Up to this point we've glossed over the actual data types used as the input and output of the map and reduce classes. Let's take a look at them now.
If you browse the Hadoop API for the org.apache.hadoop.io package, you'll see some familiar classes such as Text and IntWritable along with others with the Writable suffix.
This package also contains the Writable interface specified as follows:
import java.io.DataInput ;
import java.io.DataOutput ;
import java.io.IOException ;
public interface Writable
{
void write(DataOutput out) throws IOException ;
void readFields(DataInput in) throws IOException ;
}The main purpose of this interface is to provide mechanisms for the serialization and deserialization of data as it is passed across the network or read and written from the disk. Every data type to be used as a value input or output from a mapper or reducer (that is, V1, V2, or V3) must implement this interface.
Data to be...