How would I be able to determine if a string contains only one digit?

I would need to find if the user input for password contains only 1 digit and I am having trouble what the if statement would be.

I currently only have:

import java.util.Scanner;

public static boolean onlyOneDigit (String password) {
    if ( ........ ) {
        System.out.print("Password requirement not met");
        return true;
    } else {
        return false;
    }
}

public static void main(String[] args) {
    do {
        System.out.print("Enter a password");
        String password = input.next();
    } while (onlyOneDigit(password)); 

    {
        System.out.print("Password updated");
    }
}