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Java 8 Stream: groupingBy with multiple Collectors

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野性不改
野性不改 2020-12-15 08:20

I want to use a Java 8 Stream and Group by one classifier but have multiple Collector functions. So when grouping, for example the average and the sum of one field (or maybe

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  • 不思量自难忘°
    不思量自难忘° (楼主)
    2020-12-15 09:11

    For the concrete problem of summing and averaging, use collectingAndThen along with summarizingDouble:

    Map result = persons.stream().collect(
        groupingBy(Person::getGroup, 
                collectingAndThen(summarizingDouble(Person::getAge), 
                        dss -> new Data((long)dss.getAverage(), (long)dss.getSum()))));

    For the more generic problem (collect various things about your Persons), you can create a complex collector like this:

    // Individual collectors are defined here
List> collectors = Arrays.asList(
        Collectors.averagingInt(Person::getAge),
        Collectors.summingInt(Person::getAge));

@SuppressWarnings("unchecked")
Collector, List> complexCollector = Collector.of(
    () -> collectors.stream().map(Collector::supplier)
        .map(Supplier::get).collect(toList()),
    (list, e) -> IntStream.range(0, collectors.size()).forEach(
        i -> ((BiConsumer) collectors.get(i).accumulator()).accept(list.get(i), e)),
    (l1, l2) -> {
        IntStream.range(0, collectors.size()).forEach(
            i -> l1.set(i, ((BinaryOperator) collectors.get(i).combiner()).apply(l1.get(i), l2.get(i))));
        return l1;
    },
    list -> {
        IntStream.range(0, collectors.size()).forEach(
            i -> list.set(i, ((Function)collectors.get(i).finisher()).apply(list.get(i))));
        return list;
    });

Map> result = persons.stream().collect(
        groupingBy(Person::getGroup, complexCollector)); 



    Map values are lists where first element is the result of applying the first collector and so on. You can add a custom finisher step using Collectors.collectingAndThen(complexCollector, list -> ...) to convert this list to something more appropriate.
    
    
    
             
    
                                                        
    
            
            
    
              
                
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