數據：

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,K

求哪些人兩兩之間是互粉好友，形如：A的好友有B，B的好友有A 。 那麽A和B就是互粉好友。

思路：

對每一行數據進行組合輸出 （person-person，1），

然後再Reducer階段進行統計，等於2的就是互粉好友對；

問題：將B-A轉換成A-B這種形式；

方案：比較兩個字符之間的大小，小的在前；

import java.io.IOException;
 
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
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;
 
 
public class EachOtherFriend {
 
    public static class MyMapper extends Mapper<LongWritable, Text, Text, IntWritable>{
        @Override
        protected void map(LongWritable key, Text value,Context context) throws IOException, InterruptedException {
            String[] lines = value.toString().split("
 
:");
            char person = lines[0].charAt(0); 
            for (String str : lines[1].split(",")) {
                char friend = str.charAt(0);
                String per_per = "";
                if(person > friend){
                    per_per += friend+"-"+person;
                }else{
                    per_per 
 
+= person+"-"+friend;
                }
                context.write(new Text(per_per), new IntWritable(1));
                
            }
        }
    }
    
    
    public static class MyReducer extends Reducer<Text, IntWritable, Text, NullWritable>{
        @Override
        protected void reduce(Text key, Iterable<IntWritable> values,Context context)
                throws IOException, InterruptedException {
            
            int num = 0;
            for (IntWritable it : values) {
                num++;
            }
            if(num > 1){
                context.write(key, null);
            }
        }
    }
    
    public static void main(String[] args) throws Exception {
        Configuration conf = new Configuration();
        
        Job job = Job.getInstance(conf);
        
        job.setJarByClass(EachOtherFriend.class);
        job.setMapperClass(MyMapper.class);
        job.setReducerClass(MyReducer.class);
        
        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(IntWritable.class);
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(NullWritable.class);
        
        FileInputFormat.setInputPaths(job, new Path("G:/files/input"));
        FileOutputFormat.setOutputPath(job, new Path("G:/files/output"));
        
        boolean isDone = job.waitForCompletion(true);
        System.exit(isDone ? 0:1);
        
    }
}

MapReduce--求哪些人兩兩之間是互粉好友