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From: Jan Burse <janburse@fastmail.fm>
Newsgroups: comp.lang.java.programmer
Subject: Re: setSize ArrayList, when will it come?
Date: Wed, 10 Aug 2011 00:36:03 +0200
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Tom Anderson schrieb:
> On Tue, 9 Aug 2011, Patricia Shanahan wrote:
>
>> On 8/9/2011 9:58 AM, Roedy Green wrote:
>> ...
>>> Further, I would hope ArrayList.addAll would be smart enough to grow
>>> the array only once, if needed.
>> ...
>>
>> It does the following:
>>
>> 1. Grow to the needed size.
>>
>> 2. Call the other collection's toArray method.
>>
>> 3. System.arraycopy the toArray result into the ArrayList's elementData.
>>
>> This double copy is going to be faster than the one at a time approach
>> only if large numbers of nulls are being added. If that is the case,
>> the structure is probably too sparse for ArrayList to be a good choice.
>
> It would be nice if ArrayList used a loop to do the copy for small added
> collections; it could cut over to the array method for larger addends.
>
> Anyway, with List.addAll and Collections.nCopies, we can write:
>
> <T> void setSize(List<T> list, int size) {
> int change = size - list.size();
> if (change > 0) {
> list.addAll(Collections.nCopies(change, null));
> }
> else if (change < 0) {
> list.subList(size, list.size()).clear();
> if (list instanceof ArrayList) ((ArrayList)list).trimToSize();
> }
> // else do nothing
> }
>
> I haven't tried that, but it should work.
>
> So, Jan, less whining, more coding, please.
>
> tom
>

This is not efficient. You don't get it what the
problem is. I really really need a highly efficient
setSize() specialized, otherwise my stuff will not work.