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jAgg is a Java 5.0 API that supports “group by” operations on Lists of Java objects: aggregate operations such as count, sum, max, min, avg, and many more. It also allows custom aggregate operations.

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package net.sf.jagg;

import net.sf.jagg.model.WindowClause;

/**
 * This interface allows for the functionality necessary to implement
 * analytic functions, based on aggregate functions.  Subclasses define all of
 * the steps of the {@link AggregateFunction Aggregation algorithm}, plus these
 * methods:
 * 
 * Deletion, with the delete method.  This removes an element
 * from the state of the AnalyticFunction.  This is used when a
 * sliding window applies to an analytic function and an window "slides past"
 * an element, such that an element is no longer being considered in the
 * current value of the analytic function.
 * Whether it accepts the windowing clause, with the
 * takesWindowClause method.  Window clauses may not make sense
 * with certain analytic functions.  If a window clause is supplied to such an
 * analytic function, then an error will result.
 * 
 *
 * @see AggregateFunction
 *
 * @author Randy Gettman
 * @since 0.9.0
 */
public interface AnalyticFunction extends AggregateFunction
{
   /**
    * Removes the given value from the aggregation.  This assumes that the
    * value to be removed was already processed with the iterate
    * method.  E.g., a "sum" aggregation will subtract this object's property
    * value from a sum object.
    *
    * @param value The value to delete.
    */
   public abstract void delete(Object value);

   /**
    * Determines whether this AnalyticFunction can operate with a
    * window clause.  Most will return true, but some
    * analytic-only functions don't take a window clause.  For those that
    * return false, they must also return a non-null
    * WindowClause
    *
    * @return Whether this AnalyticFunction can operate with a
    *    window clause.
    * @see WindowClause
    */
   public abstract boolean takesWindowClause();

   /**
    * For AnalyticFunctions that don't take a window clause, this
    * method returns the implicit WindowClause.
    * 
    * @return A WindowClause if takesWindowClause
    *    returns false, else null.
    * @see #takesWindowClause
    * @see WindowClause
    */
   public abstract WindowClause getWindowClause();
}