Question

My Map class is

 public static class MapClass extends Mapper<LongWritable, Text, Text, LongWritable> {

        public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
            // your map code goes here
            String[] fields = value.toString().split(",");
            String year = fields[1];
            String claims = fields[8];

            if (claims.length() > 0 && (!claims.startsWith("\""))) {
                context.write(new Text(year), new LongWritable(Long.parseLong(claims)));
            }
        }
    }

My Reduce class Looks like

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


        public void reduce(Text key, Iterable<LongWritable> values, Context context) throws IOException, InterruptedException {
            // your reduce function goes here
            context.write(key, new Text("hello"));
        }
    }

The dataset looks like

3070801,1963,1096,,"BE","",,1,,269,6,69,,1,,0,,,,,,,
3070802,1963,1096,,"US","TX",,1,,2,6,63,,0,,,,,,,,, 

When I run program with configuration

        Job job = new Job();
        job.setJarByClass(TopKRecords.class);

        job.setMapperClass(MapClass.class);
        job.setReducerClass(Reduce.class);

        FileInputFormat.setInputPaths(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));

        job.setJobName("TopKRecords");
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(Text.class);

I see error as

java.io.IOException: Type mismatch in value from map: expected org.apache.hadoop.io.Text, recieved org.apache.hadoop.io.LongWritable
    at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.collect(MapTask.java:1019)
    at org.apache.hadoop.mapred.MapTask$NewOutputCollector.write(MapTask.java:691)
    at org.apache.hadoop.mapreduce.TaskInputOutputContext.write(TaskInputOutputContext.java:80)
    at com.hadoop.programs.TopKRecords$MapClass.map(TopKRecords.java:35)
    at com.hadoop.programs.TopKRecords$MapClass.map(TopKRecords.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)

What is wrong going here?

I dont see any reason for mismatch here

Mapper<LongWritable, Text, Text, LongWritable>
Reducer<Text, LongWritable, Text, Text>

UPDATE

After setting the following, things started to work

        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(LongWritable.class);

Answer 1

You also need the following line during your setup:

job.setMapOutputValueClass(LongWritable.class);

From the Hadoop 20.2 Javadoc:

This allows the user to specify the map output value class to be
different than the final output value class.

For clarity, you could also add:

job.setMapOutputKeyClass(Text.class);

but it is not necessary in this case.