Question

I have written a MapReduce program, code is below:

import java.io.IOException;
import java.util.Iterator;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.*;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.conf.*;
import org.apache.hadoop.io.*;
import org.apache.hadoop.mapred.Reporter;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.input.TextInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.mapreduce.lib.output.TextOutputFormat;

public class MaxTemperature {

public class MaxTemperatureMapper extends Mapper <LongWritable, Text, Text, IntWritable>{
    private static final int MISSING = 9999;

    public void map(LongWritable key, Text Value, Context context, Reporter reporter) throws IOException, InterruptedException{
        String line = Value.toString();
        String year = line.substring(15,19);
        int airTemperature;
        if(line.charAt(87)=='+')
            airTemperature = Integer.parseInt(line.substring(88,92));
        else
            airTemperature = Integer.parseInt(line.substring(87,91));
        String quality = line.substring(92,93);
        if(airTemperature!=MISSING && quality.matches("[01459]"))
            context.write(new Text(year), new IntWritable(airTemperature));
    }
}

public class MaxTemperatureReducer extends Reducer<Text, IntWritable, Text, IntWritable>{
    public void reduce (Text Key, Iterator<IntWritable> Values, Context context, Reporter reporter) throws IOException, InterruptedException{

        int maxValue = Integer.MIN_VALUE;
        while(Values.hasNext())
            maxValue = Math.max(maxValue, Values.next().get());
        context.write(Key, new IntWritable(maxValue));

    }
}

public static void main(String[] args) throws Exception {
    if(args.length!=2){
        System.err.println("Usage: WeatherTemperature  <input path> <output path>");
        System.exit(-1);
    }
    Configuration conf = new Configuration();
    Job job = new Job(conf, "Maximum Temperature Calculator");
    job.setJarByClass(MaxTemperature.class);

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

    job.setMapperClass(MaxTemperatureMapper.class);
    job.setReducerClass(MaxTemperatureReducer.class);

//  job.setInputFormatClass(TextInputFormat.class);
    //job.setOutputFormatClass(TextOutputFormat.class);
    FileInputFormat.addInputPath(job, new Path(args[0]));
    FileOutputFormat.setOutputPath(job, new Path(args[1]));
    job.waitForCompletion(true);
    }
}

I run the .jar of this program using the following command:

hadoop jar weather.jar MaxTemperature input output

And I’m getting the following error:

12/06/13 00:52:05 INFO mapred.JobClient: Task Id :     attempt_201206121354_0007_m_000000_0, Status : FAILED
java.lang.RuntimeException: java.lang.NoSuchMethodException:      MaxTemperature$MaxTemperatureMapper.<init>()
    at org.apache.hadoop.util.ReflectionUtils.newInstance(ReflectionUtils.java:115)
    at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:602)
    at org.apache.hadoop.mapred.MapTask.run(MapTask.java:323)
    at org.apache.hadoop.mapred.Child$4.run(Child.java:270)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:396)
    at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1177)
    at org.apache.hadoop.mapred.Child.main(Child.java:264)
Caused by: java.lang.NoSuchMethodException: MaxTemperature$MaxTemperatureMapper.<init>()
    at java.lang.Class.getConstructor0(Class.java:2706)
    at java.lang.Class.getDeclaredConstructor(Class.java:1985)
    at org.apache.hadoop.util.ReflectionUtils.newInstance(ReflectionUtils.java:109)
    ... 7 more

What does this error mean? And what am I doing wrong, how do I rectify it? Thanks!

Answer 1

Your mapper and reducer classes need to be defined static, otherwise the compile creates a constructor with a single argument (the parent MaxTemperature class). Hence there now is not a default constructor.

public static class MaxTemperatureMapper extends Mapper<....

public static class MaxTemperatureReducer extends Reducer<....