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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;

import net.sf.jagg.model.WindowClause;

/**
 * A DependentAnalyticFunction is an AnalyticFunction
 * that depends on the results of multiple other AnalyticFunctions
 * that have different WindowClauses.  The Analytic
 * class detects DependentAnalyticFunctions, loads the functions
 * they're dependent on, processes them normally, and feeds the values back to
 * this function to calculate the value depending on the these values.
 *
 * @author Randy Gettman
 * @since 0.9.0
 */
public interface DependentAnalyticFunction extends AnalyticFunction
{
   /**
    * Returns the number of AnalyticFunctions on which this
    * DependentAnalyticFunction depends.  This controls the domain
    * of index values that getAnalyticFunction and
    * getWindowClause can take.
    *
    * @return The number of AnalyticFunctions on which this
    * DependentAnalyticFunction depends.
    */
   public int getNumDependentFunctions();

   /**
    * Returns an AnalyticFunction on which this
    * DependentAnalyticFunction depends.
    * @param index A 0-based index, ranging from 0 through
    *    getNumDependentFunctions() - 1.
    * @return An AnalyticFunction.
    * @see #getNumDependentFunctions
    */
   public AnalyticFunction getAnalyticFunction(int index);

   /**
    * Returns a WindowClause that is associated with the
    * AnalyticFunction at the same index.
    * @param index A 0-based index, ranging from 0 through
    *    getNumDependentFunctions() - 1.
    * @return A WindowClause that is associated with the
    * AnalyticFunction at the same index.
    * @see #getAnalyticFunction
    * @see #getNumDependentFunctions
    */
   public WindowClause getWindowClause(int index);

   /**
    * Sets the current value of the AnalyticFunction at the given
    * index.  Analytic calls this getNumDependentFunctions()
    * times prior to calling terminate(), so that terminate
    * can make a calculation based on these values.
    * @param index A 0-based index, ranging from 0 through
    *    getNumDependentFunctions() - 1.
    * @param value The current value of the AnalyticFunction.
    * @see #terminate
    */
   public void setValue(int index, Object value);
}