The instances of Hadoop MapReduce key types should have the ability to compare against each other for sorting purposes. In order to be used as a key type in a MapReduce computation, a Hadoop Writable data type should implement the org.apache.hadoop.io.WritableComparable<T> interface. The WritableComparable interface extends the org.apache.hadoop.io.Writable interface and adds the compareTo() method to perform the comparisons.
In this recipe, we modify the LogWritable data type of the Implementing a custom Hadoop Writable data type recipe to implement the WritableComparable interface.
The following are the steps to implement a custom Hadoop WritableComparable data type for the HTTP server log entries, which uses the request hostname and timestamp for comparison.
Modify the
LogWritableclass to implement theorg.apache.hadoop.io.WritableComparableinterface:public class LogWritable implements WritableComparable<LogWritable> {...