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automatically initialized to null?

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  automatically initialized to null? bob smith <bob@coolfone.comze.com> - 2012-08-08 12:23 -0700
    Re: automatically initialized to null? markspace <-@.> - 2012-08-08 13:20 -0700
      Re: automatically initialized to null? Mike Winter <usenet@michael-winter.me.invalid> - 2012-08-08 21:32 +0100
        Re: automatically initialized to null? Lew <lewbloch@gmail.com> - 2012-08-08 17:36 -0700
          Re: automatically initialized to null? Jim Gibson <jimsgibson@gmail.com> - 2012-08-08 18:06 -0700
      Re: automatically initialized to null? bob smith <bob@coolfone.comze.com> - 2012-08-09 09:12 -0700
        Re: automatically initialized to null? Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-08-09 14:34 -0400
          Re: automatically initialized to null? bob smith <bob@coolfone.comze.com> - 2012-08-10 08:53 -0700
    Re: automatically initialized to null? Roedy Green <see_website@mindprod.com.invalid> - 2012-08-09 02:38 -0700

#17431 — automatically initialized to null?

I always forget stuff like this…

If I have

Bitmap[] bmps = new Bitmap[256];

Are all 256 positions automatically initialized to null?

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#17436

On 8/8/2012 12:23 PM, bob smith wrote:
> I always forget stuff like this…
>
> If I have
>
> Bitmap[] bmps = new Bitmap[256];
>
> Are all 256 positions automatically initialized to null?
>


Yes.  I'm sure that's in the JLS someplace.  And just about every noob 
tutorial and "Learn Java" book.  But yes, all uninitialized static or 
instance fields and all arrays are set to null.  Note this excludes 
local variables.

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#17437

On 08/08/2012 21:20, markspace wrote:
> On 8/8/2012 12:23 PM, bob smith wrote:
>> I always forget stuff like this…
>>
>> If I have
>>
>> Bitmap[] bmps = new Bitmap[256];
>>
>> Are all 256 positions automatically initialized to null?
>
> Yes.  I'm sure that's in the JLS someplace.

"[If] there is no array initializer, and [...] a single [array
dimension expression] appears, a one-dimensional array is created
of the specified length, and each component of the array is
initialized to its default value (§4.12.5)."
       -- 15.10.1 Run-time evaluation of Array Creation Expressions

"For all reference types (§4.3), the default value is null."
                              -- 4.12.5 Initial Values of Variables

:-)

-- 
Mike Winter
Replace ".invalid" with ".uk" to reply by e-mail.

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#17442

Mike Winter wrote:
> markspace wrote:
>> bob smith wrote:
>>> I always forget stuff like this…
>>>
>>> If I have
>>>
>>> Bitmap[] bmps = new Bitmap[256];
>>>
>>> Are all 256 positions automatically initialized to null?
> 
>> Yes.  I'm sure that's in the JLS someplace.
> 
> "[If] there is no array initializer, and [...] a single [array
> dimension expression] appears, a one-dimensional array is created
> of the specified length, and each component of the array is
> initialized to its default value (§4.12.5)."
>        -- 15.10.1 Run-time evaluation of Array Creation Expressions
> 
> "For all reference types (§4.3), the default value is null."
>                               -- 4.12.5 Initial Values of Variables
> 
> :-)

See? The JLS is easy to use and very authoritative, and highly useful.

Pllptpptt! to the naysayers.

-- 
Lew

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#17443

In article <90ca1dd2-f3eb-49d8-882c-de7f7a8e5d42@googlegroups.com>, Lew
<lewbloch@gmail.com> wrote:

> Mike Winter wrote:
> > markspace wrote:
> >> bob smith wrote:
> >>> I always forget stuff like thisŠ
> >>>
> >>> If I have
> >>>
> >>> Bitmap[] bmps = new Bitmap[256];
> >>>
> >>> Are all 256 positions automatically initialized to null?
> > 
> >> Yes.  I'm sure that's in the JLS someplace.
> > 
> > "[If] there is no array initializer, and [...] a single [array
> > dimension expression] appears, a one-dimensional array is created
> > of the specified length, and each component of the array is
> > initialized to its default value (§4.12.5)."
> >        -- 15.10.1 Run-time evaluation of Array Creation Expressions
> > 
> > "For all reference types (§4.3), the default value is null."
> >                               -- 4.12.5 Initial Values of Variables
> > 
> > :-)
> 
> See? The JLS is easy to use and very authoritative, and highly useful.

Not really. That extract of paragraph 15.10.1 is embedded in a long
section discussing the allocation of arrays. It is statement 9 of 11 as
quoted here:


"15.10.1. Run-time Evaluation of Array Creation Expressions

At run-time, evaluation of an array creation expression behaves as
follows:

  € If there are no dimension expressions, then there must be an array
initializer. 

  A newly allocated array will be initialized with the values provided
by the array initializer as described in §10.6. 

