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public static class Map extends MapReduceBase
	implements Mapper<LongWritable, Text, Text, IntWritable> {

	private final static IntWritable one = new IntWritable(1);
	private Text word = new Text();
	
	public void map(LongWritable key, Text value,
			OutputCollector<Text, IntWritable> output,
			Reporter reporter) throws IOException {
		String line = value.toString();
		StringTokenizer tokenizer = new StringTokenizer(line);
		while(tokenizer.hasMoreTokens()) {
			word.set(tokenizer.nextToken());
			output.collect(word, one);
		}
	}
}

public static class Reduce extends MapReduceBase
	implements Reducer<Text, IntWritable, Text, IntWritable> {
	
	public void reduce(Text key, Iterator<IntWritable> values,
			OutputCollector<Text, IntWritable> output,
			Reporter reporter) throws IOException {
		int sum = 0;
		while(values.hasNext()) {
			sum += values.next().get();
		}
		output.collect(key, new IntWritable(sum));
	}
}

public static void main(String[] args) throws Exception {
	JobConf conf = new JobConf(WordCount.class);
	conf.setJobName("wordcount");
	
	conf.setOutputKeyClass(Text.class);
	conf.setOutputValueClass(IntWritable.class);
	
	conf.setMapperClass(Map.class);
	conf.setCombinerClass(Reduce.class);
	conf.setReducerClass(Reduce.class);
	
	conf.setInputFormat(TextInputFormat.class);
	conf.setOutputFormat(TextOutputFormat.class);
	
	FileInputFormat.setInputPaths(conf, new Path(args[0]));
	FileOutputFormat.setOutputPath(conf, new Path(args[1]));
	
	JobClient.runJob(conf);
}

$ bin/hadoop jar wordcount.jar org.myorg.WordCount input_dir output_dir