why there is an error unhandled exception type NullDenominatorException

The error "unhandled exception type NullDenominatorException" occurs because the `Fraction` constructor throws a checked exception `NullDenominatorException`, but the calling code does not handle the exception.

In Java, a checked exception must be either caught or declared to be thrown. If a method throws a checked exception, the calling code must either handle the exception using a try-catch block or declare that it throws the exception itself.

To fix the error, you can either catch the exception in a try-catch block or declare that the calling method throws the exception. Here's an example of catching the exception in a try-catch block:

try {
    Fraction f = new Fraction(1, 0);
} catch (NullDenominatorException e) {
    System.err.println("Error: " + e.getMessage());
}

In this example, we try to create a new Fraction object with a denominator of 0. Since the `Fraction` constructor throws a `NullDenominatorException`, we catch the exception in a try-catch block and print an error message.

Alternatively, you can declare that the calling method throws the exception. For example:

public static void main(String[] args) throws NullDenominatorException {
    Fraction f = new Fraction(1, 0);
}

In this example, we declare that the `main` method throws a `NullDenominatorException`, so we don't need to catch the exception within the method. However, the exception will need to be caught or declared to be thrown by any methods that call `main`.