Github user tdas commented on the pull request:
https://github.com/apache/spark/pull/126#issuecomment-37501724
@rxin It overrides stuff to make sure such things like traversing entire
HashMap does not happen. They are meant for being drop-in replacements of scala
HashMaps when applying certain added features like maintaining timestamps
(TimeStampedHashMaps and TimeStampedWeakValueHashMap), or bounding the number
of key-value pairs (BoundedHashMap). They are meant to be thread-safe, and used
for metadata (not really for super-heavy usage like AppendOnlyHashMap for
processing data). They are useful for easily replacing various scala HashMaps
used for storing metadata, when we want to apply different policies for
managing that metadata (TTL based cleanup vs bounded cache behavior).
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