String prefix =fileName.substring(index+1,fileName.length());这里的疑问
String prefix =fileName.substring(index+1,fileName.length());
"1. 字符串 str 中字符的索引从0开始,范围为 0 到 str.length()-1"
按道理,应该是这样写的:String prefix =fileName.substring(index+1,fileName.length()-1);
毕竟索引区间是这个:[0,fileName.length()-1]