map-reduce/java/parallel/WordCount.java

import java.io.IOException;
import java.util.HashMap;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;

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();

        /* map function */
        public void map(Object key, Text value, Context context) throws IOException, InterruptedException {
            String line = value.toString();
            String str = new String("");
            for (int i = 0; i < line.length(); i++) {
                if (Character.isLetter(line.charAt(i)) || str.length() > 0 && line.charAt(i) == '\'') {
                    str += Character.toLowerCase(line.charAt(i)); // extend word
                } else if (str.length() > 0) {
                    word.set(str);
                    context.write(word, one);
                    str = "";
                }
            }
            if (str.length() > 0) {
                word.set(str);
                context.write(word, one);
            }
        }
    }

    public static class IntSumReducer extends Reducer<Text, IntWritable, Text, IntWritable> {
        private HashMap<String, Integer> record = new HashMap<String, Integer>(); // store count in map, used for final sort
        private Text word = new Text();
        private IntWritable result = new IntWritable();

        /* reduce function */
        public void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {
            int sum = 0;
            for (IntWritable val : values) {
                sum += val.get();
            }
            record.put(key.toString(), record.getOrDefault(key.toString(), 0) + sum); // update map, don't write context here
        }

        /* cleanup function executed after reducing */
        public void cleanup(Context context) throws IOException, InterruptedException {
            ArrayList<HashMap.Entry<String, Integer>> list = new ArrayList<HashMap.Entry<String, Integer>>(record.entrySet()); // get answer from map
            Collections.sort(list, new Comparator<HashMap.Entry<String, Integer>>() {
                public int compare(HashMap.Entry<String, Integer> o1, HashMap.Entry<String, Integer> o2) { // compare function
                    if (o1.getValue().equals(o2.getValue())) {
                        return o1.getKey().compareTo(o2.getKey());
                    }
                    return o1.getValue() < o2.getValue() ? 1 : -1;
                }
            });
            for (HashMap.Entry<String, Integer> itr : list) {
                word.set(itr.getKey());
                result.set(itr.getValue());
                context.write(word, result); // write context here
            }
        }
    }

    public static void main(String[] args) throws Exception {
        Configuration conf = new Configuration();
        Job job = Job.getInstance(conf, "word count");
        job.setJarByClass(WordCount.class);
        job.setMapperClass(TokenizerMapper.class); // map class
        job.setCombinerClass(IntSumReducer.class); // combine class
        job.setReducerClass(IntSumReducer.class); // reduce class
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(IntWritable.class);
        FileOutputFormat.setOutputPath(job, new Path(args[0])); // output directory
        for (int i = 1; i < args.length; i++) {
            FileInputFormat.addInputPath(job, new Path(args[i])); // input files
        }
        System.exit(job.waitForCompletion(true) ? 0 : 1);
    }
}