資料去重源資料：
2012-3-1 a
2012-3-2 b
2012-3-3 c
2012-3-4 d
2012-3-5 a
2012-3-6 b
2012-3-7 c
2012-3-3 c
2012-3-1 b
2012-3-2 a
2012-3-3 b
2012-3-4 d
2012-3-5 a
2012-3-6 c
2012-3-7 d
2012-3-3 c


最終結果：
2012-3-1 a
2012-3-1 b
2012-3-2 a
2012-3-2 b
2012-3-3 b
2012-3-3 c
2012-3-4 d
2012-3-5 a
2012-3-6 b
2012-3-6 c
2012-3-7 c
2012-3-7 d

思路：將兩列資料作為key值，在reduce階段，每次輸出一個，就可以達到去重的目的

/**
 * @author: lpj   
 * @date: 2018年3月16日 下午7:16:47
 * @Description:
 */
package lpj.reduceWork;

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
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 DistinctNumMR {
	
	public static void main(String[] args) throws Exception {
		Configuration conf = new Configuration();
//		conf.addResource("hdfs-site.xml");//使用配置檔案
//		System.setProperty("HADOOP_USER_NAME", "hadoop");//使用叢集
		FileSystem fs = FileSystem.get(conf);//預設使用本地
		
		Job job = Job.getInstance(conf);
		job.setJarByClass(DistinctNumMR.class);
		job.setMapperClass(DistinctNumMR_Mapper.class);
		job.setReducerClass(DistinctNumMR_Reducer.class);
		
		job.setMapOutputKeyClass(Text.class);
		job.setMapOutputValueClass(NullWritable.class);
		job.setOutputKeyClass(Text.class);
		job.setOutputValueClass(NullWritable.class);
//		
//		String inputpath = args[0];
//		String outpath = args[1];
		
		Path inputPath = new Path("d:/a/homework3.txt");
		Path outputPath = new Path("d:/a/homework3");
		if (fs.exists(inputPath)) {
			fs.delete(outputPath, true);
		}
		
		FileInputFormat.setInputPaths(job, inputPath);
		FileOutputFormat.setOutputPath(job, outputPath);
		boolean isdone = job.waitForCompletion(true);
		System.exit(isdone ? 0 : 1);
	}
	
	public static class DistinctNumMR_Mapper extends Mapper<LongWritable, Text, Text, NullWritable>{
		Text kout = new Text();
		Text valueout = new Text();
		@Override
		protected void map(LongWritable key, Text value,Context context)throws IOException, InterruptedException {
			//2012-3-3 c
			String [] reads = value.toString().trim().split(" ");
			String kk = reads[0] + "\t" + reads[1];
			kout.set(kk);
			context.write(kout, NullWritable.get());
			
		}
	}
	public static class DistinctNumMR_Reducer extends Reducer<Text, NullWritable,Text, NullWritable>{
		Text kout = new Text();
		Text valueout = new Text();
		@Override
		protected void reduce(Text key, Iterable<NullWritable> values, Context context)throws IOException, InterruptedException {
			context.write(key, NullWritable.get());
		}
		
	}

}
 

執行結果：

2012-3-1	a
2012-3-1	b
2012-3-2	a
2012-3-2	b
2012-3-3	b
2012-3-3	c
2012-3-4	d
2012-3-5	a
2012-3-6	b
2012-3-6	c
2012-3-7	c
2012-3-7	d