Github user zsxwing commented on the pull request:
https://github.com/apache/spark/pull/4006#issuecomment-71419194
> (1) make StreamingListenerEvent extend SparkListenerEvent
I don't like it because if so, we will get many meaningless "match may not
be exhaustive." warnings from the compiler when we use pattern matching.
My original idea was that creating a parent trait for
StreamingListenerEvent and SparkListenerEvent and also a parent trait for
SparkListener and StreamingListener. However, it still needs `ListenerHelper` ,
an event process template, to remove duplicate codes.
When I was writing `ListenerHelper`, I found I need the type parameters to
check the type at the compile time. If not using type parameters, E.g., assume
`Listener` is the parent trait of SparkListener and StreamingListener, then the
method signature of `addListener` will be `def addListener(listener:
Listener)`. The subclasses cannot change it to accept a more specific
`Listener`, so they need to do the type check at the runtime to reject a wrong
Listener.
Therefore, I added the type parameters and found I could remove the parent
traits of StreamingListenerEvent/SparkListenerEvent and
SparkListener/StreamingListener. I prefer less classes, so I removed them.
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