public?class?HelloWorld?{ ????public?static?void?main(String[]?args)?{ //?創(chuàng)建一個空的StringBuilder對象 StringBuilder?str=?new?StringBuilder(); ???????? //?追加字符串 str.append("jaewkjldfxmopzdm"); ????????//?從后往前每隔三位插入逗號 for?(int?i?=?str.length()-3;i>0;i=i-3) ????????str.insert(i,','); ???????? ???????? ????????//?將StringBuilder對象轉(zhuǎn)換為String對象并輸出 System.out.print(str.toString()); } }for (int i = str.length()-3;i>0;i=i-3)什么意思？