StringBuilder
1.创建对象
StringBuilder sb = new StringBuilder();

2.添加元素append()
sb.append(1);
sb.append(dkjfak);

3.反转reverse()
sb.reverse();

4.获取长度length()
int len = sb.length();

5.变回字符串toString()
(因为sb是StringBuilder类型的，并不是字符串)
String str = sb.toString();