需求：
现有一份QQ好友数据，其中数据第一列代表用户QQ昵称，第二列代表好友QQ昵称。要求编写MapReduce程序实现QQ好友推荐，例如A的好友是B，B的好友是C，即A与C有共同的好友B，则可以向A推荐C，向C推荐A。通过MapReduce完成推荐后，会发现存在给A推B，也给B推A的重复情况，请继续完成处理，避免这个重复操作。

数据：
A,B
A,C
A,E
B,D
B,A
B,F
C,E
C,A

共同好友依次为A，B，C

即要求运行结果为：A -> C,A -> D…

代码如下：

import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;

import java.io.IOException;


public class QqFriendsMapper extends Mapper<LongWritable, Text,Text,Text> {
    private String spliter="";

    @Override
    protected void setup(Mapper<LongWritable, Text, Text, Text>.Context context) throws IOException, InterruptedException {
        spliter = context.getConfiguration().get(spliter);
    }

    @Override
    protected void map(LongWritable key, Text value, Mapper<LongWritable, Text, Text, Text>.Context context) throws IOException, InterruptedException {
        String line = value.toString();
        String[] split = line.split(",");
        context.write(new Text(split[0]),new Text(split[1]));
        context.write(new Text(split[1]),new Text(split[0]));
    }
}

import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Reducer;

import java.io.IOException;
import java.util.HashSet;
import java.util.Iterator;


public class QqFriendsReducer extends Reducer<Text, Text, Text, Text> {
    private Text outkey = new Text();
    private Text outvalue = new Text();

    @Override
    protected void reduce(Text key, Iterable<Text> values, Reducer<Text, Text, Text, Text>.Context context) throws IOException, InterruptedException {
        HashSet<String> set = new HashSet<>();
        for (Text value : values) {
            set.add(value.toString());
        }
        Iterator<String> i = set.iterator();
        Iterator<String> j = set.iterator();
        if (set.size() > 1) {
            while (i.hasNext()) {
                String qqname = i.next();
                while (j.hasNext()) {
                    String otherqqname = j.next();
                    if (!qqname.equals(otherqqname)) {
                        outkey.set(qqname);
                        outvalue.set(otherqqname);
                        context.write(outkey, outvalue);

                    }
                }
            }
        }
    }
}

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

import java.io.IOException;


public class QqFriendsDriver {
    public static void main(String[] args) throws IOException, InterruptedException, ClassNotFoundException {
        Configuration conf = new Configuration();
        Job job = Job.getInstance(conf);

        job.setJarByClass(QqFriendsDriver.class);

        job.setMapperClass(QqFriendsMapper.class);
        job.setReducerClass(QqFriendsReducer.class);

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

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

        FileInputFormat.setInputPaths(job, new Path("C:\\Users\\XR\\Desktop\\大数据开发\\hadoop\\作业\\qqfriends.txt"));
        FileOutputFormat.setOutputPath(job, new Path("C:\\Users\\XR\\Desktop\\大数据开发\\hadoop\\作业\\qqresult1"));

        boolean b = job.waitForCompletion(true);
        System.exit(b ? 0 : 1);

    }
}

输出·结果·为：

B C
B E
A D
A F
A E
A C