Hadoop I/O - Hadoop: The Definitive Guide - page 132

ByteArrayOutputStream out = new ByteArrayOutputStream ();
DataOutputStream dataOut = new DataOutputStream ( out );
writable . write ( dataOut );
dataOut . close ();
return out . toByteArray ();
}
An integer is written using four bytes (as we see using JUnit 4 assertions):
byte [] bytes = serialize ( writable );
assertThat ( bytes . length , is ( 4 ));
The bytes are written in big-endian order (so the most significant byte is written to the
stream first, which is dictated by the java.io.DataOutput interface), and we can see
their hexadecimal representation by using a method on Hadoop's StringUtils :
assertThat ( StringUtils . byteToHexString ( bytes ), is ( "000000a3" ));
Let's try deserialization. Again, we create a helper method to read a Writable object
from a byte array:
public static byte [] deserialize ( Writable writable , byte [] bytes )
throws IOException {
ByteArrayInputStream in = new ByteArrayInputStream ( bytes );
DataInputStream dataIn = new DataInputStream ( in );
writable . readFields ( dataIn );
dataIn . close ();
return bytes ;
}
We construct a new, value-less IntWritable , and then call deserialize() to read
from the output data that we just wrote. Then we check that its value, retrieved using the
get() method, is the original value, 163:
IntWritable newWritable = new IntWritable ();
deserialize ( newWritable , bytes );
assertThat ( newWritable . get (), is ( 163 ));
WritableComparable and comparators
IntWritable implements the WritableComparable interface, which is just a
subinterface of the Writable and java.lang.Comparable interfaces:
package org . apache . hadoop . io ;
public interface WritableComparable < T > extends Writable ,