Java: Using Try/Catch Exception to check if user input is Double

Question

I am writing a simple program that allows a user to enter two separate doubles for a foot and inch measurement. The program is intended to take these values and convert them to centimeters and output them. Additionally I am to include two exceptions: one to make sure the numeric values are positive and not negative (this one I have completed) and another to make sure the input entered is a double value and not a string value (this one I am having a hard time with). So if a user enters an input... for example 'Bill' instead of a number, it is to display an error message and ask the user to re-enter the input values again.

It seems like perhaps I would be best off gathering the user input as a string (rather than doubles as I currently am), which I convert to doubles and return them as doubles to their corresponding methods: getFootValue() and getInchValue() -- but I am not too sure.

How should I go about implementing this by way of a custom exception? I cannot simply utilize the InputMismatchException, I need to make my own titled NonDigitNumberException().

Here is what I have so far...

import java.util.Scanner; 

public class Converter 
{
    private double feet;
    private double inches;

    public Converter(double feet, double inches) 
    {
        this.feet = feet;
        this.inches = inches;

    }

    public double getFootValue() 
    {
            return feet;
    }

    public double getInchValue()
    {
        return inches; 
    }

    public double convertToCentimeters()
    {
        double inchTotal;

        inchTotal = (getFootValue() * 12) + getInchValue();

        return inchTotal * 2.54;
    }

    public String toString() 
    {
        return ("Your result is: " + convertToCentimeters());
    }
}


import java.util.Scanner; 
import java.util.InputMismatchException;

public class TestConverter
{
    public static void main(String[] args) 
    {
        /* Create new scanner for user input */
        Scanner keyboard = new Scanner(System.in);

        do
        {
            try
            {
                /* Get the feet value */
            System.out.print("Enter the foot value: ");
                double feet = keyboard.nextDouble();
            if (feet < 0) throw new NegativeNumberException();

            /* Get the inches value */
            System.out.print("Enter the inch value: ");
                double inches = keyboard.nextDouble();  
            if (inches < 0) throw new NegativeNumberException();    

            else
            {
                 Converter conversion = new Converter(feet, inches);    

                /* Print the converted result */
                System.out.println(conversion);
                break;
            }
            } catch(InputMismatchException ignore){}
            catch(NegativeNumberException error)
            {
                System.out.println("A negative-numeric value was entered, please enter only positive-numeric values...");
            }

        }while(true);

        /* Close the keyboard */
         keyboard.close();

    }
}

class NegativeNumberException extends Exception 
{
    public NegativeNumberException() 
    {
        super();
    }
    public NegativeNumberException(String errorMessage) 
    {
        super(errorMessage);
    }
}

Thanks for any help!

Why are you throwing an exception at all? You could just print the error message and continue instead. — azurefrog
– azurefrog, Commented Feb 28, 2017 at 21:32
Have you considered using Scanner.hasNextDouble() to test whether the next token can be scanned as a double? — John Bollinger
– John Bollinger, Commented Feb 28, 2017 at 21:32
Or else simply handle the InputMismatchException that occurs when the input cannot be scanned as a double in some way more useful than just ignoring it. Catching an exception and ignoring it is almost never the right thing to do. — John Bollinger
– John Bollinger, Commented Feb 28, 2017 at 21:36
I must use a custom exception to test whether it is a double or string value being entered -- unfortunately. This is per my instructors requests. — Kevin Gombos
– Kevin Gombos, Commented Feb 28, 2017 at 22:02
Thanks for the tip on the handle, I am rather new to using exceptions so I wasn't too sure how to go about ignoring the error that closes the program when a user enters a string instead of a double and using the custom exception instead. — Kevin Gombos
– Kevin Gombos, Commented Feb 28, 2017 at 22:02

Ousmane D. · Accepted Answer · 2017-03-01 00:51:05Z

1

You're over complicating things. You can simply use the Scanner.hasNextDouble() method.

Example:

Assuming this code is inside your main method.

public class Main {
  public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    System.out.println("enter value");
    double myValue = 0;
    if(scanner.hasNextDouble()){
      myValue = scanner.nextDouble();
    }else{
      System.out.println("Wrong value entered");
    }
  }
}

you can then go on and use myValue with your Converter class.

UPDATE

It seems that you must create your own exception class according to what you have told me within the comments. So, I have decided to implement that for you and hopefully, you can be able to carry on from here.

