Java – ClassCastException: java.lang.Exception: java.lang.ClassCastException in mapred

I’m writing a mapreduce application that accepts input in (key, value) format and displays only the same data as the reducer output.

Here is the sample input:

1500s   1
1960s   1
Aldus   1

In the code below, I specify the input format using <<>> and the delimiter as a tab character in main(). When I run the code, I get the error message:

java.lang.Exception: java.lang.ClassCastException: org.apache.hadoop.io.Text cannot be cast to org.apache.hadoop.io.LongWritable
at org.apache.hadoop.mapred.LocalJobRunner$Job.runTasks(LocalJobRunner.java:462)
at org.apache.hadoop.mapred.LocalJobRunner$Job.run(LocalJobRunner.java:522)
Caused by: java.lang.ClassCastException: org.apache.hadoop.io.Text cannot be cast to org.apache.hadoop.io.LongWritable
at cscie63.examples.WordDesc$KVMapper.map(WordDesc.java:1)
at org.apache.hadoop.mapreduce.Mapper.run(Mapper.java:145)
at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:787)
at org.apache.hadoop.mapred.MapTask.run(MapTask.java:341)
at org.apache.hadoop.mapred.LocalJobRunner$Job$MapTaskRunnable.run(LocalJobRunner.java:243)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)

Tried different debugging methods but to no avail.

public class WordDesc {

public static class KVMapper 
        extends Mapper<Text, LongWritable, Text, LongWritable>{
    private final static IntWritable one = new IntWritable(1);
    private Text word = new Text();
    public void map(Text key, LongWritable value , Context context
                    ) throws IOException, InterruptedException {
      context.write(key,value);
      }
    }

public static class KVReducer 
       extends Reducer<Text,LongWritable,Text,LongWritable> {

private IntWritable result = new IntWritable();
    public void reduce(Text key, LongWritable value, 
                       Context context
                       ) throws IOException, InterruptedException {
      context.write(key, value);
    }
  }

public static void main(String[] args) throws Exception {
    Configuration conf = new Configuration();
    conf.set("mapreduce.input.keyvaluelinerecordreader.key.value.separator", "\t");
            String[] otherArgs = new GenericOptionsParser(conf, args).getRemainingArgs();
    if (otherArgs.length < 2) {
      System.err.println("Usage: wordcount <in> [<in>...] <out>");
      System.exit(2);
    }
    Job job = new Job(conf, "word desc");
    job.setInputFormatClass(KeyValueTextInputFormat.class);
    job.setJarByClass(WordDesc.class);
    job.setMapperClass(KVMapper.class);
    job.setCombinerClass(KVReducer.class);
    job.setReducerClass(KVReducer.class);
    job.setOutputKeyClass(Text.class);
    job.setOutputValueClass(LongWritable.class);
    for (int i = 0; i < otherArgs.length - 1; ++i) {
      FileInputFormat.addInputPath(job, new Path(otherArgs[i]));
    }
    FileOutputFormat.setOutputPath(job,
      new Path(otherArgs[otherArgs.length - 1]));
    System.exit(job.waitForCompletion(true) ? 0 : 1);
  }
}

I guess this line job.setInputFormatClass(KeyValueTextInputFormat.class); The program that tells you treats your input as Text. Therefore, you receive this exception when you ask that the input value be LongWritable.

A quick fix is to read your input as text, and then, if you want to use LongWritable, parse it using the following method:

public static class KVMapper 
    extends Mapper<Text, Text, Text, LongWritable>{
    private final static LongWritable val = new LongWritable();
    public void map(Text key, Text value, Context context) {
      val.set(Long.parseLong(value.toString()));
      context.write(key,val);
    }
}

Here’s what it does: value is Text, then value.toString() gives the String representation of the Text, and then Long.parseLong() parses the string as long. Finally, val.set() converts it to LongWritable.

By the way, I don’t think you need to use Reducer for this… You can make the Reduce task faster by setting the number of reduce tasks to 0.