Java 8 stream join and return multiple values

Question

I\'m porting a piece of code from .NET to Java and stumbled upon a scenario where I want to use stream to map & reduce.

class Content
{
  private String          


        

Answer 1

The most generic way to deal with such tasks would be to combine the result of multiple collectors into a single one.

Using the jOOL library, you could have the following:

Content content = 
    Seq.seq(contentList)
       .collect(
         Collectors.mapping(Content::getA, Collectors.joining(", ")),
         Collectors.mapping(Content::getB, Collectors.joining(", ")),
         Collectors.mapping(Content::getC, Collectors.joining(", "))
       ).map(Content::new);

This creates a Seq from the input list and combines the 3 given collectors to create a Tuple3, which is simply a holder for 3 values. Those 3 values are then mapped into a Content using the constructor new Content(a, b, c). The collector themselves are simply mapping each Content into its a, b or c value and joining the results together separated with a ", ".

---

Without third-party help, we could create our own combiner collector like this (this is based of StreamEx pairing collector, which does the same thing for 2 collectors). It takes 3 collectors as arguments and performs a finisher operation on the result of the 3 collected values.

public interface TriFunction {
    R apply(T t, U u, V v);
}

public static  Collector combining(Collector c1, Collector c2, Collector c3, TriFunction finisher) {

    final class Box {
        A a; B b; C c;
        Box(A a, B b, C c) {
            this.a = a;
            this.b = b;
            this.c = c;
        }
    }

    EnumSet c = EnumSet.noneOf(Characteristics.class);
    c.addAll(c1.characteristics());
    c.retainAll(c2.characteristics());
    c.retainAll(c3.characteristics());
    c.remove(Characteristics.IDENTITY_FINISH);

    return Collector.of(
            () -> new Box<>(c1.supplier().get(), c2.supplier().get(), c3.supplier().get()),
            (acc, v) -> {
                c1.accumulator().accept(acc.a, v);
                c2.accumulator().accept(acc.b, v);
                c3.accumulator().accept(acc.c, v);
            },
            (acc1, acc2) -> {
                acc1.a = c1.combiner().apply(acc1.a, acc2.a);
                acc1.b = c2.combiner().apply(acc1.b, acc2.b);
                acc1.c = c3.combiner().apply(acc1.c, acc2.c);
                return acc1;
            },
            acc -> finisher.apply(c1.finisher().apply(acc.a), c2.finisher().apply(acc.b), c3.finisher().apply(acc.c)),
            c.toArray(new Characteristics[c.size()])
           );
}

and finally use it with

Content content = contentList.stream().collect(combining(
    Collectors.mapping(Content::getA, Collectors.joining(", ")),
    Collectors.mapping(Content::getB, Collectors.joining(", ")),
    Collectors.mapping(Content::getC, Collectors.joining(", ")), 
    Content::new
));