Find mutual friends

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
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.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

public class SharedFriendsStepOne {

	static class SharedFriendsStepOneMapper extends Mapper<LongWritable, Text, Text, Text> {

		@Override
		protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
			// A:B,C,D,F,E,O
			String line = value.toString();
			String[] person_friends = line.split(":");
			String person = person_friends[0];
			String friends = person_friends[1];

			for (String friend : friends.split(",")) {

				// output <friend, person>
				context.write(new Text(friend), new Text(person));
			}

		}

	}

	static class SharedFriendsStepOneReducer extends Reducer<Text, Text, Text, Text> {

		@Override
		protected void reduce(Text friend, Iterable<Text> persons, Context context) throws IOException, InterruptedException {

			StringBuffer sb = new StringBuffer();

			for (Text person : persons) {
				sb.append(person).append(",");

			}
			context.write(friend, new Text(sb.toString()));
		}

	}

	public static void main(String[] args) throws Exception {

		Configuration conf = new Configuration();

		Job job = Job.getInstance(conf);
		job.setJarByClass(SharedFriendsStepOne.class);

		job.setOutputKeyClass(Text.class);
		job.setOutputValueClass(Text.class);
		
		job.setMapperClass(SharedFriendsStepOneMapper.class);
		job.setReducerClass(SharedFriendsStepOneReducer.class);

		FileInputFormat.setInputPaths(job, new Path("D:/srcdata/friends"));
		FileOutputFormat.setOutputPath(job, new Path("D:/temp/out"));

		job.waitForCompletion(true);

	}

}

import java.io.IOException;
import java.util.Arrays;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
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.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

public class SharedFriendsStepTwo {

	static class SharedFriendsStepTwoMapper extends Mapper<LongWritable, Text, Text, Text> {

		// The data obtained is the output result of the previous step
		// A I,K,C,B,G,F,H,O,D,
		// friend, person, person
		@Override
		protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {

			String line = value.toString();
			String[] friend_persons = line.split("\t");

			String friend = friend_persons[0];
			String[] persons = friend_persons[1].split(",");

			Arrays.sort(persons);

			for (int i = 0; i < persons.length - 1; i++) {
				for (int j = i + 1; j < persons.length; j++) {
					// issue <person-person, friend>, so that all friends of the same "person-person" pair will go to the same reduce
					context.write(new Text(persons[i] + "-" + persons[j]), new Text(friend));
				}

			}

		}

	}

	static class SharedFriendsStepTwoReducer extends Reducer<Text, Text, Text, Text> {

		@Override
		protected void reduce(Text person_person, Iterable<Text> friends, Context context) throws IOException, InterruptedException {

			StringBuffer sb = new StringBuffer();

			for (Text friend : friends) {
				sb.append(friend).append(" ");

			}
			context.write(person_person, new Text(sb.toString()));
		}

	}

	public static void main(String[] args) throws Exception {

		Configuration conf = new Configuration();

		Job job = Job.getInstance(conf);
		job.setJarByClass(SharedFriendsStepTwo.class);

		job.setOutputKeyClass(Text.class);
		job.setOutputValueClass(Text.class);

		job.setMapperClass(SharedFriendsStepTwoMapper.class);
		job.setReducerClass(SharedFriendsStepTwoReducer.class);

		FileInputFormat.setInputPaths(job, new Path("D:/temp/out/part-r-00000"));
		FileOutputFormat.setOutputPath(job, new Path("D:/temp/out2"));

		job.waitForCompletion(true);

	}

}

Test file:

A:B,C,D,F,E,O
B:A,C,E,K
C:F,A,D,I
D:A,E,F,L
E:B,C,D,M,L
F:A,B,C,D,E,O,M
G:A,C,D,E,F
H:A,C,D,E,O
I:A,O
J:B,O
K:A,C,D
L:D,E,F
M:E,F,G
O:A,H,I,J

Output result:

A	I,K,C,B,G,F,H,O,D,
B	A,F,J,E,
C	A,E,B,H,F,G,K,
D	G,C,K,A,L,F,E,H,
E	G,M,L,H,A,F,B,D,
F	L,M,D,C,G,A,
G	M,
H	O,
I	O,C,
J	O,
K	B,
L D, E,
M	E,F,
O	A,H,I,J,F,

A-B	C E 
AC 	FD 
AD 	EF 
A-E	B C D 
A-F	C D B E O 
AG 	DEFC 
AH 	EOCD 
AI 	O 
A-K	D 
A-L	F E 
BC 	A 
BD 	EA 
B-E	C 
B-F	E A C 
BG 	CEA 
B-H	E C A 
BI 	A 
BK 	A 
W 	E 
C-D	F A 
C-E	D 
C-F	D A 
C-G	F A D 
CH 	AD 
CI 	A 
C-K	D A 
C-L	F 
D-F	E A 
DG 	AEF 
DH 	AE 
DI 	A 
DK 	A 
D-L	F E 
E-F	C D B 
EG 	DC 
E-H	D C 
E-K	D 
F-G	C E D A 
FH 	CADEO 
FI 	AO 
F-K	D A 
FL 	E 
G-H	D E C A 
GI 	A 
TG 	AD 
GL 	FE 
HI 	AO 
HK 	AD 
HL 	E 
IK 	A