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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.util;

import java.util.Comparator;

/**
 * This adapter class compares Comparables.  It compares its
 * objects exactly like T's compareTo method (which
 * exists because T is Comparable).
 *
 * @author Randy Gettman
 * @since 0.1.0
 */
public class ComparableComparator> implements Comparator
{
   /**
    * Compares the given objects to determine order.  Fulfills the
    * Comparator contract by returning a negative integer, 0, or a
    * positive integer if o1 is less than, equal to, or greater
    * than o2.
    * Nulls compare equal to each other, and a null compares greater than
    * non-nulls.
    *
    * @param o1 The left-hand-side object to compare.
    * @param o2 The right-hand-side object to compare.
    * @return A negative integer, 0, or a positive integer if o1
    *    is less than, equal to, or greater than o2.
    */
   public int compare(T o1, T o2)
   {
      if (o1 == null)
      {
         if (o2 == null)
            return 0;
         else
            return 1;
      }
      else if (o2 == null)
         return -1;
      return o1.compareTo(o2);
   }
}