AggregateFunction Interface

To accomplish the aggregate functionality desired, jAgg uses a cache of objects that implement the AggregateFunction interface to perform the actual aggregation calculations. These objects are analogous to the Oracle object types that can be created for custom aggregation functions. The AggregateFunction interface does the following:

  1. Defines the interface that the Aggregation class expects.
  2. Calculates an aggregate value based on all values in the original List of Objects that have equivalent “group by” properties.

Implementing classes implement the methods init, iterate, merge, and terminate that do the actual aggregation. They also implement the method replicate to return a new AggregateFunction of the same type, but uninitialized.

Typically, AggregateFunction is not implemented directly. The abstract class Aggregator implements this interface and and supplies common functionality. All jAgg built-in aggregators subclass Aggregator.


The following methods are declared by the AggregateFunction interface. The example followed in the explanations for each method below are for a function that computes an average of property values.

  • void init() - Initialize the state of this AggregateFunction. Example: reset an internal count and an internal sum to 0.
  • void iterate(Object value) - Extract the property from the value and add the property to the function's state. Example: add the property to the running sum and increment the running count.
  • void merge(AggregateFunction func) - Merge the state of another AggregateFunction into this one. Example: Add the other function's sum and count to the internal sum and count, respectively.
  • Object terminate() - Return the result, based on the internal state accumulated so far. Example: Divide the internal sum by the internal count, returning a Double.
  • AggregateFunction replicate() - Return an uninitialized copy of this aggregate function.
  • There are a couple more methods to implement that are typically provided by the abstract implementing class Aggregator.