AnalyticFunction Interface
To accomplish the analytic functionality desired, jAgg uses a cache of objects
that implement the
AnalyticAggregator class to
perform the actual analytic calculations. These AnalyticAggregators wrap objects
that implement the AnalyticFunction interface. These objects are
analogous to the Oracle object types that can be created for custom aggregation
functions. The AnalyticFunction interface does the following:
- Defines the interface that the AnalyticAggregator and Analytic classes expect.
- Extends the AggregateFunction
interface.
- Calculates an analytic value based on a sliding window of values in the
original List of Objects, for every value in that List of Objects.
Implementing classes implement all of the methods from the AggregateFunction
interface. They also implement the method delete to remove a
value from the window that was previously added using iterate.
AnalyticFunctions aren't responsible for maintaining or using the
partition, order by, or window clauses. The
AnalyticAggregator class handles them. However, certain analytic functions can
specify that they don't take a user-given window clause. With the
takesWindowClause method, AnalyticFunctions can specify
whether they accept a user-given window clause. If they don't, then they can
specify their own window clause with the getWindowClause method.
Typically, AnalyticFunction is implemented in one of 3 ways.
- Aggregators that can easily supply a delete method
will simply implement AnalyticFunction and be done.
- A few Aggregators that need more complicated processing to
implement delete may wish to have a separate class that
subclasses the original Aggregator that implements the
AnalyticFunction interface.
MaxAnalyticAggregator and
MinAnalyticAggregator take this route. By convention, the names of the
classes that take this route end in "AnalyticAggregator".
- Certain AnalyticFunctions are analytic-only; they make no sense in
an aggregate function context. Such analytic functions usually inherit
directly from Aggregator and implement AnalyticFunction
directly. By convention, the names of the classes that take this route end in
"Analytic".
Methods
The following methods are declared by the AnalyticFunction interface.
The example followed in the explanations for each method below are for a function that
computes an average of property values.
- All AggregateFunction
methods are implemented.
- void delete(Object value) - Extract the property from the value
and remove the property from the function's state. Example: subtract the property
from the running sum and decrement the running count.
- boolean takesWindowClause() - Determines whether this
AnalyticFunction takes a user-given window clause or supplies its own.
Example: It can take a user-given window clause, so this method returns
true.
- WindowClause getWindowClause() - If this AnalyticFunction
doesn't accept a user-given window clause, then this method supplies its own window
clause. Example: It accepts a user-given window clause, so this method returns
null.
