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Re: Bulk Array Element Allocation, is it faster?

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Roedy Green wrote:
>  Lew wrote, quoted or indirectly quoted someone who said :
>> Foo#bar will be cleared to 'null' upon first construction, then initialized to 'null' by the explicit initialization. 
>> Both steps happen.  Both steps are required to happen, and they're required to happen separately.
> 
> I doubt that. The JLS never says exactly how something works, just

> what the net effect must be.  It is part of what makes it so difficult

Let me settle your doubt, then, because the JLS does say exactly that.

JLS §12.5:
http://java.sun.com/docs/books/jls/third_edition/html/execution.html#12.5
"Whenever a new class instance is created, memory space is allocated for it with room for all the instance variables declared in the class type and all the instance variables declared in each superclass of the class type, including all the instance variables that may be hidden (§8.3). If there is not sufficient space available to allocate memory for the object, then creation of the class instance completes abruptly with an OutOfMemoryError. Otherwise, all the instance variables in the new object, including those declared in superclasses, are initialized to their default values (§4.12.5).
"Just before a reference to the newly created object is returned as the result, the indicated constructor is processed to initialize the new object using the following procedure:
"...
"4. Execute the instance initializers and instance variable initializers for this class, assigning the values of instance variable initializers to the corresponding instance variables, in the left-to-right order in which they appear textually in the source code for the class. If execution of any of these initializers results in an exception, then no further initializers are processed and this procedure completes abruptly with that same exception. Otherwise, continue with step 5. (In some early implementations, the compiler incorrectly omitted the code to initialize a field if the field initializer expression was a constant expression whose value was equal to the default initialization value for its type.)"


> to follow.  A JLS for programmers rather than implementors might
> describe a simple reference implementation, and say, "As a programmer
> you can presume it works like this, but actual implementations will be
> more sophisticated and may not give exactly the same behaviour as this
> simple explanation. For those fine points, see the implementors JLS."

The JLS is extremely explicit about the order and result of steps to load, allocate and initialize classes and instances.  In this particular, non-hypothetical case, it requires members to get their default values, then separately to process explicit initializers, even (and especially) if the initialization is to the same default value.


> So for example it could describe how a simple GC works. 
> 
> In most cases the second init to null could be dropped by a clever
> optimiser because it would have the same net effect.

Except that that is, as you see from the cited passage, explicitly forbidden.

> Of course I defer to you and Patricia on divining the meaning of the
> JLS.

You don't need to.  Just read the cited passage for yourself.

-- 
Lew

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Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 03:17 +0200
  Re: Bulk Array Element Allocation, is it faster? Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-09-24 21:37 -0400
    Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 10:39 +0200
  Re: Bulk Array Element Allocation, is it faster? Patricia Shanahan <pats@acm.org> - 2011-09-24 18:41 -0700
    Re: Bulk Array Element Allocation, is it faster? Robert Klemme <shortcutter@googlemail.com> - 2011-09-25 12:57 +0200
      Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 13:38 +0200
        Re: Bulk Array Element Allocation, is it faster? Robert Klemme <shortcutter@googlemail.com> - 2011-09-25 14:16 +0200
          Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 16:04 +0200
            Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 16:11 +0200
              Re: Bulk Array Element Allocation, is it faster? Patricia Shanahan <pats@acm.org> - 2011-09-25 09:30 -0700
            Re: Bulk Array Element Allocation, is it faster? Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-09-25 10:59 -0400
              Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 17:14 +0200
                Re: Bulk Array Element Allocation, is it faster? Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-09-25 15:26 +0000
                Re: Bulk Array Element Allocation, is it faster? Lew <lewbloch@gmail.com> - 2011-09-25 11:02 -0700
                Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 20:19 +0200
                Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 20:21 +0200
                Re: Bulk Array Element Allocation, is it faster? Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-09-25 17:48 -0400
                Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-26 00:10 +0200
  Re: Bulk Array Element Allocation, is it faster? Roedy Green <see_website@mindprod.com.invalid> - 2011-09-24 19:05 -0700
    Re: Bulk Array Element Allocation, is it faster? Lew <lewbloch@gmail.com> - 2011-09-24 23:22 -0700
      Re: Bulk Array Element Allocation, is it faster? Roedy Green <see_website@mindprod.com.invalid> - 2011-09-25 00:23 -0700
        Re: Bulk Array Element Allocation, is it faster? Lew <lewbloch@gmail.com> - 2011-09-25 11:17 -0700
        Re: Bulk Array Element Allocation, is it faster? Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-09-25 14:00 -0500
    Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 10:47 +0200
      Re: Bulk Array Element Allocation, is it faster? Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-09-25 08:07 -0400
        Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 19:21 +0200
          Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 19:41 +0200
          Re: Bulk Array Element Allocation, is it faster? Lew <lewbloch@gmail.com> - 2011-09-25 11:28 -0700
            Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 21:25 +0200
              Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 23:50 +0200
              Re: Bulk Array Element Allocation, is it faster? Lew <lewbloch@gmail.com> - 2011-09-25 15:43 -0700
                Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-26 01:06 +0200
                Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-26 01:21 +0200
                Re: Bulk Array Element Allocation, is it faster? Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-09-26 06:00 +0000
                Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-26 14:52 +0200
                Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-26 15:02 +0200

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