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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.Date;
import java.util.List;

/**
 * A DateDiscriminator discriminates Dates.
 *
 * @author Randy Gettman
 * @since 0.6.0
 */
public class DateDiscriminator extends ChainedDiscriminator
{
   /**
    * Returns an appropriate ChainedExtractor.
    * @param elements The List of elements.
    * @param extractor A ChainedExtractor that returns appropriate
    *    labels.
    * @return An appropriate ChainedExtractor.
    */
   protected  ChainedExtractor getChainedExtractor(List elements, Extractor extractor)
   {
      return new DateChainedExtractor(extractor);
   }

   /**
    * A DateChainedExtractor extracts the milliseconds value of
    * the Date.
    * @param  The type of element.
    */
   protected class DateChainedExtractor extends ChainedExtractor
   {
      /**
       * Create an EnumChainedExtractor that uses the given
       * Extractor to retrieve the array.
       * @param extractor An Extractor whose label is a
       *    long.
       */
      public DateChainedExtractor(Extractor extractor)
      {
         super(extractor);
      }

      /**
       * The label is the milliseconds value of the Date.
       * @param element The element.
       * @return The milliseconds value of the Date.
       */
      public Long getLabel(E element)
      {
         return myExtractor.getLabel(element).getTime();
      }

      /**
       * The discrimination is complete when the chained Extractor
       * is complete.
       * @param element The element.
       * @return true if complete, false otherwise.
       */
      public boolean isComplete(E element)
      {
         // Only check the ordinal once!
         return myExtractor.isComplete(element) || myIndex >= 1;
      }
   }

   /**
    * Returns the Discriminator that discriminates on the
    * milliseconds value of the Date.
    * @param elements The list of elements.
    * @param extractor The ChainedExtractor that was obtained from
    *    getChainedExtractor.
    * @param index The index of the loop.
    * @return A Discriminator that discriminates on the
    *    milliseconds value of the Date.
    */
   protected  Discriminator getDiscriminator(List elements, ChainedExtractor extractor,
      int index)
   {
      return new LongDiscriminator();
   }
}