算法提高 身份证号码升级

import java.io.BufferedInputStream;
import java.util.Scanner;

public class Main {

    static int[] coefficient = {7,9,10,5,8,4,2,1,6,3,7,9,10,5,8,4,2};
    static String[] last = {"1","0", "x","9","8","7","6","5","4","3","2"};

    public static String Update(String old_id){
        StringBuffer str = new StringBuffer();
        str.append(old_id.substring(0,6));
        str.append("19"+old_id.substring(6));
        int sum = 0;
        for ( int i = 0 ; i < str.length() ; i++){
            sum += coefficient[i]*(int)(str.charAt(i) - '0');
        }
        sum %= 11;
        str.append(last[sum]);
        return str.toString();
    }

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Scanner in = new Scanner(new BufferedInputStream(System.in));
        String old_id = in.next();
        String new_id = Update(old_id);
        System.out.println(new_id);
        in.close();
    }

}