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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 Discriminable object knows which properties in itself to
 * be discriminated.  
 *
 * @author Randy Gettman
 * @since 0.5.0
 */
public interface Discriminable
{
   /**
    * Returns a List of properties on which to discriminate on
    * this object.  Such properties must map to an actual method name on the
    * object in the bean naming convention, e.g. a property of "value" maps to
    * a method "getValue()" that takes no arguments.  All properties must map
    * to methods whose returns types are themselves Discriminable,
    * or of a Java built-in type:
    * 
    * 

    * Boolean
    * 
Character
    * 
CharSequence (all implementing classes such as
    *    String, CharBuffer,
    *    StringBuffer, and StringBuilder)
    * 
Number (includes BigDecimal,
    *    BigInteger, Byte, Double,
    *    Float, Integer, Long, and
    *    Short)
    * 
Any Date
    * 
Any Calendar
    * 
Any Enum
    * 
Any Collection
    * 
Any List
    * 
Any primitive type: boolean, char,
    *    byte, short, int,
    *    long, float, double.
    * 
An array of any above type.
    * 
    * 
    * If a Discriminable object is also
    * Comparable, then the properties listed here should also be
    * the same properties used in compareTo, for consistency.
    * The List of properties returned must be the same for any
    * and every instance of each Discriminable class.  For this
    * reason, it is recommended to return a static final List.
    *
    * @return A List of properties.
    */
   public List getDiscriminableProperties();
}