Pass zero-sized array, save allocation?

Question

In this code sample from page 114 of The Well-Grounded Java Developer, the last line:

Update[] updates = lu.toArray(new Update[0]);

contains the note: Pass zero-sized array, save allocation

List<Update> lu = new ArrayList<Update>();
String text = "";
final Update.Builder ub = new Update.Builder();
final Author a = new Author("Tallulah");

for (int i=0; i<256; i++) {
  text = text + "X";
  long now = System.currentTimeMillis();
  lu.add(ub.author(a).updateText(text).createTime(now).build());
  try {
    Thread.sleep(1);
  } catch (InterruptedException e) {
  }
}

Collections.shuffle(lu);
Update[] updates = lu.toArray(new Update[0]);

What allocation is this saving, exactly?

The javadoc for List#toArray(T[] a) mentions:

If the list fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this list.

Which is what I remembered: if the array you pass to toArray(T[] a) can't fit everything in the list, a new array is allocated. Plainly, there are 256 elements in the list, which cannot fit in an array of size 0, therefore a new array must be allocated inside the method, right?

So is that note incorrect? Or is there something else it means?

Actually, it is better, imho, to do lu.toArray(new Update[lu.size()]), but... — fge
– fge, Commented Jan 11, 2013 at 14:56
Probably a typo, and should mean safe allocation? In the sense that it is left up to toArray() to "safely" create the proper-sized and proper-typed array ... — Andreas Fester
– Andreas Fester, Commented Jan 11, 2013 at 15:02
See what J.Bloch has to say on this: informit.com/articles/article.aspx?p=31551&seqNum=5 (at the end of the article) — MicSim
– MicSim, Commented Jan 11, 2013 at 17:03

Peter Lawrey · Accepted Answer · 2013-01-11 16:47:04Z

3

Plainly, there are 256 elements in the list, which cannot fit in an array of size 0, therefore a new array must be allocated inside the method, right?

yes.

You can use

 private static final Update NO_UPDATES = { }

 lu.toArray(NO_UPDATES);

however this will should only help if you expect the list to be typically 0 length.

Generally, I would the same approach as fge

 lu.toArray(new Update[lu.size()]);

In your specific case you know the size in advance so you can do

Update[] updates = new Update[256];
String text = "";
final Update.Builder ub = new Update.Builder();
final Author a = new Author("Tallulah");

long now = System.currentTimeMillis();
for (int i=0; i<updates.length; i++) 
  updates[i] = ub.author(a).updateText(text += 'X').createTime(now++).build();

Collections.shuffle(Arrays.asList(updates));

edited Jan 11, 2013 at 16:47

answered Jan 11, 2013 at 15:02

Peter Lawrey

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2 Comments

Tom Tresansky Over a year ago

The code could (and should) use a properly-sized array, yes. But the question was to explain the note.

Martijn Verburg Over a year ago

I'll grudgingly upvote Peter now ;-) - waves at Peter - great convo the other night BTW, I'm still looking for a slower SSD for those tests!

Tom Tresansky · Accepted Answer · 2013-01-11 19:03:39Z

2

Going off of @Andreas comment on the question, I think it is a typo, and should say:

Pass zero-sized array, safe allocation.

Because if you passed nothing to the method, you'll end up calling the List#toArray() no-argument overload!

This would return an Object[] (though it would contain nothing but Update instances) and would require changing the type of the updates variable, so the last line would become:

Object[] updates = lu.toArray();

And then every time you wanted to iterate over and use the elements in that array, you'd have to cast them to Update.

Supplying the array calls the List#toArray(T[] a) method, which returns a <T> T[]. This array is reified to know it is an array of Update instances.

So supplying an empty array of Updates results in an Update[] coming back from the toArray call, not an Object[]. This is a much more type-safe allocation! The word "save" in the note must be a typo!

...this consumed way too much mental effort. Will post link to this in the book's forums so they can correct it.

answered Jan 11, 2013 at 19:03

Tom Tresansky

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3 Comments

Tom Tresansky Over a year ago

Link to post at manning's site: manning-sandbox.com/thread.jspa?threadID=54973&stqc=true

Martijn Verburg Over a year ago

Hi! I'm the co-author - thanks for picking up this type - indeed it should say safe - hope you're enjoying the rest of the book!

Tom Tresansky Over a year ago

Wow, was not expecting that! Am definitely enjoying the book!

irreputable · Accepted Answer · 2013-01-11 15:15:01Z

0

It saves allocation, comparing to toArray(new Update[255]) or toArray(new Update[1000])

answered Jan 11, 2013 at 15:15

irreputable

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1 Comment

Tom Tresansky Over a year ago

Can you explain further?. I don't think it does save anything. An array of size 255 is too small, and thus would have to be re-allocated inside the method call. An array of 1000 would not have to be allocated, but wastes 744 reference slots in memory. An array of 256 is the proper size to alloc, per @Peter Lawrey's answer, however this does not explain the note.

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