= map 的 key 為 LongWritable ，然而此key轉型為Text會遇到些run-time-error =

 原因：Type mismatch in key from map: expected org.apache.hadoop.io.LongWritable, recieved org.apache.hadoop.io.Text 

 Map 的 key 從 longWritable 強制轉型到 String，似乎會遇到一些錯

 * keyvalue.java

{{{
#!java
package nchc.keyvalue;

import org.apache.hadoop.fs.Path;
import org.apache.hadoop.mapred.FileInputFormat;
import org.apache.hadoop.mapred.FileOutputFormat;
import org.apache.hadoop.mapred.JobClient;
import org.apache.hadoop.mapred.JobConf;

public class keyvalue{
	public static void main(String[] args) {
		String[] argv = {"input","output","1","1"};
		args = argv;
		
		if (args.length < 4) {
			System.out.println("keyvalue <inDir> <outDir> <m> <r>");
			return;
		}
		
		JobConf conf = new JobConf(keyvalue.class);
		conf.setJobName("keyValue");
		FileInputFormat.setInputPaths(conf, args[0]);
		FileOutputFormat.setOutputPath(conf, new Path(args[1]));
		conf.setNumMapTasks(Integer.parseInt(args[2]));
		conf.setNumReduceTasks(Integer.parseInt(args[3]));
		conf.setMapperClass(kvM.class);
		conf.setReducerClass(kvR.class);

		long start = System.nanoTime();
		try {
			JobClient.runJob(conf);
		} catch (Exception e) {
			e.printStackTrace();
		}
		long period = System.nanoTime() - start;
		System.err.println(period*(1e-9) + " secs.");
	}
}

}}}

 * kvm.java (有runtime error)

{{{

package nchc.keyvalue;

import java.io.IOException;

import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapred.MapReduceBase;
import org.apache.hadoop.mapred.Mapper;
import org.apache.hadoop.mapred.OutputCollector;
import org.apache.hadoop.mapred.Reporter;

public class kvM extends MapReduceBase implements
		Mapper<LongWritable, Text, Text, Text> {

	public void map(LongWritable key, Text value,
			OutputCollector<Text, Text> output, Reporter report)
			throws IOException {
		Text keyv = new Text(key.toString());
		output.collect(keyv, value);
	}

}
}}}

 * kvr.java (有runtime error)

{{{

package nchc.keyvalue;

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

import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapred.MapReduceBase;
import org.apache.hadoop.mapred.OutputCollector;
import org.apache.hadoop.mapred.Reducer;
import org.apache.hadoop.mapred.Reporter;

public class kvR extends MapReduceBase implements
		Reducer< Text, Text, Text, Text> {
	public void reduce(Text key, Iterator<Text> values,
			OutputCollector<Text, Text> output, Reporter report)
			throws IOException {
		while (values.hasNext()) {
			Text keyv = new Text("< "+key+" , ");
			Text val = new Text(values.next()+">");
			output.collect(keyv, val);
		}
	}
}
}}}

----------
改成以下的map 與 reduce 檔就可以正常運作

 * kvM.java

{{{
#!java
package nchc.keyvalue;

import java.io.IOException;

import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapred.MapReduceBase;
import org.apache.hadoop.mapred.Mapper;
import org.apache.hadoop.mapred.OutputCollector;
import org.apache.hadoop.mapred.Reporter;

public class kvM extends MapReduceBase implements
		Mapper<LongWritable, Text, LongWritable, Text> {

	public void map(LongWritable key, Text value,
			OutputCollector<LongWritable, Text> output, Reporter report)
			throws IOException {
		output.collect(key, value);
	}

}
}}}

 * kvR.java

{{{
#!java
package nchc.keyvalue;

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

import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapred.MapReduceBase;
import org.apache.hadoop.mapred.OutputCollector;
import org.apache.hadoop.mapred.Reducer;
import org.apache.hadoop.mapred.Reporter;

public class kvR extends MapReduceBase implements
		Reducer< LongWritable, Text, Text, Text> {
	public void reduce(LongWritable key, Iterator<Text> values,
			OutputCollector<Text, Text> output, Reporter report)
			throws IOException {
		while (values.hasNext()) {
			Text keyv = new Text("< "+key+" , ");
			Text val = new Text(values.next()+">");
			output.collect(keyv, val);
		}
	}
}
}}}

然而就看不到key被reduce起來

 * output/part-00000
{{{
< 0 , 	This eBook is for the use of anyone anywhere at no cost and with>
< 0 , 	This eBook is for the use of anyone anywhere at no cost and with>
< 66 , 	almost no restrictions whatsoever.  You may copy it, give it away or>
< 66 , 	almost no restrictions whatsoever.  You may copy it, give it away or>
< 136 , 	re-use it under the terms of the Project Gutenberg License included>
< 136 , 	re-use it under the terms of the Project Gutenberg License included>
< 205 , 	with this eBook or online at www.gutenberg.net>
< 205 , 	with this eBook or online at www.gutenberg.org>

.... (省略)
}}}