这是我在mapreduce中的字数计算程序，我已经成功地创建了jar文件，但是当我在hadoop上执行时，我遇到了以下错误。

package com.niit;

import java.io.IOException;
import java.util.StringTokenizer;

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.input.TextInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.mapreduce.lib.output.TextOutputFormat;

public class WordCount {
    public static class Map extends Mapper<LongWritable,Text,Text,IntWritable> {
        public void map(LongWritable key, Text value,Context context) throws IOException,InterruptedException{
            String line = value.toString();
            StringTokenizer tokenizer = new StringTokenizer(line);
            while (tokenizer.hasMoreTokens()) {
                value.set(tokenizer.nextToken());
                context.write(value, new IntWritable(1));
            }
        }
    }

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

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

        Configuration conf= new Configuration();
        Job job = new Job(conf,"My Word Count Program");
        job.setJarByClass(WordCount.class);
        job.setMapperClass(Map.class);
        job.setReducerClass(Reduce.class);
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(IntWritable.class);
        job.setInputFormatClass(TextInputFormat.class);
        job.setOutputFormatClass(TextOutputFormat.class);
        Path outputPath = new Path(args[1]);
        //Configuring the input/output path from the filesystem into the job
        FileInputFormat.addInputPath(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));
        //deleting the output path automatically from hdfs so that we don't have to delete it explicitly
        outputPath.getFileSystem(conf).delete(outputPath);
        //exiting the job only if the flag value becomes false
        System.exit(job.waitForCompletion(true) ? 0 : 1);
    }
}

这是我收到的错误信息：

Command: yarn jar WordCount-1.0.jar com.niit.WordCount /usr/lib/hadoop-2.2.0/workspace/input/simple.txt /usr/lib/hadoop-2.2.0/workspace/output
    Exception in thread "main" java.lang.NoSuchMethodException: com.niit.WordCount.main([Ljava.lang.String;)
            at java.lang.Class.getMethod(Class.java:1665)
            at org.apache.hadoop.util.RunJar.main(RunJar.java:206)