Hadoop Throws ClassCastException for the keytype of java.nio.ByteBuffer
This is where the exception is being thrown. Here is the code from SVN.
public RawComparator getOutputKeyComparator() { Class<? extends RawComparator> theClass = getClass("mapred.output.key.comparator.class", null, RawComparator.class); if (theClass != null) return ReflectionUtils.newInstance(theClass, this); return WritableComparator.get(getMapOutputKeyClass().asSubclass(WritableComparable.class));}If mapred.output.key.comparator.class property is not defined on JobConf then the key must implement the WritableComparable interface. ByteBuffer class doesn't implement the WritableComparable interface, so the exception.
BTW, WritableComparable interface is sub-interface of Writable and Comparable classes.