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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.msd;

import java.util.List;

/**
 * A Discriminator is able to distinguish elements of a given
 * List of objects of a certain type.  It partitions the given
 * list into a List of equivalence classes.  An
 * equivalence class is represented by a List of objects
 * that compare equal, according to this Discriminator.
 *
 * For example, in an unsorted list, one may have different values scattered
 * randomly throughout the list:
 * {100, 23, 5, 23, 10, 10, 5, 23, 6}
 * The discrimination process returns a new List of
 * Lists, each list containing an equivalence class.  Each
 * equivalence class list contains all values that compare equal to each other.
 * They are not necessarily in sorted order.  For example, integer
 * discrimination of the above list yields the following result:
 * {{100}, {6}, {23, 23, 23}, {5, 5}, {10, 10}}
 * This algorithm is stable, meaning that values that do compare
 * equal to each other remain in the same order as before, i.e. the first 5
 * remains before the second 5 in the equivalence class list.
 *
 * @param  The type being discriminated.
 *
 * @author Randy Gettman
 * @since 0.5.0
 */
public interface Discriminator
{
   /**
    * Partitions the given List of elements into another
    * List, in which all of the elements from the given list exist
    * in the new list, and all elements that compare equal are adjacent to each
    * other.
    * @param elements A List of elements.
    * @param workspace The MsdWorkspace used in the discrimination process.
    * @return A List of Lists containing all
    *    equivalence classes.  Each equivalence class list contains all values
    *    that compare equal to each other.
    */
   public List> discriminate(List elements, MsdWorkspace workspace);

   /**
    * Partitions the given List of elements into another
    * List, in which all of the elements from the given list exist
    * in the new list, and all elements that compare equal are adjacent to each
    * other, according to the given Extractor.
    * @param elements A List of elements.
    * @param extractor An Extractor that gives labels for
    *    each element.
    * @param workspace The MsdWorkspace used in the discrimination process.
    * @return A List of Lists containing all
    *    equivalence classes.  Each equivalence class list contains all
    *    elements that compare equal to each other.
    * @param  The type of element that is being discriminated by the type
    *    <E>
    */
   public  List> discriminate(List elements, Extractor extractor, MsdWorkspace workspace);
}