cascading.operation.aggregator
Class Count

java.lang.Object
  cascading.operation.BaseOperation<Long[]>
      cascading.operation.aggregator.Count

All Implemented Interfaces:

Aggregator<Long[]>, Operation<Long[]>, Serializable

public class Count
extends BaseOperation<Long[]>
implements Aggregator<Long[]>

Class Count is an Aggregator that calculates the number of items in the current group.

Note the resulting value for count is always a long. So any comparisons should be against a long value.

See Also:

Serialized Form

Field Summary

static String

FIELD_NAME Field COUNT

Fields inherited from class cascading.operation.BaseOperation

fieldDeclaration, numArgs, trace

Fields inherited from interface cascading.operation.Operation

ANY

Constructor Summary

Count()
Constructor Count creates a new Count instance using the defalt field declaration of name 'count'.

Count(Fields fieldDeclaration)
Constructor Count creates a new Count instance and returns a field with the given fieldDeclaration name.

Method Summary

void	aggregate(FlowProcess flowProcess, AggregatorCall<Long[]> aggregatorCall) Method aggregate is called for each TupleEntry value in the current grouping.
void	complete(FlowProcess flowProcess, AggregatorCall<Long[]> aggregatorCall) Method complete will be issued last after every TupleEntry has been passed to the Aggregator.aggregate(cascading.flow.FlowProcess, AggregatorCall) method.
protected Tuple	getResult(AggregatorCall<Long[]> aggregatorCall)
void	start(FlowProcess flowProcess, AggregatorCall<Long[]> aggregatorCall) Method start initializes the aggregation procedure and is called for every unique grouping.

Methods inherited from class cascading.operation.BaseOperation

cleanup, getFieldDeclaration, getNumArgs, getTrace, prepare, printOperationInternal, toString, toStringInternal

Methods inherited from class java.lang.Object

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

Methods inherited from interface cascading.operation.Operation

cleanup, getFieldDeclaration, getNumArgs, prepare

Field Detail

FIELD_NAME

public static final String FIELD_NAME

Field COUNT

See Also:

Constant Field Values

Constructor Detail

Count

public Count()

Constructor Count creates a new Count instance using the defalt field declaration of name 'count'.

Count

public Count(Fields fieldDeclaration)

Constructor Count creates a new Count instance and returns a field with the given fieldDeclaration name.

Parameters:

fieldDeclaration - of type Fields

Method Detail

start

public void start(FlowProcess flowProcess,
                  AggregatorCall<Long[]> aggregatorCall)

Description copied from interface: Aggregator

Method start initializes the aggregation procedure and is called for every unique grouping.

The AggregatorCall context should be initialized here if necessary.

The first time this method is called for a given 'process', the AggregatorCall context will be null. This method should set a new instance of the user defined context object. When the AggregatorCall context is not null, it is up to the developer to create a new instance, or 'recycle' the given instance. If recycled, it must be re-initialized to remove any previous state/values.

For example, if a Map is used to hold the intermediate data for each subsequent Aggregator.aggregate(cascading.flow.FlowProcess, AggregatorCall) call, new HashMap() should be set on the AggregatorCall instance when OperationCall.getContext() is null. On the next grouping, start() will be called again, but this time with the old Map instance. In this case, map.clear() should be invoked before returning.

Specified by:

start in interface Aggregator<Long[]>

Parameters:

flowProcess - of type FlowProcess

aggregatorCall - of type AggregatorCall

aggregate

public void aggregate(FlowProcess flowProcess,
                      AggregatorCall<Long[]> aggregatorCall)

Description copied from interface: Aggregator

Method aggregate is called for each TupleEntry value in the current grouping.

TupleEntry entry, or entry.getTuple() should not be stored directly in the context. A copy of the tuple should be made via the new Tuple( entry.getTuple() ) copy constructor.

Specified by:

aggregate in interface Aggregator<Long[]>

Parameters:

flowProcess - of type FlowProcess

aggregatorCall - of type AggregatorCall

complete

public void complete(FlowProcess flowProcess,
                     AggregatorCall<Long[]> aggregatorCall)

Description copied from interface: Aggregator

Method complete will be issued last after every TupleEntry has been passed to the Aggregator.aggregate(cascading.flow.FlowProcess, AggregatorCall) method. Any final calculation should be completed here and passed to the outputCollector.

Specified by:

complete in interface Aggregator<Long[]>

Parameters:

flowProcess - of type FlowProcess

aggregatorCall - of type AggregatorCall

getResult

protected Tuple getResult(AggregatorCall<Long[]> aggregatorCall)