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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;

/**
 * A IntegerDiscriminator discriminates Lists of
 * Integers.
 *
 * @author Randy Gettman
 * @since 0.5.0
 */
public class IntegerDiscriminator extends NumberDiscriminator
{
   /**
    * Returns a portion extractor appropriate for integers.
    * @param e An Extractor that returns appropriate labels.
    * @return An appropriate PortionExtractor.
    */
   protected  PortionExtractor getPortionExtractor(Extractor e)
   {
      return new PortionExtractor(e) {
         /**
          * The label is the specific portion of the element, cast as an
          * integer.
          * @param element The element.
          * @return The specific portion of the element, cast as an integer.
          */
         public int getLabel(E element)
         {
            return (myExtractor.getLabel(element) >> (myIndex * PORTION_BITS)) & PORTION_MASK;
         }

         /**
          * Complete after 2 portions.
          * @param element The element.
          * @return true if the index is at least 2,
          *    false otherwise.
          */
         public boolean isComplete(E element)
         {
            return myExtractor.isComplete(element) || myIndex >= 2;
         }
      };
   }
}