WordCount.java

/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/**
* Changes:
* Comments and identations added
* Structure improved
*/

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.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 WordCount {

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

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

		/**
		 * The "Mapper" function
		 * @param key - Input key - The line offset in the file - ignored.
		 * @param value - Input Value - This is the line itself.
		 * @param context - Provides access to the OutputCollector and Reporter.
		 * @throws IOException
		 * @throws InterruptedException 
		 */
		public void map(Object key, Text value, Context context) throws IOException, InterruptedException {
			StringTokenizer itr = new StringTokenizer(value.toString());
			while (itr.hasMoreTokens()) {
				word.set(itr.nextToken());
				// Record the output in the Context object
				context.write(word, one);
			}
		}
	}

	public static class IntSumReducer extends Reducer<Text,IntWritable,Text,IntWritable> {
		private IntWritable result = new IntWritable();
		
		/**
		 * The "Reducer" function
		 * @param key - Input key - Name of the region
		 * @param values - Input Value - Iterator over quake magnitudes for region
		 * @param context - Used for collecting output
		 * @throws IOException
		 * @throws InterruptedException 
		 */
		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 {
		// Create the job specification object
		Configuration conf = new Configuration();
		Job job = Job.getInstance(conf, "word count");
		job.setJarByClass(WordCount.class);
		
		// Set the Mapper and Reducer classes
		job.setMapperClass(TokenizerMapper.class);
		job.setCombinerClass(IntSumReducer.class);
		job.setReducerClass(IntSumReducer.class);
		
		// Specify the type of output keys and values
		job.setOutputKeyClass(Text.class);
		job.setOutputValueClass(IntWritable.class);
		
		// Setup input and output paths
		FileInputFormat.addInputPath(job, new Path(args[0]));
		FileOutputFormat.setOutputPath(job, new Path(args[1]));
		
		// Wait for the job to finish before terminating
		System.exit(job.waitForCompletion(true) ? 0 : 1);
	}
}