Re: Array of a Class Type that contains an array where the sizes of the array are not known at compile time.

Newsgroups	comp.lang.java.help
Date	2013-01-14 20:04 -0800
References	<f2b43fa0-1da2-4dc4-981b-bb756186050b@googlegroups.com> <atKIs.36232$TH3.9358@newsfe12.iad> <fcf46194-fde7-4096-b2d1-490586ccb83a@googlegroups.com> <kd1v4d$u9n$1@dont-email.me> <2c39444f-5585-4373-a851-6c5aeefa21b3@googlegroups.com>
Message-ID	<9e7b5131-a40b-4487-9173-c8c886a99b9d@googlegroups.com> (permalink)
Subject	Re: Array of a Class Type that contains an array where the sizes of the array are not known at compile time.
From	GCRhoads@volcanomail.com

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On Monday, January 14, 2013 11:00:14 PM UTC-5, gcrh...@volcanomail.com wrote:
> On Monday, January 14, 2013 4:59:38 PM UTC-5, Eric Sosman wrote:
> 
> > On 1/14/2013 4:36 PM, GCRhoads@volcanomail.com wrote:
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> > 
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> > >[...]
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> > 
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> > > Sure I can write.
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> > 
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> > >
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> > 
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> > > conf = new Config[ m ]
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> > 
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> > > for (int i=0; i < m; i++)
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> > 
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> > >     conf[i] = new Config( n );
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> > 
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> > >
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> > 
> 
> > > but I need to ensure that the memory returned by the successive calls to
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> > 
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> > > "new Config( n )" occupy contiguous memory locations.  I know how to do this in
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> > 
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> > > C or C++ but the solution uses pointer arithmetic which Java doesn't have.
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> > 
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> > >
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> > 
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> > > How can I do this in Java?
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> > 
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> > 
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> > 
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> >      You cannot.
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> > 
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> >      To begin with, the memory location of a particular object is
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> > 
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> > not necessarily constant during the object's lifetime.  The garbage
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> > 
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> > collector may move it from place to place, may even move it more
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> > 
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> > than once.  So even if you somehow managed to get two instances to
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> > 
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> > occupy adjacent chunks of memory it wouldn't last: They'd wander.
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> > 
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> > 
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> >      Second, if two objects' memory areas happen to be adjacent at
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> > 
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> > some time, how could you detect it?  Or, how could you detect that
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> > 
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> > they are not adjacent?  Can you think of *any* Java code that would
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> > 
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> > behave differently if objects X and Y were or were not adjacent?
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> > 
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> > If not, why do you care where the objects happen to reside?
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> > 
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> > -- 
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> > 
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> > Eric Sosman
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> > 
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> > esosman@comcast-dot-net.invalid
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> 
> 
> I care because it has a large effect on the efficiency of the program.  With contiguous memory locations, the program's memory accesses will have a very high locality of reference resulting in a much faster average memory access time due to the computer's memory cache.

Also, garbage collection shouldn't be a problem in this case because there won't be any.  All the memory gets allocated at the beginning of the program and everything remains accessible throughout the entire execution of the program.

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