这java编程题咋写呀?

public class Money {
public static void main(String[] args) {
String str="1234567890";
StringBuilder sb=new StringBuilder(str);
//标识位，用来判断是否达到3位
int flag=0;
for(int i=str.length()-1;i>=0;i--){
flag++;
if(flag==3)
{
//从后往前算，flag等于3就插入一个逗号
sb.insert(i,",");
//重置为0
flag=0;
}
}

System.out.println(str);
System.out.println(sb.toString());
}
}

import java.math.BigDecimal;
import java.text.DecimalFormat;
import java.util.Scanner;

public class CommaTest {
public static void main(String[] args) {
System.out.println("请输入数字字符串：");
Scanner in = new Scanner(System.in);
BigDecimal numbers = in.nextBigDecimal();
in.close();

DecimalFormat df = new DecimalFormat("#,###");
String str = df.format(numbers);
System.out.println(str);
}
}