Hi Tariq On further analysis I noticed a odd behavior in this context.
If we use the default InputFormat (TextInputFormat) but specify the Key type in mapper as IntWritable instead of Long Writable. The framework is supposed throw a class cast exception.Such an exception is thrown only if the key types at class level and method level are the same (IntWritable) in Mapper. But if we provide the Input key type as IntWritable on the class level but LongWritable on the method level (map method), instead of throwing a compile time error, the code compliles fine . In addition to it on execution the framework triggers Identity Mapper instead of the custom mapper provided with the configuration. This seems like a bug to me . Filed a jira to track this issue https://issues.apache.org/jira/browse/MAPREDUCE-4507 Regards Bejoy KS
