org.apache.hadoop.io.Writable is a Java interface. Any key or
value type in the Hadoop Map-Reduce framework implements this interface.
Implementations typically implement a static read(DataInput) method
which constructs a new instance, calls readFields(DataInput) and returns
org.apache.hadoop.io.WritableComparable is a Java interface. Any
type which is to be used as a key in the Hadoop Map-Reduce framework
should implement this interface. WritableComparable objects can be
compared to each other using Comparators.
answered By  0
WritableComparator is a general-purpose implementation of RawComparator for WritableComparable classes. It provides two main functions. First, it provides a default implementation of the raw compare() method that deserializes the objects to be compared from the stream and invokes the object compare() method. Second, it acts as a factory for RawComparator instances (that Writable implementations have registered).