Github user carsonwang commented on the pull request:
https://github.com/apache/spark/pull/11090#issuecomment-180652254
Reusing a `Calendar` object when the method will be called frequently is
something recommended by "Effective Java" mentioned
[here](http://www.informit.com/articles/article.aspx?p=1216151&seqNum=5). The
more the method is invoked, the more performance improvement we get.
`stringToTimestamp` has the same problem but it uses different `TimeZone`s
instead of only a GMT `TimeZone`. We need cache the `TimeZone`s in a hash map
as I did in #11071. It is a little more involved and there is a case that the
`Calendar` cannot be reused. So I'll keep it as is and make this PR as small as
possible for now. If it is worth doing, I'll be happy to submit a new PR to
address it.
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