How come... part 1: importing statically

[Samir Talwar]

Features don't exist by default.

`System.out` is special. Not in the good way. It's a hold-over from
procedural code that flies in the face of object-oriented code. `out` is a
static field, which means all manner of things, but what it generally means
is that in good object-oriented code (which may or may not be the same as
"good code"; that's a different discussion), it's rare. Mutable static
fields are generally considered unpleasant and should be avoided.

So given that this is a pattern we're trying to avoid in good OO code, what
would be the point of implementing this feature?

— Samir.


On Sat, Jan 4, 2014 at 2:21 PM, Timo Kinnunen <timo.kinnunen at gmail.com>wrote:

> Hi,
>
>
> Don’t worry, I’ll only post things that deal with lambdas that have me
> scratching my head to this list. I have a feeling that there might be more,
> but let’s hope there isn’t :)
>
>
> Now, on to this issue.
>
>
> Picking my words deliberately and carefully in order to make my point,
> import static allows me to reference a method as if it had been declared in
> my class:
>
>
> import static java.lang.String.valueOf;
>
> [. . .]
>
> Stream<String> s1 = IntStream.of(1, 2).mapToObj((int i) -> valueOf(i));
>
>
> Lambdas are - still, hopefully - intended to be less instances of
> anonymous classes and more like anonymous code. When passed around they
> manifest as pointers to code, or the internal functions containing the
> code, references to internal methods if you will. The above Stream can be
> declared also like this:
>
>
> Stream<String> s2 = IntStream.of(1, 2).mapToObj(String::valueOf);
>
>
> This pointer-to-code nature makes it natural to convert from a method
> reference to a lambda, like above. And this nature is so strong that
> converting from a reference to some object’s instance method is just as
> easy as if it was a static method, in this case a PrintStream method of an
> object in the static field System.out, like you mentioned:
>
>
> s1.forEach(System.out::println);
>
>
> As before, a strong pointer-to-code similarity between lambdas and methods
> of both instance and static kinds let’s us neatly come back full circle and
> also write the above like. . . OK, so how come this is an error suddenly?
>
>
>
>
> import static java.lang.System.out.println;
>
> [. . .]
>
>
> s2.forEach((String s) -> println(s));
>
>
>
> Where is the Zen in this?
>
>
>
>
>
> --
> Have a nice day,
> Timo.
>
> Sent from Windows Mail
>
>
>
>
>
> From: Richard Warburton
> Sent: Saturday, January 4, 2014 16:00
> To: Timo Kinnunen
> Cc: lambda-dev at openjdk.java.net
>
>
>
>
>
>
> Hi,
>
>
>
>
> Arrays.asList("--(!)Error loading\n").stream().forEach(System.out::printf);
>
> but I can’t write:
>
> import static java.lang.System.out::printf;
>
> . . .
>
> printf("--(!)Error loading\n");
>
>
> What’s up with that?
>
>
>
>
> import static let's you import static fields or methods with an
> abbreviated syntax.  printf isn't a static field or method - its an
> instance method of the PrintStream class.  On the other hand "out" is a
> static field of the System class, so you can do:
>
>
>
>
> import static java.lang.System.out;
>
>
>
>
>
> out.printf("--(!)Error loading\n");
>
>
>
>
> What you asking for is a situation where "import static" imports things
> that aren't static. This would almost inevitably lead to an alternative
> "How come ..." question!
>
>
>
>
> (This is part 1 in my How come... series, more parts as they come.)
>
>
>
>
>
> I will observe that this question didn't really have anything to do with
> lambda expressions and stack overflow deals very well with general Java
> questions.
>
>
>
>
> regards,
>
>
>   Richard Warburton
>
>
>   http://insightfullogic.com
>
>
>   @RichardWarburto
>
>