java中设置千分位分隔符

好像明白了需要不停循环使用n%1000，还是想请教一下各位，这是一道程序填空题

import java.util.*;

public class T {
    public static void main(String[] args) {
        int ri, repeat;
        long n, t, m;
        Scanner in = new Scanner(System.in);
        repeat = in.nextInt();
        for (ri = 1; ri <= repeat; ri++) {
            n = in.nextLong();
            var ns = n + "";
            List<String> sss = new ArrayList<String>();
            for (int i = 0; i < ns.length() / 3; i++) { // 从最后一位开始往前循环,向sss中添加3位数字
                sss.add(ns.substring(ns.length() - (i + 1) * 3, ns.length() - i * 3));
            }
            sss.add(ns.substring(0, ns.length() % 3)); // 添加开头
            for (int i = sss.size() - 1; i >= 0; i--) {
                if (i == 0) {
                    System.out.println(sss.get(i));
                } else {
                    if (sss.get(i) != "") {
                        System.out.print(sss.get(i) + ",");
                    }
                }
            }
        }
    }
}