org.apache.crunch

Class MapFn<S,T>

java.lang.Object

org.apache.crunch.DoFn<S,T>

org.apache.crunch.MapFn<S,T>

All Implemented Interfaces:

Serializable

Direct Known Subclasses:

CompositeMapFn, ExtractKeyFn, IdentityFn, PairMapFn, PGroupedTableType.PairIterableMapFn

public abstract class MapFn<S,T>
extends DoFn<S,T>

A DoFn for the common case of emitting exactly one value for each input record.

See Also:

Serialized Form

Constructor Summary

Constructors

Constructor and Description
MapFn()

Method Summary

Methods

Modifier and Type	Method and Description
abstract T	map(S input) Maps the given input into an instance of the output type.
void	process(S input, Emitter<T> emitter) Processes the records from a PCollection.
float	scaleFactor() Returns an estimate of how applying this function to a PCollection will cause it to change in side.

Methods inherited from class org.apache.crunch.DoFn

cleanup, configure, initialize, setContext

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

MapFn

public MapFn()

Method Detail

map

public abstract T map(S input)

Maps the given input into an instance of the output type.

process

public void process(S input,
           Emitter<T> emitter)

Description copied from class: DoFn

Processes the records from a PCollection.

Note: Crunch can reuse a single input record object whose content changes on each DoFn.process(Object, Emitter) method call. This functionality is imposed by Hadoop's Reducer implementation: The framework will reuse the key and value objects that are passed into the reduce, therefore the application should clone the objects they want to keep a copy of.

Specified by:

process in class DoFn<S,T>

Parameters:

input - The input record.

emitter - The emitter to send the output to

scaleFactor

public float scaleFactor()

Description copied from class: DoFn

Returns an estimate of how applying this function to a PCollection will cause it to change in side. The optimizer uses these estimates to decide where to break up dependent MR jobs into separate Map and Reduce phases in order to minimize I/O.

Subclasses of DoFn that will substantially alter the size of the resulting PCollection should override this method.

Overrides:

scaleFactor in class DoFn<S,T>