saffron
Interface AggregationExtender

All Known Implementing Classes:

LocaleMin, Median, Nth

public interface AggregationExtender

An AggregationExtender allows end-users to define their own aggregate functions.

An aggregation extender supports one or more sets of argument types, T. T can be a single type (such as String or int), several types (such as String, int), or empty. For each set T, it declares 4 methods:

• Object start(T dummy) creates an accumulator
• Object next(T dummy, Object accumulator) adds a value to an accumulator, and returns the accumulator
• Object merge(T dummy, Object accumulator0, Object accumulator1) merges the contents of accumulator1 into accumulator0, and returns the accumulator
• T2 result(T dummy, Object accumulator) retrieves the result from an accumulator

(The merge method is actually optional. If it is not present, ExtenderAggregation.canMerge() returns false.)

Consider the query

Locale locale = Locale.getDefault();
 Collator collator = Collator.getInstance(locale);
 LocaleMin localeMin = new LocaleMin(collator);
 select {localeMin.aggregate(emp.name)}
 group by {emp.deptno}
 from emps as emp

Here T is String, and so LocaleMin.start(String) will be called when a group starts, LocaleMin.next(String,Object) on each row, and so forth.

Note that the aggregation object is evaluated each time a group starts. If each department has its own locale, one could write

select {new LocaleMin(Collator.getInstance(emp.dept.locale))
                 .aggregate(emp.name) }
 group by {emp.dept}
 from emps as emp

Since:

26 January, 2001

Version:

$Id: //open/saffron/src/main/saffron/AggregationExtender.java#1 $

Author:

jhyde

Field Summary

static String	METHOD_AGGREGATE The name of the "aggregate" method.
static String	METHOD_MERGE The name of the "merge" method.
static String	METHOD_NEXT The name of the "next" method.
static String	METHOD_RESULT The name of the "result" method.
static String	METHOD_START The name of the "start" method.

Field Detail

METHOD_AGGREGATE

public static final String METHOD_AGGREGATE

The name of the "aggregate" method.

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Constant Field Values

METHOD_START

public static final String METHOD_START

The name of the "start" method.

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Constant Field Values

METHOD_NEXT

public static final String METHOD_NEXT

The name of the "next" method.

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Constant Field Values

METHOD_RESULT

public static final String METHOD_RESULT

The name of the "result" method.

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Constant Field Values

METHOD_MERGE

public static final String METHOD_MERGE

The name of the "merge" method.

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Constant Field Values