Hive - Hadoop: The Definitive Guide - page 617

public static class MaximumIntUDAFEvaluator implements UDAFEvaluator {
private IntWritable result ;
public void init () {
result = null ;
}
public boolean iterate ( IntWritable value ) {
if ( value == null ) {
return true ;
}
if ( result == null ) {
result = new IntWritable ( value . get ());
} else {
result . set ( Math . max ( result . get (), value . get ()));
}
return true ;
}
public IntWritable terminatePartial () {
return result ;
}
public boolean merge ( IntWritable other ) {
return iterate ( other );
}
public IntWritable terminate () {
return result ;
}
}
}
The class structure is slightly different from the one for UDFs. A UDAF must be a sub-
class of org.apache.hadoop.hive.ql.exec.UDAF (note the “A” in UDAF) and
contain one or more nested static classes implementing
org.apache.hadoop.hive.ql.exec.UDAFEvaluator . In this example, there
is a single nested class, MaximumIntUDAFEvaluator , but we could add more evalu-
ators, such as MaximumLongUDAFEvaluator , MaximumFloatUDAFEvaluator ,
and so on, to provide overloaded forms of the UDAF for finding the maximum of a col-
lection of longs, floats, and so on.