check number of a given character occurence in a String

Question

I want to write a simple class to process Strings (may be very long strings up to 1mil characters in it). String will basically consists of two characters \"a\" and \"b\" that

Answer 1

You should definitely not using regexp for this problem: generally speaking, regexp is not good when you need to count anything. You cannot even write a regexp to check if brackets in an expression are balanced.

For this problem a simple counter will be sufficient: increment on a, decrement on b, check for zero in the end to know the answer to your problem.

boolean check(String s) {
    int count = 0;
    for (int i = 0 ; i != s.length() ; i++) {
        if (s.charAt(i) == 'a') {
            count++;
        } else { /* it is b */
            count--;
        }
    }
    return count == 0;
}