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Re: Why is that in JDK8: value used in lambda expression shuld be effectively final?

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Am Freitag, 4. Januar 2013 22:32:38 UTC+1 schrieb Steven Simpson:
> On 03/01/13 14:45, Saxo wrote:
> 
> I'm still interested in seeing an example that you would normally write 
> with free variables, demonstrating that lambdas are inadequate for the 
> things you'd like to do. 

Groovy:

def list = [1, 2, 3]
def sum = 0
list.each { sum += it }
println(sum)

Kotlin:

val list = arrayListOf(1, 2, 3)
var sum = 0;
list.forEach { i -> sum += i }
println(sum)

Scala:

val list = List(1, 2, 3)          
var sum = 0        	  
list.foreach(sum += _)
println(sum)

Works in all these languages that support closures (and I guess in any other language that supports closures as well), but the same does not work with JDK8 lambdas.

Reality can be tough... There is always the workaround of defining a single element array final:

final int sumArray[] = new int[] { 0 };
ints.forEach(i -> {sumArray[0] += i;}); 
println(sumArray[0]); 

I wrote in my blog:

The lambda specification (JSR 335) also says so explicitly: "For both lambda bodies and inner classes, local variables in the enclosing context can only be referenced if they are final or effectively final. A variable is effectively final if it is never assigned to after its initialization.".

And I provided the link to JSR 335: http://cr.openjdk.java.net/~dlsmith/jsr335-0.6.1/B.html

I took some effort in my blog to describe the matter and provided the links to the respective resources on the JDK8 lambda homepage :-).

OTOH, JDK8 lambdas are still enormously useful. No doubt about that. An imense amount of pre-JDK8 boilerplate code can be thrown out of myriads of systems, frameworks and applications. It's a big step ahead. Nevertheless, it can be said that JDK8 lambdas don't go all the way to be called true closures. Apparently, JDK8 lambdas can be assigned to a Runnable. Maybe this was a deliberate choice to stay backwards compatible. 

-- Oliver

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Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? jeti789@web.de - 2013-01-02 07:08 -0800
  Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? jeti789@web.de - 2013-01-02 07:52 -0800
  Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Steven Simpson <ss@domain.invalid> - 2013-01-02 17:33 +0000
    Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Saxo <jeti789@web.de> - 2013-01-02 11:31 -0800
      Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Lew <lewbloch@gmail.com> - 2013-01-02 12:52 -0800
        Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-01-02 20:58 -0400
          Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Lew <lewbloch@gmail.com> - 2013-01-02 17:40 -0800
            Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-01-03 20:43 -0400
        Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Saxo <jeti789@web.de> - 2013-01-02 23:44 -0800
          Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Patricia Shanahan <pats@acm.org> - 2013-01-03 07:57 -0800
            Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Lew <lewbloch@gmail.com> - 2013-01-03 13:22 -0800
              Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Patricia Shanahan <pats@acm.org> - 2013-01-03 16:14 -0800
      Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Steven Simpson <ss@domain.invalid> - 2013-01-02 21:46 +0000
        Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Lew <lewbloch@gmail.com> - 2013-01-02 17:35 -0800
        Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Saxo <jeti789@web.de> - 2013-01-03 02:13 -0800
          Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Saxo <jeti789@web.de> - 2013-01-03 06:45 -0800
            Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Steven Simpson <ss@domain.invalid> - 2013-01-04 21:32 +0000
              Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2013-01-04 14:38 -0800
                Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Steven Simpson <ss@domain.invalid> - 2013-01-04 23:45 +0000
                Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2013-01-04 17:24 -0800
                Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Steven Simpson <ss@domain.invalid> - 2013-01-05 13:20 +0000
              Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Saxo <jeti789@web.de> - 2013-01-05 01:50 -0800
                Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Steven Simpson <ss@domain.invalid> - 2013-01-05 11:21 +0000
                Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Saxo <jeti789@web.de> - 2013-01-05 04:27 -0800
                Re: Why is that in JDK8: value used in lambda expression shuld be effectively final? Steven Simpson <ss@domain.invalid> - 2013-01-05 12:58 +0000

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