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List:       openjdk-lambda-dev
Subject:    =?utf-8?Q?Re:_How_come..._part_1:_importing_statically?=
From:       Timo_Kinnunen <timo.kinnunen () gmail ! com>
Date:       2014-01-04 14:21:00
Message-ID: 52c85400.456c0e0a.0893.7124 () mx ! google ! com
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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@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


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