Java 8 Streams: How to call once the Collection.stream() method and retrieve an array of several aggregate values with different fields

Question

I\'m starting with the Stream API in Java 8.

Here is my Person object I use:

public class Person {

    private String firstName;
    private String last         


        

Answer 1

Without third-party libraries you may create a universal collector which combines the results of any number of specified collectors into single Object[] array:

/**
 * Returns a collector which combines the results of supplied collectors
 * into the Object[] array.
 */
@SafeVarargs
public static  Collector multiCollector(
        Collector... collectors) {
    @SuppressWarnings("unchecked")
    Collector[] cs = (Collector[]) collectors;
    return Collector. of(
        () -> Stream.of(cs).map(c -> c.supplier().get()).toArray(),
        (acc, t) -> IntStream.range(0, acc.length).forEach(
            idx -> cs[idx].accumulator().accept(acc[idx], t)),
        (acc1, acc2) -> IntStream.range(0, acc1.length)
            .mapToObj(idx -> cs[idx].combiner().apply(acc1[idx], acc2[idx])).toArray(),
        acc -> IntStream.range(0, acc.length)
            .mapToObj(idx -> cs[idx].finisher().apply(acc[idx])).toArray());
}

For your concrete problem you'll also need a filtering() collector (which will be added in JDK-9, see JDK-8144675):

public static  Collector filtering(
        Predicate filter, Collector downstream) {
    BiConsumer accumulator = downstream.accumulator();
    Set characteristics = downstream.characteristics();
    return Collector.of(downstream.supplier(), (acc, t) -> {
        if(filter.test(t)) accumulator.accept(acc, t);
    }, downstream.combiner(), downstream.finisher(), 
        characteristics.toArray(new Collector.Characteristics[characteristics.size()]));
}

Now you can build a collector which will generate the final result:

Collector collector = 
    multiCollector(
        filtering(p -> p.getFirstName().equals("John"), counting()),
        collectingAndThen(mapping(Person::getAge, 
            maxBy(Comparator.naturalOrder())), Optional::get),
        collectingAndThen(mapping(Person::getHeight, 
            minBy(Comparator.naturalOrder())), Optional::get),
        averagingDouble(Person::getWeight));

Object[] result = personsList.stream().collect(collector);
System.out.println(Arrays.toString(result));