这样也可以

public class TestString {
    public static void main(String[] args) {
        // 创建一个空的StringBuilder对象
        StringBuilder str = new StringBuilder();
        // 追加字符串
        str.append("jaewkjldfxmopzdmab");
        // 从后往前每隔三位插入逗号
        int num = str.length();
        while (num>3) {
            num -= 3;
            str.insert(num, ",");
        }
        // 将StringBuilder对象转化为String对象并打印
        System.out.println(str.toString());
    }

}