OrphanPages.java

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.NullWritable;
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;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;

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

// >>> Don't Change
public class OrphanPages extends Configured implements Tool {

    public static void main(String[] args) throws Exception {
        int res = ToolRunner.run(new Configuration(), new OrphanPages(), args);
        System.exit(res);
    }
// <<< Don't Change

    @Override
    public int run(String[] args) throws Exception {
	Job job = Job.getInstance(this.getConf(), "Orphan Page");
	job.setOutputKeyClass(IntWritable.class);
	job.setOutputValueClass(NullWritable.class);

	job.setMapOutputKeyClass(IntWritable.class);
	job.setMapOutputValueClass(IntWritable.class);

        job.setMapperClass(LinkCountMap.class);
        job.setReducerClass(OrphanPageReduce.class);

        FileInputFormat.setInputPaths(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));

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

    public static class LinkCountMap extends Mapper<Object, Text, IntWritable, IntWritable> {
        @Override
        public void map(Object key, Text value, Context context) throws IOException, InterruptedException {
		final String line = value.toString();
		final String pages[] =  line.split("[:]");

		context.write(new IntWritable(Integer.parseInt(pages[0].trim())), new IntWritable(0));

		final String links[] = pages[1].split("[ ]");
		for(String l : links) {
			if (!l.trim().isEmpty()) {
				Integer linkId = Integer.parseInt(l.trim());
				context.write(new IntWritable(linkId), new IntWritable(1));
			}
		}
        }
    }

    public static class OrphanPageReduce extends Reducer<IntWritable, IntWritable, IntWritable, NullWritable> {
        @Override
        public void reduce(IntWritable key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {
		int numLink = 0;
                for (IntWritable val : values) {
                        numLink += val.get();
                }

		if (0 == numLink) {
	                context.write(key, NullWritable.get());
		}
        }
    }
}