nextFloat();
                break;
            } catch(InputMismatchException e) {
                System.out.println("Please enter a number");
            }
      }
}

See https://www.w3schools.com/java/java_user_input.asp

answered Mar 10, 2021 at 17:56

Casey

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8 Comments

L0k3 d20uz4 Over a year ago

its saying it can't find the InputMismatchException symbol

Dave Newton Over a year ago

@L0k3d20uz4 Did you import it?

L0k3 d20uz4 Over a year ago

ok nevermind its kinda working now so maybe I made a mistake. So now i'm running it and purposely typing in a string. It is saying the "input a number" message, but then in prints the "what is the diameter" question infinitely.

L0k3 d20uz4 Over a year ago

@DaveNewton no I did not import anything, but it is working now. What should I import if I need to import anything?

Dave Newton Over a year ago

@L0k3d20uz4 Anything that isn't being imported, like InputMismatchException. SO has numerous questions on dealing with scanners, and you may wish to look at the Scanner documentation.

|

OhleC · Accepted Answer · 2021-03-10 17:59:00Z

0

Every keyboard input is a string at fist. If you need a number, you need to convert it. Since you're using a scanner, input.nextInt() would do that for you (and throw an exception if the input isn't parseable as a number). There's also input.hasNextInt(), which will consume some input and tell you whether it's an integer, in which case nextInt() won't throw.

You can also use Integer.parseInt() to convert a String into an integer (again, this will throw an exception if the input isn't a number).

(There are of course also corresponding versions of these functions for the other numeric types)

answered Mar 10, 2021 at 17:59

OhleC

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1 Comment

L0k3 d20uz4 Over a year ago

im using the ".hasNextDouble()", but it is only working when I type an input in a second time.

L0k3 d20uz4 · Accepted Answer · 2021-03-11 01:31:31Z

".next()" is a string. If you want a double use ".nextDouble". Instead of using if statements, use a try catch which will catch an exception if the user enters the wrong data type.

import java.util.*;
import java.util.InputMismatchException;
public class Main {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        double diameter;

        while (true) {
        System.out.print(
            "What is the diameter of the sphere (cm): ");
        try {
            diameter = input.nextDouble();
        break;  
        } catch (InputMismatchException mme) {
        System.out.println("double value not entered");
        // clear the scanner input
         input.nextLine();
        }
        }
        System.out.println("The diameter is " + diameter);
  }
}

Credit: @WJS

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