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) 2018 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.Function0;
import org.eclipse.collections.api.block.procedure.Procedure;
import org.eclipse.collections.api.block.procedure.Procedure2;
import org.eclipse.collections.api.map.MutableMapIterable;


This procedure is used to apply an aggregate function like sum on a grouped set of data. The values in the
grouping must be mutable using this procedure. An example would be using an AtomicInteger which can be mutated
vs. an Integer which cannot.

/**
 * This procedure is used to apply an aggregate function like sum on a grouped set of data. The values in the
 * grouping must be mutable using this procedure. An example would be using an AtomicInteger which can be mutated
 * vs. an Integer which cannot.
 */
public final class MutatingAggregationProcedure<T, K, V> implements Procedure<T>
{
    private static final long serialVersionUID = 1L;
    private final MutableMapIterable<K, V> map;
    private final Function<? super T, ? extends K> groupBy;
    private final Function0<? extends V> zeroValueFactory;
    private final Procedure2<? super V, ? super T> mutatingAggregator;

    public MutatingAggregationProcedure(
            MutableMapIterable<K, V> map,
            Function<? super T, ? extends K> groupBy,
            Function0<? extends V> zeroValueFactory,
            Procedure2<? super V, ? super T> mutatingAggregator)
    {
        this.map = map;
        this.groupBy = groupBy;
        this.zeroValueFactory = zeroValueFactory;
        this.mutatingAggregator = mutatingAggregator;
    }

    @Override
    public void value(T each)
    {
        K key = this.groupBy.valueOf(each);
        V value = this.map.getIfAbsentPut(key, this.zeroValueFactory);
        this.mutatingAggregator.value(value, each);
    }
}