在编写WordCount程序时，我遇到了一个奇怪的问题：在HDFS上可以创建output文件夹，但里面却没有程序运行后的结果文件。如图所示，output3文件夹里面的文件数为0：http://s1/mw690/002aBHAXzy6S1S7wrW870&690

这让我百思不得其解。折腾了半天以后，偶然间发现我的Map端和Reduce端的函数误写为了Map（）和Reduce（），改为map( )和reduce( )即可。本人编写的程序如下：

import java.io.IOException;
import java.util.Iterator;

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 TxtCounter{
 public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {
  Configuration conf=new Configuration();
  String inputPathString="hdfs://master:9000/inputFile";
  String outputPathString="hdfs://master:9000/output2";
  
  Job job=new Job(conf,TxtCounter.class.getSimpleName());
  job.setMapperClass(TxtMapper.class);
  job.setCombinerClass(TxtReducer.class);
  job.setReducerClass(TxtReducer.class);
  job.setOutputKeyClass(Text.class);
  job.setOutputValueClass(IntWritable.class);
  
  FileInputFormat.addInputPath(job, new Path(inputPathString));
  FileOutputFormat.setOutputPath(job, new Path(outputPathString));
  
  job.waitForCompletion(true);
  
 }
 
 static class TxtMapper extends Mapper{
  protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException{
   String[] strs=value.toString().split(" ");
   for(String str:strs){
    context.write(new Text(str), new IntWritable(1));
   }
  }
 }
 
 static class TxtReducer extends Reducer, Text, IntWritable>{
  protected void reduce(Text key, Iterable values,Context context) throws IOException, InterruptedException{
   int sum=0;
   Iterator it=values.iterator();
   while(it.hasNext()){
    IntWritable value=it.next();
    sum+=value.get();
   }
   context.write(key, new IntWritable(sum));
  }
 }
}