The “Aggregations” utility class is a convenience entry point for a developer attempting to present a view of raw data in an application. It has six public, static, overloaded methods called “groupBy”. All methods take a List of Ts as a parameter. All of these methods create an Aggregation object using the Builder, then defer to that Aggregation object for aggregate calculation. If super-aggregates such as rollups or cubes are desired, then one must use the Aggregation object (and Builder) directly.
public static <T extends Comparable<? super T>> List<AggregateValue<T>>
groupBy(List<T> values, List<Aggregator> aggregators)
public static <T extends Comparable<? super T>> List<AggregateValue<T>>
groupBy(List<T> values, List<Aggregator> aggregators, int parallelism)
public static <T> List<AggregateValue<T>>
groupBy(List<T> values, List<String> properties, List<Aggregator> aggregators)
public static <T> List<AggregateValue<T>>
groupBy(List<T> values, List<String> properties, List<Aggregator> aggregators, boolean useMsd)
public static <T> List<AggregateValue<T>>
groupBy(List<T> values, List<String> properties, List<Aggregator> aggregators, int parallelism)
public static <T> List<AggregateValue<T>>
groupBy(List<T> values, List<String> properties, List<Aggregator> aggregators, int parallelism, boolean useMsd)
The first two methods operate on a List of Comparables. This will group together all Objects that compare equal using that Object's "compareTo" method.
The last four methods operate on any List of Objects. The extra parameter "properties" is a List of property Strings.
The developer has the option of specifying a degree of parallelism to speed up the processing by taking advantage of multiple processors that may be available on the host. Because of the expected CPU-intensive nature of the required calculations, jAgg won’t spawn more Threads than there are processors on the host.
On the "groupBy" methods that take a List of property Strings, the developer has the option using Multiset Discrimination instead of sorting to group items with identical properties.This option defaults to false, i.e. always sort the items. If for any reason Multiset Discrimination fails, the it falls back to sorting.
Each “groupBy” method returns a List of AggregateValue objects. The AggregateValue class, parameterized with type T, contains an object of type T and maintains a HashMap of aggregate values associated with each object of type T.
The developer has the choice of “group by” properties to use. Here, “properties” is used in a bean-like context, in that property “xyz” corresponds to a “getXyz” method (or an "isXyz" method) on an object of type T. A “property” may also specify an explicit method call with parameters that are Numbers, Strings, or Enums. The pre-defined property “.” indicates that the values to aggregate are represented by the List objects themselves. This allows aggregation of simple objects, like Numbers and Strings. It is possible to use "nested" properties, with a "." character separating nested properties, e.g. "item.objectB.displayName". If a method isn't found, e.g. for the property "property", a getProperty() method isn't found, then jAgg will look for a get(String) or a get(Object) method and pass in the property name as the parameter, e.g. get("property").
Here are the choices the developer has in how to specify “group by” properties:
The developer supplies a List of AggregateFunctions that specify which aggregate operations are to be performed, on which properties. For example:
List<AggregateFunction> aggregators = new ArrayList<AggregateFunction>();
aggregators.add(new SumAggregator("numBillableEvents"));
aggregators.add(new AvgAggregator("totalAmt"));
aggregators.add(new StdDevAggregator("getValue(MyEnum.MY_VALUE)"));