class Main {
    public static void main(String[] args) {
        // Character helpers test a single char
        System.out.println(Character.isDigit('5'));
        System.out.println(Character.isLetter('a'));
        System.out.println(Character.isWhitespace(' '));
        // test a whole string with a stream
        System.out.println("123".chars().allMatch(Character::isDigit));
    }
}

Character.isDigit and friends