stringbuffer与stringbuilder

StringBuffer vs StringBuidler算是面试中露脸频率非常高的两兄弟

A mutable sequence of characters. This class provides an API compatible

• with StringBuffer, but with no guarantee of synchronization.
  This class is designed for use as a drop-in replacement for
  $StringBuffer$ in places where the string buffer was being
  used by a single thread (as is generally the case). Where possible,
  it is recommended that this class be used in preference to
  StringBuffer as it will be faster under most implementations.