Question

The javascript code that is generated by GWT for HashMap.iterator() and HashSet.iterator() are very very slow. Is there a way to speed it up?

Answer 1

Very very slow? You’ll have to be a little bit more specific. Here’s a little example that sums up 100000 numbers:

final int N = 100000;

final long startInsert = System.currentTimeMillis();

final HashSet<Integer> set = new HashSet<Integer>(N);
for (int i = 0; i < N; i ++)
  set.add(i);
final long stopInsert = System.currentTimeMillis();

RootPanel.get().add(new Label(
    "Time to insert: " + (stopInsert - startInsert) + "ms"));

final long startIterate = System.currentTimeMillis();
final Iterator<Integer> iterator = set.iterator();

int sum = 0;
while (iterator.hasNext()) {
  final Integer integer = iterator.next();
  sum += integer;
}

final long stopIterate = System.currentTimeMillis();
RootPanel.get().add(new Label("Sum: " + sum + 
    ", Time to iterate: " + (stopIterate - startIterate) + "ms"));

Let’s try this on a Core2 Duo:

Here’s the output in Firefox 15 (compiled mode):

Time to insert: 490ms
Sum: 4999950000, Time to iterate: 766ms 

And Chrome 21 (compiled mode):

Time to insert: 130ms
Sum: 4999950000, Time to iterate: 105ms

That’s slower than in dev mode:

Firefox 15 (dev mode):

Time to insert: 16ms
Sum: 704982704, Time to iterate: 12ms

Chrome 21 (dev mode):

Time to insert: 59ms
Sum: 704982704, Time to iterate: 10ms

But considering, that this is JavaScript vs Java, the results are actually pretty good.

_{(BTW, in case somebody wonders why the sum is different in Java (704982704) compared to JavaScript (4999950000)… this is expected, see https://developers.google.com/web-toolkit/doc/latest/DevGuideCodingBasicsCompatibility#language)}