Reducer class
The Reducer base class
works very similarly to the Mapper class, and usually requires only
sub classes to override a
single reduce method. Here is the cut-down class definition:
public class
Reducer<K2, V2, K3, V3>
{
void reduce(K1 key,
Iterable<V2> values,Context context)
throws IOException,
InterruptedException
{……
……….
}}
Notice the class
definition in terms of the broader data flow (the reduce method accepts K2/V2 as reduce
input key, reduce input value and
provides K3/V3 as reduce output key, reduce output value) while the actual
reduce method takes only a single key and its associated list of values. The Context
object is again the mechanism to output the result of the method.
reduce( ) method
The reduce method takes
an input key/value pair in its parameters. First one is reduce input key(map
output key), second one is reduce input value(map output value) and the other
parameter is an instance of the Context class that provides various mechanisms
to communicate with the Hadoop framework.
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