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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;
import java.util.ArrayList;

/**
 * A BooleanDiscriminator discriminates Lists of
 * Booleans.
 *
 * @author Randy Gettman
 * @since 0.5.0
 */
public class BooleanDiscriminator extends AbstractDiscriminator
{
   /**
    * Partitions the given List of values into another
    * List, in which all of the values from the given list exist
    * in the new list, and all values 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 values
    *    that compare equal to each other.
    */
   public  List> discriminate(List elements, Extractor extractor, MsdWorkspace workspace)
   {
      List trues = new ArrayList();
      List falses = new ArrayList();
      int size = elements.size();
      for (int i = 0; i < size; i++)
      {
         E element = elements.get(i);
         if (extractor.getLabel(element))
            trues.add(element);
         else
            falses.add(element);
      }
      List> equivClasses = new ArrayList>();
      if (!trues.isEmpty())
         equivClasses.add(trues);
      if (!falses.isEmpty())
         equivClasses.add(falses);

      return equivClasses;
   }
}