Hadoop: LongWritable cannot be cast to org.apache.hadoop.io.IntWritable

Question

I want to take a mean value of a temperature given in an input file and my Mapper and Reducer synatax seems fine to me but I am still getting the following error:

 Unable to load realm info from SCDynamicStore
    13/02/17 08:03:28 INFO mapred.JobClient: Task Id : attempt_201302170552_0009_m_000000_1, Status : FAILED
    java.lang.ClassCastException: org.apache.hadoop.io.LongWritable cannot be cast to org.apache.hadoop.io.IntWritable
        at org.apache.hadoop.examples.TempMeasurement$TempMapper.map(TempMeasurement.java:26)
        at org.apache.hadoop.mapreduce.Mapper.run(Mapper.java:144)
        at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:764)
        at org.apache.hadoop.mapred.MapTask.run(MapTask.java:370)
        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)

My Mapper function is this:

public static class TempMapper extends Mapper<IntWritable, Text, IntWritable, FloatWritable>{

@Override
protected void map(IntWritable key, Text value, Context context)
                throws IOException, InterruptedException {

    //code for getting date and temperature

    String temp = columns.get(3);
    context.write(new IntWritable(year), new FloatWritable(Float.valueOf(temp)));
}
}

And Reduce is:

  public static class IntSumReducer
       extends Reducer<IntWritable, FloatWritable, IntWritable ,FloatWritable> {
    private FloatWritable result = new FloatWritable();

    public void reduce(IntWritable key, Iterable<FloatWritable> values,
                       Context context
                       ) throws IOException, InterruptedException {

      //code for making calculations    

      context.write(key, result);
    }
  }

Input file is as:

11111 , 0,19900101, 44.04 ,
11112, 0, 19900102, 50.00,
11113, 3, 19910203, 30.00,

Any help would be appreciated

Answer 1

The key class of a mapper that maps text files is always LongWritable. That is because it contains the byte offset of the current line and this could easily overflow an integer.

Basically you need to change your code to this:

public static class TempMapper extends Mapper<LongWritable, Text, IntWritable, FloatWritable>{

  @Override
  protected void map(LongWritable key, Text value, Context context)
                throws IOException, InterruptedException {
       //code for getting date and temperature
       String temp = columns.get(3);
       context.write(new IntWritable(year), new FloatWritable(Float.valueOf(temp)));
  }
}