需求：

有一個文件保存瞭如下信息：

A:B,C,D,F,E,O（代表A的好友有BCDFEO）
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

求出兩兩之間有共同好友的"用戶對"，及他倆的共同好友
比如:
A--B C ,E

思路：

先將文件內容輸出爲：<好友擁有該好友的人1,擁有該好友的人2, ....>的格式
然後將擁有該好友的人兩兩配對，作爲map的key,好友作爲value，reduce且調整格式後輸出的則爲< A--B 共同好友1，共同好友2，....>的格式

第一步代碼：

package com.bpf.mr.friend;

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 FriendStepOne {

    static class FriendStepOneMapper 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[] p_friend = line.split(":");
            //System.out.println(p_friend[1]);
            String p = p_friend[0];
            String[] friends = p_friend[1].split(",");
            
            //輸出<好友，人>
            for (String friend : friends) {
               
                context.write(new Text(friend), new Text(p));
            }
        }
    }
    
    static class FriendStepOneRuducer extends Reducer<Text, Text, Text, Text>{
        @Override
        protected void reduce(Text key, Iterable<Text> persons, Context context) throws IOException, InterruptedException {
            StringBuffer b = new StringBuffer();
            for (Text person : persons) {
                
                b.append(person.toString() + ",");
            }
            
            context.write(key, new Text(b.toString()));
        }
    }
    
    public static void main(String[] args) throws Exception {
        Configuration conf = new Configuration();
        Job job = Job.getInstance(conf);
        
        job.setJarByClass(FriendStepOne.class);
        
        job.setMapperClass(FriendStepOneMapper.class);
        job.setReducerClass(FriendStepOneRuducer.class);
        
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(Text.class);
        
        FileInputFormat.setInputPaths(job, new Path("D:\\測試數據\\輸入"));
        FileOutputFormat.setOutputPath(job, new Path("D:\\測試數據\\輸出"));
                
        boolean res = job.waitForCompletion(true);
        System.exit(res?0:1);
    
        
    }
}

結果：

第二步代碼：

package com.bpf.mr.friend;

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 FriendStepTwo {

    static class FriendStepOneMapper extends Mapper<LongWritable, Text, Text, Text>{
        @Override
        protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
            //A I,K,C,B,G,F,H,O,D
            String line = value.toString();
            String[] f_p = line.split("\t");
            String f = f_p[0];
            String[] p = f_p[1].split(",");
            Arrays.sort(p);
            for(int i = 0; i < p.length - 1; i ++) {
                for(int j = i + 1; j < p.length; j ++) {
                    context.write(new Text(p[i] + "--" + p[j]), new Text(f));
                }
            }
        }
    }
    
    static class FriendStepOneRuducer extends Reducer<Text, Text, Text, Text>{
        @Override
        protected void reduce(Text key, Iterable<Text> friends, Context context) throws IOException, InterruptedException {
            StringBuffer b = new StringBuffer();
            for (Text friend : friends) {
                b.append(friend.toString() + ",");
            }
            
            context.write(key, new Text(b.toString()));
        }
    }
    
    public static void main(String[] args) throws Exception {
        Configuration conf = new Configuration();
        Job job = Job.getInstance(conf);
        
        job.setJarByClass(FriendStepTwo.class);
        
        job.setMapperClass(FriendStepOneMapper.class);
        job.setReducerClass(FriendStepOneRuducer.class);
        
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(Text.class);
        
        FileInputFormat.setInputPaths(job, new Path("D:\\測試數據\\輸出"));
        FileOutputFormat.setOutputPath(job, new Path("D:\\測試數據\\再輸出"));
                
        boolean res = job.waitForCompletion(true);
        System.exit(res?0:1);
    
        
    }
}

結果：