Updated State of the Collections

[Jose]

Thanks for the explanations!
 
I think I'm starting to get the idea:
 
- I don't have to see flatMap as function of two arguments, because I'm only
allowed to provide one. 
- The first argument is just a workaround to access (but not to set) an
internal variable of the flapMap method.
- The way this argument is used in the flatMap body defines a concrete
flavor of the "old" flatMap version. 
 
I'm correct?
 
 
 

  _____  

De: Michael Hixson [mailto:michael.hixson at gmail.com] 
Enviado el: viernes, 16 de noviembre de 2012 11:14
Para: Jose
CC: lambda-dev at openjdk.java.net
Asunto: Re: Updated State of the Collections


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