Understanding LongWritable

Question

How I can understand LongWritable type? What is it? Can anybody link to a schema or other helpful page.

Answer 1

Hadoop needs to be able to serialise data in and out of Java types via DataInput and DataOutputobjects (IO Streams usually). The Writable classes do this by implementing two methods `write(DataOuput) and readFields(DataInput).

Specifically LongWritable is a Writable class that wraps a java long.

Most of the time (especially just starting out) you can mentally replace LongWritable -> Long i.e. it's just a number. If you get to defining your own datatypes you will start to become every familiar with implementing the writable interface:

Which looks some thing like:

public interface Writable {

       public void write(DataOutput out) throws IOException;

       public void readFields(DataInput in) throws IOException;
}