StringBuffer 其实提供了很多有用的方法，
以前用的多是 append，
其实还有：
append(double)
delete(int, int)
deleteCharAt(int)
replace(int, int, String)
substring(int)
subSequence(int, int)
substring(int, int)
insert(int, char[], int, int)
insert(int, Object)
insert(int, String)
insert(int, char[])
insert(int, CharSequence)
insert(int, CharSequence, int, int)
insert(int, boolean)
insert(int, char)
insert(int, int)
insert(int, long)
insert(int, float)
insert(int, double)
indexOf(String)
indexOf(String, int)
lastIndexOf(String)
lastIndexOf(String, int)
reverse()