simpledb.materialize
Class CountFn

java.lang.Object
  simpledb.materialize.CountFn

All Implemented Interfaces:

AggregationFn

public class CountFn
extends Object
implements AggregationFn

The count aggregation function.

Author:

Edward Sciore

Constructor Summary

CountFn(String fldname)
Creates a count aggregation function for the specified field.

Method Summary

String	fieldName() Returns the field's name, prepended by "countof".
void	processFirst(Scan s) Starts a new count.
void	processNext(Scan s) Since SimpleDB does not support null values, this method always increments the count, regardless of the field.
Constant	value() Returns the current count.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

CountFn

public CountFn(String fldname)

Creates a count aggregation function for the specified field.

Parameters:

fldname - the name of the aggregated field

Method Detail

processFirst

public void processFirst(Scan s)

Starts a new count. Since SimpleDB does not support null values, every record will be counted, regardless of the field. The current count is thus set to 1.

Specified by:

processFirst in interface AggregationFn

Parameters:

s - the scan to aggregate over.

See Also:

AggregationFn.processFirst(simpledb.query.Scan)

processNext

public void processNext(Scan s)

Since SimpleDB does not support null values, this method always increments the count, regardless of the field.

Specified by:

processNext in interface AggregationFn

Parameters:

s - the scan to aggregate over.

See Also:

AggregationFn.processNext(simpledb.query.Scan)

fieldName

public String fieldName()

Returns the field's name, prepended by "countof".

Specified by:

fieldName in interface AggregationFn

Returns:

the name of the new aggregation field

See Also:

AggregationFn.fieldName()

value

public Constant value()

Returns the current count.

Specified by:

value in interface AggregationFn

Returns:

the computed aggregation value

See Also:

AggregationFn.value()