The Writable and WritableComparable interfaces

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 used as keys (K1, K2, K3) has a stricter requirement: in addition to Writable,it must also provide an implementation of the standard Java Comparable interface. This has the following specifications:

public interface Comparable
{
public int compareTO( Object obj) ;
}

The compare method returns -1, 0, or 1 depending on whether the compared object is less than, equal to, or greater than the current object.

As a convenience interface, Hadoop provides the WritableComparable interface in the org.apache.hadoop.io package.

public interface WritableComparable extends Writable, Comparable
{}

Introducing the wrapper classes

Fortunately, you don't have to start from scratch; as you've already seen, Hadoop provides classes that wrap the Java primitive types and implement WritableComparable. They are provided in the org.apache.hadoop.io package.