MapReduce平均数成绩怎么求每行的平均数

package com.hpu.hadoop.test;

import org.apache.hadoop.io.DoubleWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;

import java.io.IOException;

public class MyMapper extends Mapper<LongWritable, Text,Text, DoubleWritable> {
    private Text K;
    private DoubleWritable V;

    @Override
    protected void setup(Context context) throws IOException, InterruptedException {
        K = new Text();
        V = new DoubleWritable();
    }

    @Override
    protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
        String[] split = value.toString().split("\t");
        K.set(split[0]);
        double sum = Double.parseDouble(split[1]) + Double.parseDouble(split[2]) + Double.parseDouble(split[3]);
        V.set(sum/3);
        context.write(K,V);
    }
}

package com.hpu.hadoop.test;

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.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

import java.io.IOException;


public class MyDriver {
    public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {
        Configuration conf = new Configuration();
        Job job = Job.getInstance(conf);

        job.setJarByClass(MyDriver.class);
        job.setMapperClass(MyMapper.class);
        job.setNumReduceTasks(0);

        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(DoubleWritable.class);

        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(DoubleWritable.class);
        //指定输入、输出路径
        FileInputFormat.setInputPaths(job,new Path(""));
        FileOutputFormat.setOutputPath(job,new Path(""));

        boolean b = job.waitForCompletion(true);
        System.exit(b?0:1);

    }
}