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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 ChainedExtractor extracts a part of an element and returns it
 * as the label.  It relies on labels from a chained Extractor so
 * it can produce its own labels based on that chained Extractor's
 * labels.  At the end of the chain is an Extractor that returns
 * the element itself -- a SelfExtractor.
 *
 * @param  The element type, matching Extractor.
 * @param  The label type, matching Extractor.
 * @param  The label type from a chained Extractor,
 *    from which this can generate a new label of type L.
 *
 * @author Randy Gettman
 * @since 0.5.0
 * @see SelfExtractor
 */
public abstract class ChainedExtractor extends AbstractExtractor implements Extractor
{
   /**
    * Creates an ChainedExtractor that uses the given
    * Extractor in a chain for its labels.
    *
    * @param extractor The chained Extractor.
    */
   public ChainedExtractor(Extractor extractor)
   {
      super(extractor);
   }

   /**
    * Returns the label for a given element.
    * @param element The element.
    * @return The label.
    */
   public abstract L getLabel(E element);
}