Scanner Double Value: InputMismatchException - Why Does It Occur and How Can It Be Avoided?

Scanner Double Value Exception: InputMismatchException

Problem Introduction:

When using the Scanner class to read double values from the console, a user may encounter the InputMismatchException. This error occurs when the input does not match the expected data type, in this case, a double.

Questions:

• Why does this exception occur when reading double values using Scanner?
• How can this exception be avoided?

Answers:

1. Why does the exception occur?

The InputMismatchException is thrown because the default locale of the Scanner is not configured to use a period (.) as the decimal separator. By default, many locales use a comma (,) as the decimal separator. However, floating-point numbers in Java are expected to use a period as the decimal separator. When the Scanner tries to read a double value from an input that uses a comma as the decimal separator, it interprets it as an integer, leading to the InputMismatchException.

2. How to circumvent the exception:

To avoid the InputMismatchException, the following steps can be taken:

• Specify the Locale: Explicitly set the Scanner's locale to one that uses a period as the decimal separator. This can be done using the useLocale method:

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

• Use Regular Expressions: Instead of using the nextDouble method, employ regular expressions to validate and extract double values from user input. This approach allows for greater control over the input format.

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