MapReduce编程练习（四），统计多个输入文件学生的平均成绩，

问题描述：

在输入文件中，有多个，其中每个输入文件代表一个学生的各科成绩，其中每行的数据形式为<科目,成绩>，你需要将每个文件中的每科目的成绩进行统计，然后求平均值。

输入文件格式：

这里有三个学生：

输出文件格式：

实例代码：

package com.test;

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.LongWritable;
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;

public class StudentAverage {

	public static void main(String[] args) throws IllegalArgumentException, IOException, ClassNotFoundException, InterruptedException {
		@SuppressWarnings("deprecation")
		Job job = new Job(new Configuration(), "StudentAverage");
		job.setJarByClass(StudentAverage.class);
		job.setMapperClass(Map.class);
		job.setReducerClass(Reduce.class);
		job.setMapOutputKeyClass(Text.class);
		job.setMapOutputValueClass(IntWritable.class);
		job.setOutputKeyClass(Text.class);
		job.setOutputValueClass(IntWritable.class);
		FileInputFormat.setInputPaths(job, new Path("hdfs://localhost:9000/Student/input"));
		FileOutputFormat.setOutputPath(job, new Path("hdfs://localhost:9000/Student/output"));
		job.waitForCompletion(true);
		System.out.println("运行结束！");
	}

	public static class Map extends Mapper<LongWritable, Text, Text, IntWritable>{
		protected void map(LongWritable key, Text value,
				org.apache.hadoop.mapreduce.Mapper<LongWritable, Text, Text, IntWritable>.Context context)
				throws java.io.IOException, InterruptedException {

			String[] data = value.toString().split(" ");
			context.write(new Text(data[0]), new IntWritable(Integer.parseInt(data[1])));
		};
	}

	public static class Reduce extends Reducer<Text, IntWritable, Text, IntWritable> {
		protected void reduce(Text key, java.lang.Iterable<IntWritable> values, Context context)
				throws java.io.IOException, InterruptedException {
			int average = 0;
			int sum = 0;
			for (IntWritable value : values) {
				   sum += value.get();
			}
			average = sum / 3;
		    context.write(new Text(key), new IntWritable(average));
		};
	}
}