This code is adapted from the official MapReduce Tutorial. It needs to be in a file WordCount.java (with an appropriate package declared if you like):

// adapted from https://hadoop.apache.org/docs/current/hadoop-mapreduce-client/hadoop-mapreduce-client-core/MapReduceTutorial.html

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

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.conf.Configured;
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.TextInputFormat;
import org.apache.hadoop.mapreduce.lib.output.TextOutputFormat;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;

public class WordCount extends Configured implements Tool {

	public static class TokenizerMapper
	extends Mapper<LongWritable, Text, Text, IntWritable>{

		private final static IntWritable one = new IntWritable(1);
		private Text word = new Text();

		@Override
		public void map(LongWritable key, Text value, Context context
				) throws IOException, InterruptedException {
			StringTokenizer itr = new StringTokenizer(value.toString());
			while (itr.hasMoreTokens()) {
				word.set(itr.nextToken());
				context.write(word, one);
			}
		}
	}

	public static class IntSumReducer
	extends Reducer<Text, IntWritable, Text, IntWritable> {
		private IntWritable result = new IntWritable();

		@Override
		public void reduce(Text key, Iterable<IntWritable> values,
				Context context
				) throws IOException, InterruptedException {
			int sum = 0;
			for (IntWritable val : values) {
				sum += val.get();
			}
			result.set(sum);
			context.write(key, result);
		}
	}

	public static void main(String[] args) throws Exception {
		int res = ToolRunner.run(new Configuration(), new WordCount(), args);
		System.exit(res);
	}

	@Override
	public int run(String[] args) throws Exception {
		Configuration conf = this.getConf();
		Job job = Job.getInstance(conf, "word count");
		job.setJarByClass(WordCount.class);

		job.setInputFormatClass(TextInputFormat.class);

		job.setMapperClass(TokenizerMapper.class);
		job.setCombinerClass(IntSumReducer.class);
		job.setReducerClass(IntSumReducer.class);

		job.setOutputKeyClass(Text.class);
		job.setOutputValueClass(IntWritable.class);
		job.setOutputFormatClass(TextOutputFormat.class);
		TextInputFormat.addInputPath(job, new Path(args[0]));
		TextOutputFormat.setOutputPath(job, new Path(args[1]));

		return job.waitForCompletion(true) ? 0 : 1;
	}
}

Notes on changes

I have updated the original example so it “extends Configured implements Tool”. This allows it to take the standard collection of command-line arguments for free and is good practice.

I have also been a little more explicit: setting the InputFormat and OutputFormat to the default, and giving the specific types for the Mapper.