  The value of the array initializer becomes the value of the array
creation expression. 

  € Otherwise, there is no array initializer, and: 

    - First, the dimension expressions are evaluated, left-to-right. If
any of the expression evaluations completes abruptly, the expressions
to the right of it are not evaluated. 

    - Next, the values of the dimension expressions are checked. If the
value of any DimExpr expression is less than zero, then a
NegativeArraySizeException is thrown. 

    - Next, space is allocated for the new array. If there is
insufficient space to allocate the array, evaluation of the array
creation expression completes abruptly by throwing an OutOfMemoryError.


    - Then, if a single DimExpr appears, a one-dimensional array is
created of the specified length, and each component of the array is
initialized to its default value (§4.12.5). 

    - Otherwise, if n DimExpr expressions appear, then array creation
effectively executes a set of nested loops of depth n-1 to create the
implied arrays of arrays. 

A multidimensional array need not have arrays of the same length at
each level."

From:
<http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.10.
1>


If you think that text is easy to find and parse to figure out how
array elements are initialized, then my hat is off to you. I was unable
to find it by scanning the table of contents. I would have expected it
to be in Section 10: Arrays


> Pllptpptt! to the naysayers.

Back at you. :)

-- 
Jim Gibson

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#17459

On Wednesday, August 8, 2012 3:20:08 PM UTC-5, markspace wrote:
> On 8/8/2012 12:23 PM, bob smith wrote:
> 
> > I always forget stuff like this…
> 
> >
> 
> > If I have
> 
> >
> 
> > Bitmap[] bmps = new Bitmap[256];
> 
> >
> 
> > Are all 256 positions automatically initialized to null?
> 
> >
> 
> 
> 
> 
> 
> Yes.  I'm sure that's in the JLS someplace.  And just about every noob 
> 
> tutorial and "Learn Java" book.  But yes, all uninitialized static or 
> 
> instance fields and all arrays are set to null.  Note this excludes 
> 
> local variables.

Why does Eclipse seem to frequently tell me to explicitly initialize a reference to null?

Is it kind of being paranoid?

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#17464

On 8/9/2012 12:12 PM, bob smith wrote:
> On Wednesday, August 8, 2012 3:20:08 PM UTC-5, markspace wrote:
>> On 8/8/2012 12:23 PM, bob smith wrote:
>>
>>> If I have
>>> Bitmap[] bmps = new Bitmap[256];
>>> Are all 256 positions automatically initialized to null?
>>
>> Yes.  I'm sure that's in the JLS someplace.  And just about every noob
>> tutorial and "Learn Java" book.  But yes, all uninitialized static or
>> instance fields and all arrays are set to null.  Note this excludes
>> local variables.
>
> Why does Eclipse seem to frequently tell me to explicitly initialize a reference to null?
>
> Is it kind of being paranoid?

     Did you see the part about "excludes local variables?"  Maybe
that's what Eclipse is complaining about, and not about class or
instance fields, or array elements.

-- 
Eric Sosman
esosman@ieee-dot-org.invalid

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#17576

On Thursday, August 9, 2012 1:34:21 PM UTC-5, Eric Sosman wrote:
> On 8/9/2012 12:12 PM, bob smith wrote:
> 
> > On Wednesday, August 8, 2012 3:20:08 PM UTC-5, markspace wrote:
> 
> >> On 8/8/2012 12:23 PM, bob smith wrote:
> 
> >>
> 
> >>> If I have
> 
> >>> Bitmap[] bmps = new Bitmap[256];
> 
> >>> Are all 256 positions automatically initialized to null?
> 
> >>
> 
> >> Yes.  I'm sure that's in the JLS someplace.  And just about every noob
> 
> >> tutorial and "Learn Java" book.  But yes, all uninitialized static or
> 
> >> instance fields and all arrays are set to null.  Note this excludes
> 
> >> local variables.
> 
> >
> 
> > Why does Eclipse seem to frequently tell me to explicitly initialize a reference to null?
> 
> >
> 
> > Is it kind of being paranoid?
> 
> 
> 
>      Did you see the part about "excludes local variables?"  Maybe
> 
> that's what Eclipse is complaining about, and not about class or
> 
> instance fields, or array elements.
> 
> 
> 
> -- 
> 
> Eric Sosman
> 
> esosman@ieee-dot-org.invalid

I missed that "excludes local variables" part.  I guess that's probably what it is.

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#17449

On Wed, 8 Aug 2012 12:23:30 -0700 (PDT), bob smith
<bob@coolfone.comze.com> wrote, quoted or indirectly quoted someone
who said :

>Are all 256 positions automatically initialized to null?

see http://mindprod.com/jgloss/array.html
-- 
Roedy Green Canadian Mind Products http://mindprod.com
A new scientific truth does not triumph by convincing its opponents and making them see the light,
but rather because its opponents eventually die, and a new generation grows up that is familiar with it.
~ Max Planck 1858-04-23 1947-10-04 

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