Add convenience collect methods to the Stream interface

Remi Forax forax at
Mon Dec 10 17:26:18 UTC 2018

Hi Rob,
i will add to the answer of Brian that if you have too many .collect(toList()),
it's perhaps your application perhaps suffers of the equivalent of the n + 1 select query you have with SQL but with the Stream API.
You should try to see if returning a Stream instead of a List for some of methods is not a better option:
  public List<Employee> getAllEmployee() {
  public List<Employee> getManager(List<Employee> list) {

should be:
  public Stream<Employee> getAllEmployee() {
  public Stream<Employee> getManager(Stream<Employee> stream) {
    return stream.filter(Employee::isManager);


----- Mail original -----
> De: "Brian Goetz" <brian.goetz at>
> À: "Rob Griffin (rgriffin)" <Rob.Griffin at>, "core-libs-dev" <core-libs-dev at>
> Envoyé: Lundi 10 Décembre 2018 17:14:41
> Objet: Re: Add convenience collect methods to the Stream interface

> As will surprise no one, this was extensively discussed during the
> development of the Streams API.  (Our default position on "convenience
> methods" is hostile.  While everyone sees the benefit of convenience
> methods (it's convenient!), most people don't see the cost, which
> includes the complexity for users to understand the model by looking at
> the API; having lots of ad-hoc convenience method obscures the
> underlying model, making it harder for everyone to learn or reason
> about.  That default position seems to stand up pretty well here, as the
> stream API is pretty well factored.)
> Let's be honest: the "convenience" or concision of being able to say
> .toList() instead of .collect(toList()) is really small.  I don't think
> you'll be able to justify it by saying "but we do it a lot."
> (Digression: to whoever is about to say "then why `toArray()`?  Arrays
> are different; for better or worse, they're part of the language, and
> they lend themselves particularly poorly to the Collector API, and there
> are particular parallelization optimizations that are possible for
> arrays that can't be accessed through Collector.  End digression.)
> It is possible, however, that one could justify `toList()` on the basis
> of _discoverability_.  (Though I'm having a hard time seeing any world
> where `toSet()` makes the cut.)  New users who approach streams will not
> easily be able to figure out how to materialize a list from a stream,
> even though this is an entirely reasonable and quite common thing to
> want to do.  Having to learn about `collect()` first is asking a lot of
> users who are still wrapping their heads around streams.  Not only would
> `toList()` be more discoverable, it would provide a path to discovery of
> the rest of the `collect()` API.  This is a point in its favor.
> A significant downside of adding `toList()` is that by diluting the
> orthogonality of the existing API, it provides both incentive and
> justification for further dilution, leading to someplace we don't want
> to be.  (And, the cost of that falls heavily on the stewards, which in
> turn takes time away from far more valuable activities.)
> Put it this way: imagine we have a budget of one convenience method in
> Stream for every five years.  Is this the one we want to spend the next
> five year's budget on?  (And, who of the proponents will volunteer to
> answer the next 200 "I have an idea for a stream method" mails,
> explaining that the budget is spent?)
> So, summary:
>  - I won't outright veto `toList`, as I would for almost all other
> "convenience" streams additions, because this one actually has a valid
> non-convenience argument;
>  - But, it's still not a slam dunk.
> On 12/9/2018 5:44 PM, Rob Griffin (rgriffin) wrote:
>> Hi,
>> I have raised an enhancement request (Incident Report 913453) about adding some
>> convenience methods to the Stream interface that collect the stream and Pallavi
>> Sonal asked me to start a thread here about that.
>> More than 50% of our Stream collect calls use Collectors.toList() or
>> Collectors.toSet() as arguments so I think it would be very handy if the Stream
>> interface had default collectToList and collectToList and collectToMap methods.
>> The advantages are:
>> 	it would be easier to use code completion in IDEs. There are lot of classes
>> 	starting with Collect so finding the Collectors class is a bit of a pain.
>> 	one less method call in what is usually a long chain of calls.
>> Regards,
>> Rob Griffin
>> Software Analyst, Spotlight on SQL Server
>> Quest | R&D
>> rob.griffin at

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