public class AffinityMatrixInputReducer
extends org.apache.hadoop.mapreduce.Reducer<org.apache.hadoop.io.IntWritable,DistributedRowMatrix.MatrixEntryWritable,org.apache.hadoop.io.IntWritable,VectorWritable>
Tasked with taking each DistributedRowMatrix entry and collecting them
into vectors corresponding to rows. The input and output keys are the same,
corresponding to the row in the ensuing matrix. The matrix entries are
entered into a vector according to the column to which they belong, and
the vector is then given the key corresponding to its row.