https://github.com/eclipse/eclipse-collections/eclipse-collections: Eclipse Collections is a collections framework for Java. It has JDK-compatible List, Set and Map implementations with a rich API and set of utility classes that work with any JDK compatible Collections, Arrays, Maps or Strings. The iteration protocol was inspired by the Smalltalk collection framework.
  • Craig P. Motlin
  • Donald Raab
  • Mohammad A. Rezaei
  • Hiroshi Ito
  • Nikhil J. Nanivadekar 
/*
 * Copyright (c) 2016 Goldman Sachs.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * and Eclipse Distribution License v. 1.0 which accompany this distribution.
 * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v10.html
 * and the Eclipse Distribution License is available at
 * http://www.eclipse.org/org/documents/edl-v10.php.
 */

package org.eclipse.collections.impl.block.procedure;

import org.eclipse.collections.api.block.function.Function;
import org.eclipse.collections.api.block.function.primitive.DoubleFunction;
import org.eclipse.collections.api.block.procedure.Procedure;


Implementation of Procedure that holds on to the summation of elements seen so far, determined by the Function. 
/**
 * Implementation of {@link Procedure} that holds on to the summation of elements seen so far,
 * determined by the {@link Function}.
 */
public class SumOfDoubleProcedure<T> implements Procedure<T>, DoubleSumResultHolder
{
    private static final long serialVersionUID = 2L;

    private final DoubleFunction<? super T> function;
    private double result;
    private double compensation;

    public SumOfDoubleProcedure(DoubleFunction<? super T> function)
    {
        this.function = function;
    }

    @Override
    public double getResult()
    {
        return this.result;
    }

    @Override
    public double getCompensation()
    {
        return this.compensation;
    }

    @Override
    public void value(T each)
    {
        double adjustedValue = this.function.doubleValueOf(each) - this.compensation;
        double nextSum = this.result + adjustedValue;
        this.compensation = nextSum - this.result - adjustedValue;
        this.result = nextSum;
    }
}