Updated State of the Collections

[Michael Hixson]

The sink is provided to you by the stream - you don't create it yourself.
 So in this example:

  List<Book> books = authors.stream()
      .<Book>flatMap((sink, author) -> author.getBooks().forEach(sink))
      .into(new ArrayList<Book>());

the object behind the "sink" variable was created by the stream upon
calling .flatMap.  The flatMap method is basically saying, "I'm giving you
a sink.  It's what I'm using to build the result.  For each author, use the
sink to mark which books will appear in the result."  This lambda:

  (sink, author) -> author.getBooks().forEach(sink)

instructs the sink to include every book for every author in the result.

I agree that it's confusing.  Without looking at the method signature, I
expected flatMap to do that dumping into the sink for me.  Something like
this:

  // Note: this doesn't work
  List<Book> books = authors.stream()
      .flatMap(author -> author.getBooks())
      .into(new ArrayList<Book>());

But I see why they didn't write flatMap that way.  In my example, I have
author.getBooks() which returns a collection.  If I didn't have a method
like that, or I didn't want to create a throwaway collection just to hold
each author's books (which will all then get merged into one collection),
then if they only gave me a flatMap(A -> Collection<B>) method I'd be
hosed.  The fact that they give me the sink lets me pick and choose the
elements in the result individually from wherever I want.

-Michael


On Fri, Nov 16, 2012 at 1:40 AM, Jose <jgetino at telefonica.net> wrote:

>
>
> So flatMap function has two arguments.  For the second one you provide a
> List of authors that you defined previously.
>
> But what about the first argument?,  sink is just a variable, it has not
> being initialized at any place
>
> I find this like defining the square function as:
>
> sq=(x,y)->y*y
>
> and then using it in this way
>
> sq(x,3)==9
>
>
>
>
>
> -----Mensaje original-----
> De: Michael Hixson [mailto:michael.hixson at gmail.com]
> Enviado el: viernes, 16 de noviembre de 2012 8:30
> Para: Jose
> CC: lambda-dev at openjdk.java.net
> Asunto: Re: Updated State of the Collections
>
> flatMap caused me a lot of trouble as well.  Eventually I figured out you
> can use it without casting, declaring anonymous classes, or using a helper
> method like this:
>
> List<Author> authors = ...
> // Assume author.getBooks() returns List<Book> List<Book> books =
> authors.stream()
>     .<Book>flatMap((sink, author) -> author.getBooks().forEach(sink)
>     .into(new ArrayList<Book>());
> // Assume author.getDeceasedDate() returns Optional<Date> List<Date>
> deceasedDates = authors.stream()
>     .<Date>flatMap(sink, author) ->
> author.getDeceasedDate().ifPresent(sink))
>     .into(new ArrayList<Date>());
>
> The generic for the flatMap invocations ("<Book>.flatMap" and
> "<Date>flatMap" above) seem to be necessary.  I don't know if that was
> because of the IDE I was using (IntelliJ 12 EAP) or because that's just how
> it has to be.
>
>
>
> On Thu, Nov 15, 2012 at 10:58 PM, Jose <jgetino at telefonica.net> wrote:
>
>
>
>         Would you mind to provide an example of the use of flatMap?.
>
>         I need to adapt old b56 code like this:
>
>         Set<Polyline> lines(mapName){};
>         namesColl.flatMap(name -> lines(name)).into(...);
>
>         to the new sintax but I have no idea how to start with (do I have
> to
> provide
>         a Block.instance?)
>
>         I made some web search on flatMap and jdk8 but everyone seems to be
> talking
>         about Optional.
>         And in the repos I couln't find a single example, only discussions
> about the
>         sintax.
>
>
>
>
>         -----Mensaje original-----
>         De: lambda-dev-bounces at openjdk.java.net
>         [mailto:lambda-dev-bounces at openjdk.java.net] En nombre de Brian
> Goetz
>         Enviado el: jueves, 15 de noviembre de 2012 19:32
>         Para: lambda-dev at openjdk.java.net
>         Asunto: Updated State of the Collections
>
>         is here: http://cr.openjdk.java.net/~briangoetz/lambda/sotc3.html
>
>
>
>
>
>
>
>