check if char[] contains only one letter and one int

I have no idea how to check if char[] contains only one letter (a or b) on the first position and only one int (0-8) on the second position. for example a2, b2

I have some this, but I do not know, what should be instead of digital +=1;

private boolean isStringValidFormat(String s) {
    boolean ret = false;
    if (s == null) return false;

    int digitCounter = 0;

    char first = s.charAt(0);
    char second = s.charAt(1);

    if (first == 'a' || first == 'b') {
        if (second >= 0 && second <= '8') {
            digitCounter +=1;
        }
    }

    ret = digitCounter == 2; //only two position
    return ret;
}




` public char[] readFormat() {
    char[] ret = null;
    while (ret == null) {
        String s = this.readString();
        if (isStringValidFormat(s)) {
            ret = s.toCharArray();
        }else {
            System.out.println("Incorrect. Values must be between 'a0 - a8' and 'b0 - b8'");
        }
    }
    return new char[0];
}`

Answer

First, I would test for null and that there are two characters in the String. Then you can use a simple boolean check to test if first is a or b and the second is between 0 and 8 inclusive. Like,

private boolean isStringValidFormat(String s) {
    if (s == null || s.length() != 2) {
        return false;
    }
    char first = s.charAt(0);
    char second = s.charAt(1);
    return (first == 'a' || first == 'b') && (second >= '0' && second <= '8');
}