china：
张三 78
李四 89
王五 96
赵六 67
english
张三 80
李四 82
王五    84
赵六 86
math
张三 88
李四 99
王五 66
赵六 77

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;

import org.apache.hadoop.fs.Path;

import org.apache.hadoop.io.IntWritable;

import org.apache.hadoop.io.FloatWritable;

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.input.TextInputFormat;

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

import org.apache.hadoop.mapreduce.lib.reduce.IntSumReducer;

public class MyAverage {

    public static class FormatMapper extends Mapper<Object, Text, Text, IntWritable>{

        private IntWritable val = new IntWritable();

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

            String line[] = value.toString().split("\\s");

            val.set(Integer.parseInt(line[]));

            context.write(new Text(line[]), val);

        }

    }

    public static class AverageReducer extends Reducer<Text, IntWritable, Text, FloatWritable>{

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

            int count = ;

            int sum = ;

            for (IntWritable val : values) {

                sum += val.get();

                count ++;

            }

            float ave = (float)sum / count;

            context.write(key, new FloatWritable(ave));

        }

    }

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

        String dir_in = "hdfs://localhost:9000/in_average";

        String dir_out = "hdfs://localhost:9000/out_average";

        Path in = new Path(dir_in);

        Path out = new Path(dir_out);

        Configuration conf = new Configuration();

        Job averageJob = new Job(conf, "my_average");

        averageJob.setJarByClass(MyAverage.class);

        averageJob.setInputFormatClass(TextInputFormat.class);

        averageJob.setMapperClass(FormatMapper.class);

        averageJob.setCombinerClass(IntSumReducer.class);

        //countJob.setPartitionerClass(HashPartitioner.class);

        averageJob.setMapOutputKeyClass(Text.class);

        averageJob.setMapOutputValueClass(IntWritable.class);

        FileInputFormat.addInputPath(averageJob, in);

        averageJob.setReducerClass(AverageReducer.class);

        //averageJob.setNumReduceTasks(1);

        averageJob.setOutputKeyClass(Text.class);

        averageJob.setOutputValueClass(FloatWritable.class);

        //countJob.setOutputFormatClass(SequenceFileOutputFormat.class);

        FileOutputFormat.setOutputPath(averageJob, out);

        averageJob.waitForCompletion(true);

    }

}

张三    82.0

李四    90.0

王五    82.0

赵六    76.666664