MR找共同朋友，数据格式如下：
A B C D E F
B A C D E
C A B E
D A B E
E A B C D
F A
第一字母表示本人，其他是他的朋友，找出有共同朋友的人，和共同朋友是谁
1 import java.io.IOException; 2 import java.util.Set; 3 import java.util.StringTokenizer; 4 import java.util.TreeSet; 5 6 import org.apache.hadoop.conf.Configuration; 7 import org.apache.hadoop.fs.Path; 8 import org.apache.hadoop.io.Text; 9 import org.apache.hadoop.mapreduce.Job; 10 import org.apache.hadoop.mapreduce.Mapper; 11 import org.apache.hadoop.mapreduce.Reducer; 12 import org.apache.hadoop.mapreduce.Mapper.Context; 13 import org.apache.hadoop.mapreduce.lib.input.FileInputFormat; 14 import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat; 15 import org.apache.hadoop.util.GenericOptionsParser; 16 17 public class FindFriend { 18 19 public static class ChangeMapper extends Mapper<Object, Text, Text, Text>{ 20 @Override 21 public void map(Object key, Text value, Context context) throws IOException, InterruptedException { 22 StringTokenizer itr = new StringTokenizer(value.toString()); 23 Text owner = new Text(); 24 Set<String> set = new TreeSet<String>(); 25 owner.set(itr.nextToken()); 26 while (itr.hasMoreTokens()) { 27 set.add(itr.nextToken()); 28 } 29 String[] friends = new String[set.size()]; 30 friends = set.toArray(friends); 31 32 for(int i=0;i<friends.length;i++){ 33 for(int j=i+1;j<friends.length;j++){ 34 String outputkey = friends[i]+friends[j]; 35 context.write(new Text(outputkey),owner); 36 } 37 } 38 } 39 } 40 41 public static class FindReducer extends Reducer<Text,Text,Text,Text> { 42 public void reduce(Text key, Iterable<Text> values, 43 Context context) throws IOException, InterruptedException { 44 String commonfriends =""; 45 for (Text val : values) { 46 if(commonfriends == ""){ 47 commonfriends = val.toString(); 48 }else{ 49 commonfriends = commonfriends+":"+val.toString(); 50 } 51 } 52 context.write(key, new Text(commonfriends)); 53 } 54 } 55 56 57 public static void main(String[] args) throws IOException, 58 InterruptedException, ClassNotFoundException { 59 60 Configuration conf = new Configuration(); 61 String[] otherArgs = new GenericOptionsParser(conf, args).getRemainingArgs(); 62 if (otherArgs.length < 2) { 63 System.err.println("args error"); 64 System.exit(2); 65 } 66 Job job = new Job(conf, "word count"); 67 job.setJarByClass(FindFriend.class); 68 job.setMapperClass(ChangeMapper.class); 69 job.setCombinerClass(FindReducer.class); 70 job.setReducerClass(FindReducer.class); 71 job.setOutputKeyClass(Text.class); 72 job.setOutputValueClass(Text.class); 73 for (int i = 0; i < otherArgs.length - 1; ++i) { 74 FileInputFormat.addInputPath(job, new Path(otherArgs[i])); 75 } 76 FileOutputFormat.setOutputPath(job, 77 new Path(otherArgs[otherArgs.length - 1])); 78 System.exit(job.waitForCompletion(true) ? 0 : 1); 79 80 } 81 82 }
运行结果：
AB E:C:D
AC E:B
AD B:E
AE C:B:D
BC A:E
BD A:E
BE C:D:A
BF A
CD E:A:B
CE A:B
CF A
DE B:A
DF A
EF A
转自：http://www.aboutyun.com/thread-11107-1-1.html