Java doc 第一句话就说的明明白白的。

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.

【 在 townhope 的大作中提到: 】
: “StringBuilder和StringBuffer的区别”，
: 这是为了引出下一个问题。
: 但是我回答不知道，所以没见到过下一个问题。
: ...................
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