Hadoop I/O - Hadoop: The Definitive Guide - page 134

b2 . length ),
greaterThan ( 0 ));

Writable Classes

Hadoop comes with a large selection of Writable classes, which are available in the
org.apache.hadoop.io package. They form the class hierarchy shown in Fig-
ure 5-1 .

Writable wrappers for Java primitives

There are Writable wrappers for all the Java primitive types (see Table 5-7 ) except
char (which can be stored in an IntWritable ). All have a get() and set() meth-
od for retrieving and storing the wrapped value.

Table 5-7. Writable wrapper classes for Java primitives

Java primitive Writable implementation Serialized size (bytes)

1

boolean BooleanWritable

1

byte ByteWritable

2

short ShortWritable

4

int IntWritable

1-5

VIntWritable

4 float FloatWritable

8 long LongWritable

1-9

VLongWritable

8

double DoubleWritable

When it comes to encoding integers, there is a choice between the fixed-length formats
( IntWritable and LongWritable ) and the variable-length formats ( VIntWrit-
able and VLongWritable ). The variable-length formats use only a single byte to en-
code the value if it is small enough (between -112 and 127, inclusive); otherwise, they use
the first byte to indicate whether the value is positive or negative, and how many bytes
follow. For example, 163 requires two bytes:

byte [] data = serialize ( new VIntWritable ( 163 ));
assertThat ( StringUtils . byteToHexString ( data ), is ( "8fa3" ));