Algorithm to check if agiven is a valid hexadecimal number.

isHex.java

public class StringUtils {

    public static boolean isHexNumber(String s) {

        int cursor;

        if (s.length() == 0){
            return false;
        }

        if(s.length() == 1){
            return hasHexDigits(0,s);
        }

        if (has_0X_asPrefix(s)){
            if (s.length() > 2){
                cursor = 2;
            }
            else {
                return false;
            }
        }
        else {
            cursor = 0;
        }

        boolean isHex = hasHexDigits(cursor,s);

        return isHex;

    }

    public static boolean has_0X_asPrefix (String inputStr){

        List hexPrefix = Arrays.asList('x', 'X');

        return hexPrefix.contains(inputStr.charAt(1)) && (inputStr.charAt(0) == '0');


    }

    public static boolean hasHexDigits (int cursor, String inputStr){

        List hexDigits = Arrays.asList('a', 'b', 'c', 'd', 'e', 'f', 'A', 'B', 'C', 'D', 'E', 'F', '1', '2', '3', '4', '5', '6', '7', '8', '9', '0');

        for (;cursor < inputStr.length(); cursor++){
            if (hexDigits.contains(inputStr.charAt(cursor))){
                continue;
            }
            else {
                return false;
            }
        }
        return true;
    }
}