New to Hadoop MapReduce, getting a NoSuchMethodException on mapred.Reducer.<init>()
apache, hadoop, java, mapreduce, nosuchmethoderror
Solution
The problem is not your choice of API. Both the stable (mapred.*) and the evolving (mapreduce.*) APIs are fully supported and the framework itself carries tests for both to ensure no regressions/breakage across releases.
The problem is this line:
conf.setReducerClass(Reducer.class);
You're setting the Reducer interface there as the Reducer, when you should be setting your implementation of the Reducer interface instead. Changing it to:
conf.setReducerClass(Reduce.class);
Will fix it.
Problem
Solution: Use a better tutorial- http://hadoop.apache.org/mapreduce/docs/r0.22.0/mapred_tutorial.html I just started working with MapReduce, and I'm running into a weird bug that I haven't been able to answer through Google. I'm making a basic WordCount program, but when I run it, I get the following error during Reduce: ``` java.lang.RuntimeException: java.lang.NoSuchMethodException: org.apache.hadoop.mapred.Reducer.<init>() at org.apache.hadoop.util.ReflectionUtils.newInstance(ReflectionUtils.java:115) at org.apache.hadoop.mapred.ReduceTask.runOldReducer(ReduceTask.java:485) at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:420) at org.apache.hadoop.mapred.Child$4.run(Child.java:255) 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:1121) at org.apache.hadoop.mapred.Child.main(Child.java:249) ``` The WordCount program is the one from the Apache MapReduce tutorial. I'm running Hadoop 1.0.3 in pseudo-distributed mode on Mountain Lion, all of which I think is working fine since the examples are all executing normally. Any ideas? EDIT: Here's my code for reference: ``` package mrt; import java.io.IOException; import java.util.*; import org.apache.hadoop.fs.Path; import org.apache.hadoop.conf.*; import org.apache.hadoop.io.*; import org.apache.hadoop.mapred.*; import org.apache.hadoop.util.*; public class WordCount { public static class Map extends MapReduceBase implements Mapper<LongWritable, Text, Text, IntWritable> { private final static IntWritable one = new IntWritable(1); private Text word = new Text(); public void map(LongWritable key, Text value, OutputCollector<Text,IntWritable> output, Reporter reporter) throws IOException{ String line = value.toString(); StringTokenizer tokenizer = new StringTokenizer(line); while(tokenizer.hasMoreTokens()){ word.set(tokenizer.nextToken()); output.collect(word,one); } } } public static class Reduce extends MapReduceBase implements Reducer<Text, IntWritable, Text, IntWritable> { public void reduce(Text key, Iterator<IntWritable> values, OutputCollector<Text, IntWritable> output, Reporter reporter) throws IOException{ int sum = 0; while(values.hasNext()){ sum += values.next().get(); } output.collect(key, new IntWritable(sum)); } } public static void main(String[] args) throws Exception{ JobConf conf = new JobConf(WordCount.class); conf.setJobName("Wordcount"); conf.setOutputKeyClass(Text.class); conf.setOutputValueClass(IntWritable.class); conf.setMapperClass(Map.class); conf.setCombinerClass(Reduce.class); conf.setReducerClass(Reducer.class); conf.setInputFormat(TextInputFormat.class); conf.setOutputFormat(TextOutputFormat.class); FileInputFormat.setInputPaths(conf, new Path(args[0])); FileOutputFormat.setOutputPath(conf, new Path(args[1])); JobClient.runJob(conf); } } ```