Question

array list expansion

Asked by: kuntilanak

say I originally have an array list of size 10 and I want to expand it to a size 1000 and copy all the existing elements in the size 10 to the one that is 1000, what's a fast way to do this?

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Asked On
2009-09-16 at 17:45:08ID24738436
Topics

Java Programming Language

,

New to Java Programming

,

J2EE

Participating Experts
3
Points
500
Comments
22

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Answers

 

by: objectsPosted on 2009-09-16 at 17:48:07ID: 25351903

 

by: kuntilanakPosted on 2009-09-16 at 18:55:46ID: 25352218

I am talking about ArrayList here, not just a regular array

 

by: objectsPosted on 2009-09-16 at 19:07:36ID: 25352271

ArrayList's automatically expand, to manually expand it call ensureCapacity()

 

by: kuntilanakPosted on 2009-09-16 at 19:36:46ID: 25352420

that only expands by one right, I want it to expand to say 1000 and then 10000

 

by: objectsPosted on 2009-09-16 at 19:41:44ID: 25352444

no, it expands to whatever size you specify

 

by: kuntilanakPosted on 2009-09-16 at 19:50:26ID: 25352485

and I won't need to copy everything else? just call that?

 

by: kuntilanakPosted on 2009-09-16 at 19:50:50ID: 25352488

so when I do ensureCapacity(1000)

and I do array.size() it will return 1000 right?

 

by: objectsPosted on 2009-09-16 at 19:52:48ID: 25352496

no, size() returns how many elements are actually stored in the list

 

by: kuntilanakPosted on 2009-09-16 at 19:58:09ID: 25352515

well that's what I mean, after I do ensureCapacity(1000) then add all 1000 elements, will I get size equal to 1000?

 

by: objectsPosted on 2009-09-16 at 20:01:38ID: 25352531

yes

 

by: sciuriwarePosted on 2009-09-16 at 22:27:22ID: 25353081

The copying you are talking about is automatically done when there is not
enough space 'after' the expanding ArrayList.
In fact all the smart work was already done before you knew it.
That's the power of the ArrayList.

;JOOP!

 

by: rpnmanPosted on 2009-09-17 at 09:13:55ID: 25357819

You don't even need to call ensureCapacity() first, just add the elements.

 

by: sciuriwarePosted on 2009-09-17 at 09:29:17ID: 25358004

ensureCapacity() is needed when you want to prepare for expansion later:

physical expansion of an ArrayList might give rise to memory expansion and paging.

So, do'nt say you never need it, it has a purpose.

;JOOP!

 

by: kuntilanakPosted on 2009-09-17 at 17:18:06ID: 25362269

okay thanks all

 

by: rpnmanPosted on 2009-09-17 at 22:26:57ID: 25363086

I'll agree that it has its uses, performance wise, I was just pointing out that  is secondary to ArrayList's primary characteristic which is that it can be added to (grow) without having to declare its size in advance. As an earlier post pointed out, the ensureCapacity() call doesn't change the size at all.

 

by: objectsPosted on 2009-09-19 at 20:23:43ID: 25375778

split

 

by: sciuriwarePosted on 2009-09-19 at 22:31:45ID: 25375984

I would almost say: split

 

by: rpnmanPosted on 2009-09-20 at 07:26:56ID: 25377336

split

 

by: rpnmanPosted on 2009-09-26 at 08:10:48ID: 25430032

My recommendation:

objects ID:25352271  250 or 50%
rpnman ID:25357819  150 or 30%
sciuriware ID:25358004 100 or 20%


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