Github user StephanEwen commented on the pull request:
https://github.com/apache/flink/pull/408#issuecomment-77749391
In the core API, we have started to define all functions (map, join, ... -
here it would be SumFunction, ApplyFunction) as single-abstract-method
interfaces. That allows to use Java 8 lambas (Scala may pick this up as well).
For each function, there is a `RichFunction` variant which has the `open()` and
`close()` methods, and the `getRuntimeContext()` (here the `preSuperstep()` and
`postSuperstep()` methods).
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