chars with forEach

class IsbnVerifier {

    boolean isValid(String stringToVerify) {
        var acc = new IsbnAcc();
        stringToVerify.chars().forEach(acc::calc);
        return acc.isIsbn();
    }

}

class IsbnAcc {
    int sum = 0;
    int pos = 0;
    boolean allValidChars = true;

    boolean isIsbn() {
        return allValidChars && pos == 10 && sum % 11 == 0;
    }

    void calc(int codePoint) {
        if (Character.isDigit(codePoint)) {
            sum += (codePoint - '0') * (10 - pos);
            pos++;
            return;
        }
        if (codePoint == '-')
            return;
        if (codePoint == 'X' && pos == 9) {
            sum += 10;
            pos++;
            return;
        }
        allValidChars = false;
    }
}

This variant of validating as you go begins with instantiating an object which does the work of validating the letters and accumulating their values for the ISBN string to be verified. The object initializes its sum and pos variables to 0, and its allValidChars variable to true.

The chars() method is then called on the string. Each Character is a Unicode codepoint that is passed into the forEach()method. The forEach method passes the codepoint into the calc() method of the validating/accumulating object.

The calc() method starts by checking if the codepoint is a digit, which is the most likely condition. If so, it gets the digit value of the codepoint by subtracting the ASCII value of 0 from its own ASCII value. So, if the codepoint is a 0, then subtracting the ASCII value of 0 from itself results in 0. If the codepoint 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 codepoint 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) codepoints.
Since for this exercise all of the codepoints 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). The result of the multiplication is added to the sum variable.

The pos variable is incremented and the function returns.

If the codepoint is not a digit but a dash, the function returns without incrementing the pos variable.

If the codepoint is an X and the position is 9 (position 9 being at the end of a string whose length is 10), then 10 is added to the sum, pos is incremented, and the function returns.

If none of those legal conditions apply, then the codepoint is illegal and allValidChars is set to false.

After the forEach() is done, the isValid() function returns a call to the object's isIsbn() method. The isIsbn() method first checks if all chars are valid. If not, then the && operator short circuits and returns false from the function. If it is true, it then checks if the position is 10. If not, then the && operator short circuits and returns false from the function. If true, then the last check is if the sum is evenly divisible by 11. If all of the checks are true, the function returns true.