Github user aarondav commented on the pull request:
https://github.com/apache/spark/pull/1272#issuecomment-47614369
Out of curiosity, what motivates this change? I thought typically
ConcurrentHashMap is used for more heavyweight concurrent data structures,
while synchronizedMap() is used when concurrency is rare.
Either way, our usage is not threadsafe, since we do things like
```scala
if (map.contains(x)) map.get(x) else defaultValue
```
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