Java8 Stream as a whole maps to a Function

Question

I am wondering if there's a better way to rewrite the groupA method with lambda as part of the chaining operations ?

public class Id {
    private final int value;
    public Id(int value) {
       this.value = value;
    }

    public int value() {
       return value;
    }
}

public class Ids implements Iterable<Id> { 
   private final List<Id> ids;

   private Ids(List<Id> ids) {
      this.ids = ids;
   }

   public static Ids of(List<Id> ids) {
      return new Ids(ids);
   }

   public Ids groupA() {
      return Ids.of(ids.stream()
                       .filter(id -> id.value() > 5)
                       .collect(Collectors.toList()));
   }

   @Override
   public Iterator<Id> iterator() {
      return ids.iterator();
   }
}

Basically I want to do something like

ids.stream()
   .filter(id -> id % 10 > 5)
   .collect(Collectiors.toList())
   .andThen(Ids::of);

And wonder if that's possible

Tunaki · Accepted Answer · 2016-01-25 18:42:01Z

10

Sure. You can either just do the straightforward

Ids.of(ids.stream()
   .filter(id -> id % 10 > 5)
   .collect(Collectors.toList()))

or you could add it to the collector:

ids.stream()
   .filter(id -> id % 10 > 5)
   .collect(Collectors.collectingAndThen(Collectors.toList(), Ids::of))

edited Jan 25, 2016 at 18:42

Tunaki

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answered Jan 25, 2016 at 18:38

Louis Wasserman

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9 Comments

peter Over a year ago

when I do option2 which is the one that uses collectingAndThen that the compiler explains having problem resolving method 'of'

Louis Wasserman Over a year ago

@peter what is the exact error? It looks like the types aren't quite right, since you can't do id % 10 with id of type Id.

Holger Over a year ago

Though the static method is not necessary in this context. Here, Ids::of could be replaced by Ids::new. Besides that, @peter what’s wrong with the original approach?

Louis Wasserman Over a year ago

@peter I don't get that error when I actually plug this into a compiler. ideone.com/pr1ryb

Louis Wasserman Over a year ago

@peter the first one performs a tiny bit better, since you're not allocating the Collector or the lambda. The JIT might be able to optimize that away, but if it doesn't improve readability what's the point?

|

Tagir Valeev · Accepted Answer · 2016-01-26 04:06:01Z

2

In my StreamEx library there's a shortcut method toListAndThen which makes this looking more fluent:

StreamEx.of(ids)
  .filter(id -> id % 10 > 5)
  .toListAndThen(Ids::of);

answered Jan 26, 2016 at 4:06

Tagir Valeev

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