map with sum

import java.util.stream.IntStream;

class IsbnVerifier {

    public boolean isValid(String s) {
        String scrubbed = s.replace("-", "");

        return scrubbed.matches("^([0-9]{10}|[0-9]{9}X)$") &&
            IntStream.range(0, scrubbed.length())
            .map(pos -> {
                var chr = scrubbed.charAt(pos);
                return (chr != 'X' ? chr - '0' : 10) * (10 - pos);
            })
            .sum() % 11 == 0;
    }
}

This variant of scrubbing and validating the data first starts by importing from packages for what is needed.

The isValid() method starts by calling replace() to scrub out any dash characters.

The matches() method is then called on the scrubbed string and is passed a pattern which checks for a string of exactly ten digits or nine digits ending in an X. If the string doesn't match the pattern, then the && operator short circuits and returns false from the function.

If the string matches the pattern, then the IntStream range() method is called to generate numbers from 0 up to but not including the length of the scrubbed string. Each number is passed into a lambda which gets the character in the string at the position of the number.

If the character is not X then it is a digit whose value is determined by subtracting the ASCII value of 0 from its own ASCII value. So, if the character is a 0, then subtracting the ASCII value of 0 from itself results in 0. If the character is a 1, then subtracting the ASCII value of 0 from the ASCII value of 1 results in 1, and so on.

Another way to convert the character to a digit is to use the built-in
[`Character.digit()`](https://docs.oracle.com/javase/7/docs/api/java/lang/Character.html#digit(char,%20int))
method, which also works for [Unicode](https://docs.oracle.com/javase/tutorial/i18n/text/unicode.html) digits.
Since for this exercise all of the digits are ASCII, simple ASCII math will do. 

The digit is multiplied by 10 minus the position, with the position starting at 0. So, for the position furthest to the left, the digit is multiplied by 10 - 0 (10). For the position furthest to the right of a string whose length is 10, the digit is multiplied by 10 - 9 (1).

Each mapped value is passed into the sum() method to be added up. The function returns whether the sum() is evenly divisible by 11 or not.