Custom Exception Class

public class NonDigitNumberException extends InputMismatchException {
    public NonDigitNumberException(String message){ // you can pass in your own message
        super(message);
    }

    public NonDigitNumberException(){ // or use the default message
        super("input is not a digit");
    }
}

Negative Number Exception Class

public class NegativeNumberException extends IllegalArgumentException {
    public NegativeNumberException(String message){ // you can pass in your own message
        super(message);
    }

    public NegativeNumberException(){ // or use the default message
        super("negative number is not valid");
    }
}

Validator Method

public static double inputValidator(){
  Scanner scanner = new Scanner(System.in);
  System.out.println("enter a value"); // prompt user for input
  String getData = scanner.next(); // get input
  if(getData.length() >= 1){
        if(!Character.isDigit(getData.charAt(0)) && getData.charAt(0) != '-') throw new NonDigitNumberException();
  }
  for (int i = 1; i < getData.length(); i++) {
     if(!Character.isDigit(getData.charAt(i))) throw new NonDigitNumberException();
  }
  return Double.parseDouble(getData); // at this point the input data is correct
}

Negative Number Validator

public static boolean isNegative(double value){
   if(value < 0) throw new NegativeNumberException();
   return false;
}

Main method

 public static void main(String[] args) {
   try {
     double myValue = inputValidator();
     System.out.println(isNegative(myValue)); // check if number is negative
   }catch (NegativeNumberException e){
     e.printStackTrace();
   }
   catch (NonDigitNumberException e){
     e.printStackTrace();
   }
   catch(Exception e){
     e.printStackTrace();
   }
 }

edited Mar 1, 2017 at 0:51

answered Feb 28, 2017 at 21:34

Ousmane D.

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10 Comments

Kevin Gombos Over a year ago

I know I can do this, however, per my professor I have to utilize a custom exception that is to be titled, NonDigitNumberException which trys/catches whether or not the user entered a double value or not, and if it is a string/letter value it is to return an error message.

Ousmane D. Over a year ago

@KevinGombos okay, I will implement that and keep you updated.

Ousmane D. Over a year ago

@KevinGombos I have updated my answer to suit your latest comment. Hopefully, that does help you to carry on and finish your task. The only thing that's left is to keep prompting the user for input until the correct type(double) is entered. However, if you can't do that just keep me updated and i'd be willing to help out.

Kevin Gombos Over a year ago

This worked out tremendously. I went ahead and made two validators for each variable and they work. However. Now I noticed my NegativeNumberException() isn't working. The NonDigitNumberException() seems to be over ruling it. So when a negative number gets entered instead of displaying the catch output for NegativeNumberException it instead displays the NonDigitNumberException() output. I was wondering if there is a way to correct this? Otherwise it works great!

Ousmane D. Over a year ago

@KevinGombos sure there is. I would really appreciate it if you mark the answer as accepted if the solution has solved your issue. Also, in the meantime i will let you know how to overcome your latest problem.

|

user7605325 · Accepted Answer · 2017-02-28 21:34:32Z

0

Do you really need a custom exception? Because keyboard.nextDouble() already throws InputMismatchException if the input is not a double.

Instead of ignoring the exception, you should show an error message (saying the user didn't type a number).

answered Feb 28, 2017 at 21:34

user7605325

2 Comments

Kevin Gombos Over a year ago

Unfortunately I do have to utilize a custom exception per my professors instructions on this task. He states the following... "NonDigitNumberException (if the foot or inch input value are non numeric) In addition to checking for the NegativeNumberException, also check for a NumberFormatException for the foot or inch values that were entered incorrectly (For example: I enter a letter for an inch or foot value). When catching the NumberFormatException, you will then throw your own NonDigitNumberException which will be caught by the try/catch block."

user7605325 Over a year ago

In this case you could throw NonDigitNumberException if hasNextDouble() is false

PlutonicJ · Accepted Answer · 2017-02-28 22:24:39Z

I guess, you are mixing things up:

You have to validate the input of the user first, if it is a double. If it is not, then you are getting an InputMismatchException.

Then you have to validate the input, if it is valid for your converter (is it positive?). Here you can throw your custom exception.

And then you call your Converter, which might also throw your custom exception.

So my solution would be:

import java.util.Scanner;
import java.util.InputMismatchException;

public class TestConverter {
    public static void main(String[] args) {
        /* Create new scanner for user input */
        Scanner keyboard = new Scanner(System.in);

        do {
            double feet = -1, inches = -1;
            Exception exception;
            do {
                exception = null;
                /* Get the feet value */
                System.out.print("Enter the foot value (positive-numeric): ");
                try {
                    feet = keyboard.nextDouble();
                } catch (InputMismatchException e) {
                    keyboard.next();
                    exception = e;
                }
            } while (exception != null);
            do {
                exception = null;
                /* Get the inches value */
                System.out.print("Enter the inch value (positive-numeric): ");
                try {
                    inches = keyboard.nextDouble();
                } catch (InputMismatchException e) {
                    keyboard.next();
                    exception = e;
                }
            } while (exception != null);


            try {
                if (feet < 0) throw new NegativeNumberException();
                if (inches < 0) throw new NegativeNumberException();

                Converter conversion = new Converter(feet, inches);

                /* Print the converted result */
                System.out.println(conversion);
                break;
            }
            catch(NegativeNumberException error) {
                System.out.println("A negative-numeric value was entered, please enter only positive-numeric values...");
            }
        } while (true);

        /* Close the keyboard */
        keyboard.close();

    }
}

Output

Enter the foot value (positive-numeric): test
Enter the foot value (positive-numeric): 1234
Enter the inch value (positive-numeric): test
Enter the inch value (positive-numeric): 1234
Your result is: 40746.68

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