UnitSum.java

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.DoubleWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

import java.io.IOException;
import java.text.DecimalFormat;

public class UnitSum {
    public static class PassMapper extends Mapper<Object, Text, Text, DoubleWritable> {

        @Override
        public void map(Object key, Text value, Context context) throws IOException, InterruptedException {

            //input format: toPage\t unitMultiplication
            //target: pass to reducer
        	//inputValue: 1 \t 0.27(subPr)
        	//ouputKey = 1
        	//outputValue = 0.27
        	String idPr = value.toString().split("\t");
        	context.write(new Text(idPr[0]), new DoubleWritable(Double.parseDouble(idPr[1])));
        }
    }

    public static class SumReducer extends Reducer<Text, DoubleWritable, Text, DoubleWritable> {

        @Override
        public void reduce(Text key, Iterable<DoubleWritable> values, Context context)
                throws IOException, InterruptedException {

            //input key = toPage value = <unitMultiplication>
            //target: sum!
        	//inputKey = toId
        	//inputValue = <subpr1, subpr2...>
        	//sum(subpr*) -> outputValue
        	double sum = 0;
        	for (DoubleWritable value: values) {
        		sum += value.get();
        	}
        	context.write(key, new DoubleWritable(sum));
        }
    }

    public static void main(String[] args) throws Exception {

        Configuration conf = new Configuration();
        Job job = Job.getInstance(conf);
        job.setJarByClass(UnitSum.class);
        job.setMapperClass(PassMapper.class);
        job.setReducerClass(SumReducer.class);
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(DoubleWritable.class);
        FileInputFormat.addInputPath(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));
        job.waitForCompletion(true);
    }
}