Why Does My Scanner Throw an InputMismatchException When Reading a Double, and How Can I Fix It?

Scanner double value - InputMismatchException

The InputMismatchException is thrown when a Scanner attempts to retrieve a token that doesn't match the expected type, or when the token is out of range for the type.

In the provided code, you're using Scanner to read a double value representing the number of gallons of gas in the tank. However, Scanner's default locale is the system's default locale, which may expect a comma (",") as a decimal delimiter instead of a dot (".").

Reason for the Exception:

The exception occurs because the number entered as input (e.g., "5.1") doesn't match the expected format. The default Scanner locale expects numbers to be formatted with a comma as the decimal separator. Your input, however, uses a dot as the decimal separator, which violates the expected format.

How to Circumvent the Trouble:

To resolve this issue, you can specify a locale that uses a dot as the decimal separator using the useLocale() method. For instance:

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

By specifying Locale.US, you ensure that the Scanner uses the US locale, which uses a dot as the decimal separator. This way, the Scanner will correctly interpret the input as a double value.

Another option, if you want to keep the system's default locale but allow both comma and dot as decimal separators, is to use a regular expression to parse the input string and manually convert it to a double:

String input = scanner.nextLine();
double value = Double.parseDouble(input.replace(",", "."));

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