Tuesday, November 16, 2010

Hadoop's Hello World - Word Count using Map Reduce

The word count program in Hadoop is pretty much the Hello World of Hadoop.

The following programs counts the number of times each word appeared in input file/directory and places the results in the output directory

1. Create file WordCount.java and paste the following code in it

package org.myorg;
import java.io.IOException;
import java.util.*;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.conf.*;
import org.apache.hadoop.io.*;
import org.apache.hadoop.mapred.*;
import org.apache.hadoop.util.*;
public class WordCount {
public static class Map extends MapReduceBase implements Mapper {
private final static IntWritable one = new IntWritable(1);
private Text word = new Text();
public void map(LongWritable key, Text value, OutputCollector output, Reporter reporter) throws IOException {
String line = value.toString();
StringTokenizer tokenizer = new StringTokenizer(line);
while (tokenizer.hasMoreTokens()) {
output.collect(word, one);
public static class Reduce extends MapReduceBase implements Reducer {
public void reduce(Text key, Iterator values, OutputCollector 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);
FileInputFormat.setInputPaths(conf, new Path(args[0]));
FileOutputFormat.setOutputPath(conf, new Path(args[1]));
2. Compile, create jar and run

javac -classpath hadoop-0.20.1-dev-core.jar -d WordCount/ WordCount.java
jar -cvf WordCount.jar -C WordCount/ .
hadoop jar WordCount.jar org.myorg.WordCount in out

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