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CombineFn (Apache Crunch 0.10.0 API)

org.apache.crunch
Class CombineFn<S,T>

java.lang.Object
  extended by org.apache.crunch.DoFn<Pair<S,Iterable<T>>,Pair<S,T>>
      extended by org.apache.crunch.CombineFn<S,T>
All Implemented Interfaces:
Serializable
Direct Known Subclasses:
Aggregate.TopKCombineFn

public abstract class CombineFn<S,T>
extends DoFn<Pair<S,Iterable<T>>,Pair<S,T>>

A special DoFn implementation that converts an Iterable of values into a single value. If a CombineFn instance is used on a PGroupedTable, the function will be applied to the output of the map stage before the data is passed to the reducer, which can improve the runtime of certain classes of jobs.

Note that the incoming Iterable can only be used to create an Iterator once. Calling Iterable.iterator() method a second time will throw an IllegalStateException.

See Also:
Serialized Form

Constructor Summary
CombineFn()
           
 
Method Summary
 
Methods inherited from class org.apache.crunch.DoFn
cleanup, configure, disableDeepCopy, initialize, process, scaleFactor, setConfiguration, setContext
 
Methods inherited from class java.lang.Object
equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Constructor Detail

CombineFn

public CombineFn()


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