Nextdouble() Throws an Inputmismatchexception When I Enter a Double

nextDouble() throws an InputMismatchException when I enter a double

It might be locale dependent. Decimal numbers are e.g written as 0,5 in Sweden.

Change your code so that it says e.g.:

Scanner scan = new Scanner(System.in);
scan.useLocale(Locale.US);

Scanner#nextDouble fails to parse 5.2 (InputMismatchException)

Scanner and Locale

It is not directly obvious from the Javadoc of Scanner#nextDouble but it is Locale-sensitive.

There are various things that influence the way how Scanner expects numbers and text to look like that depend on your current set Locale.

By default, Scanner uses your current systems locale (country and language settings). So you have to enter the numbers in the way they are usually written in your current region.

Decimal separator . vs ,

In your particular case, it is the decimal separator which you entered with . (dot) but Scanner expected a , (comma) instead. This is for example the case with a Moldovan locale, since in Moldovan comma is used instead.

Hence, when you entered 5.2 it did not see any double, as specified by your current Locale. But when you enter 5,2, it works.

You can read more about this in the Javadoc of Scanner, section Localized numbers. Also see this illustration from Wikipedia:

Change locale

That said, you can use Scanner#useLocale to change the locale Scanner uses. For example:

scanner.useLocale(Locale.ENGLISH);

and then you can also work with 5.2 as input.

Scanner double value - InputMismatchException

You should precise a Locale for your Scanner.

Scanner scanner = new Scanner(System.in).useLocale(Locale.US);

From the doc :

An instance of this class is capable of scanning numbers in the
standard formats as well as in the formats of the scanner's locale. A
scanner's initial locale is the value returned by the
Locale.getDefault() method; it may be changed via the
useLocale(java.util.Locale) method


The localized formats are defined in terms of the following
parameters, which for a particular locale are taken from that locale's
DecimalFormat object, df, and its and DecimalFormatSymbols object,
dfs.

So your default locale use certainly a DecimalFormat that expect a comma as a decimal delimiter instead of a dot.

Error when using input.nextDouble in Java

I tested your code on an online IDE and it worked, then I tested on my machine and got the same error as you, then I tested again, but using commas (,) instead of dots(.) for the decimal point, and it worked. The InputMismatchException is throw because the Scanner object uses the default Locale of the JVM to parse the input value, and when it is different from the expected it causes the exception.

You can pass a locale to the scanner that uses . as a decimal separator:

input.useLocale(Locale.US);

or just use (,) when entering your values.

Also, there is another error in your code, in the last printf:

 System.out.printf("%nBMI = %d", BMI);

You should use a %f for floating points instead of a %d, that is used for integers. You can find more about how to format numeric output in the following documentation.

nextDouble() don't accept points, just commas

Your default locale uses the comma character (,) as the decimal separator, and thus fails parsing inputs with decimal points (.) as doubles. You can override this behavior by explicitly setting a Locale that accepts decimal points. E.g.:

Scanner scan = new Scanner(System.in);
scan.useLocale(Locale.US);

How do I catch InputMismatchException for doubles a, b, and c in this code?

public static void main(String[] args) {
    Scanner input = new Scanner (System.in); //scanner

    short repeat = 1;
    while (repeat == 1) {

        System.out.println("Enter your equation by entering a, b, and c."); //introduction
        System.out.println("Press enter evey time you enter a number.");

        try {
            double a = Double.parseDouble(input.nextLine());
            double b = Double.parseDouble(input.nextLine());
            double c = Double.parseDouble(input.nextLine());

            double answer1 = ( (-b) + Math.sqrt( Math.pow(b, 2) - (4*a*c) ) ) / (2*a); //answers
            double answer2 = ( (-b) - Math.sqrt( Math.pow(b, 2) - (4*a*c) ) ) / (2*a);

            System.out.println("Your answers are: " + answer1 + " and " + answer2);
        }
        catch (NumberFormatException e) {
            System.out.println("That's not a valid number.");
        }

        System.out.println("Would you like to calculate more numbers? 1 for yes, 0 for no: ");
        repeat = Short.parseShort(input.nextLine());

    }
    input.close();

}

• You have initalize the repeat variable to 1 and not to 0, since the program will not enter the while loop otherways.

• You need to put your calculations for answer1 and answer2 in the try-catch-block too, because if the user enters an invalid value, the result shouldn't get calculated.

• You should use only one Scanner instance, so you mustn't instance it in the loop.

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