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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.math.DoubleDouble;
import net.sf.jagg.model.WindowClause;

/**
 * A RatioToReportAnalytic is an
 * AbstractDependentAnalyticFunction that performs this
 * calculation:
 * sum(property) rows(0, 0) / sum(property) range()
 * I.e. The sum of values so far divided by the overall sum.
 *
 * @author Randy Gettman
 * @since 0.9.0
 */
public class RatioToReportAnalytic extends AbstractDependentAnalyticFunction
{
   /**
    * Constructs a RatioToReportAnalytic, with no property.
    *
    * @param property Should be an empty string or null.
    */
   public RatioToReportAnalytic(String property)
   {
      setProperty(property);
   }

   /**
    * Returns an uninitialized copy of this Aggregator object,
    * with the same property(ies) to analyze.
    * @return An uninitialized copy of this Aggregator object.
    */
   public RatioToReportAnalytic replicate()
   {
      return new RatioToReportAnalytic(getProperty());
   }

   /**
    * This depends on 2 functions.
    * @return 2.
    */
   public int getNumDependentFunctions()
   {
      return 2;
   }

   /**
    * Depends on two SumAggregators.
    * @param index A 0-based index, ranging from 0 through
    *    getNumDependentFunctions() - 1.
    * @return 
    *    0 => SumAggregator(getProperty())
    *    1 => SumAggregator(getProperty())
    * 
    */
   public AnalyticFunction getAnalyticFunction(int index)
   {
      String property = getProperty();
      switch(index)
      {
         case 0:
            return new SumAggregator(property);
         case 1:
            return new SumAggregator(property);
         default:
            throw new IndexOutOfBoundsException("Invalid index: " + index);
      }
   }

   /**
    * WindowClause 0 is range(, 0) and
    * WindowClause 1 is range().
    * @param index A 0-based index, ranging from 0 through
    *    getNumDependentFunctions() - 1.
    * @return 
    *    0 => rows(0, 0)
    *    1 => range()
    * 
    */
   public WindowClause getWindowClause(int index)
   {
      switch(index)
      {
         case 0:
            return new WindowClause(WindowClause.Type.ROWS, 0, 0);
         case 1:
            return new WindowClause(WindowClause.Type.RANGE, null, null);
         default:
            throw new IndexOutOfBoundsException("Invalid index: " + index);
      }
   }

   /**
    * Returns the ratio to report as a Double.
    * @return The ratio to report as a Double.
    */
   public Double terminate()
   {
      return terminateDoubleDouble().doubleValue();
   }

   /**
    * Returns the ratio to report as a DoubleDouble.
    * @return the ratio to report as a DoubleDouble.
    */
   public DoubleDouble terminateDoubleDouble()
   {
      DoubleDouble result = new DoubleDouble((Double) getValue(0));
      result.divideSelfBy((Double) getValue(1));
      return result;
   }
}