mongodb - java.lang.ClassCastException even though I am casting it to the extended class -

i trying setup mongo-hadoop driver extensions hadoop streaming jobs have written python plugin dumbo.

the dumbo project needs me use typedbyteswritable class. made new inputformat & recordreader so:

package com.mongodb.hadoop;  public class typedbytestableinputformat implements inputformat<typedbyteswritable, typedbyteswritable> {  @override public recordreader<typedbyteswritable, typedbyteswritable> getrecordreader(inputsplit split,                                                                              jobconf job,                                                                             reporter reporter) {      if (!(split instanceof mongoinputsplit))         throw new illegalstateexception("creation of new recordreader requires mongoinputsplit instance.");      final mongoinputsplit mis = (mongoinputsplit) split;      //**the following line throws error**     return (recordreader<typedbyteswritable, typedbyteswritable>) new typedbytesmongorecordreader(mis); } 

and here extended recordreader:

package com.mongodb.hadoop.input; ... ... import org.apache.hadoop.mapreduce.recordreader; ... ...  public class typedbytesmongorecordreader extends recordreader<typedbyteswritable, typedbyteswritable> {  public typedbytesmongorecordreader(mongoinputsplit mis) {     _cursor = mis.getcursor(); }  @override public void close() {     if ( _cursor != null )         _cursor.close(); } 

but when run job, throws error. not sure why, child of recordreader. doing wrong? here api doc recordreader class. thought doing correctly:

http://hadoop.apache.org/common/docs/current/api/org/apache/hadoop/mapreduce/recordreader.html

i warning on line being cast recordreader, no errors, , builds jar fine. warning:

type safety: unchecked cast typedbytesmongorecordreader recordreader<typedbyteswritable,typedbyteswritable> 

try this:

public <t extends recordreader<typedbyteswritable, typedbyteswritable>> t getrecordreader(inputsplit split, jobconf job, reporter reporter) {      if (!(split instanceof mongoinputsplit))         throw new illegalstateexception("creation of new recordreader requires mongoinputsplit instance.");      final mongoinputsplit mis = (mongoinputsplit) split;      return new typedbytesmongorecordreader(mis); // may need cast (t) - try without first }