求平均值的程式：

package my.hadoopstudy;  
   
import org.apache.hadoop.conf.Configuration;  
import org.apache.hadoop.fs.Path;   
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 org.apache.hadoop.util.GenericOptionsParser;  
   
import java.io.IOException;  
   
public class AvgCompute {  
   
    public static class MyMapper extends Mapper<Object, Text, Text, Text>{   
   
        public void map(Object key, Text value, Context context) throws IOException, InterruptedException {   
                String[] e = value.toString().split(" ",2);  
                context.write(new Text(e[0]),new Text(e[1]));
        }  
    }  
   
    public static class MyReducer extends Reducer<Text,Text,Text,Text> {  
   
        public void reduce(Text key, Iterable<Text> values, Context context) throws IOException, InterruptedException {  
            float sum = 0;  
            float count=0;
            for (Text val : values) {  
                sum += Float.parseFloat(val.toString()); 
                count +=1.0f;
            }  
            sum=sum/count;  
            context.write(key,new Text(String.valueOf(sum)));  
        }  
    }  
   
    public static void main(String[] args) throws Exception {  
        Configuration conf = new Configuration();  
        String[] otherArgs = new GenericOptionsParser(conf, args).getRemainingArgs();  
        if (otherArgs.length < 2) {  
            System.err.println("Usage: EventCount <in> <out>");  
            System.exit(2);  
        }  
        Job job = Job.getInstance(conf, "avg compute");  
        job.setJarByClass(AvgCompute.class);  
        job.setMapperClass(MyMapper.class);  
        job.setCombinerClass(MyReducer.class);  
        job.setReducerClass(MyReducer.class);  
        job.setOutputKeyClass(Text.class);  
        job.setOutputValueClass(Text.class);  
        FileInputFormat.addInputPath(job, new Path(otherArgs[0]));  
        FileOutputFormat.setOutputPath(job, new Path(otherArgs[1]));  
        System.exit(job.waitForCompletion(true) ? 0 : 1);  
    }  
}  
 

輸出：