Re: [PR] improve filtering performance [ant]

Why is the initial capacity to the ArrayList constructor commented out.
IIRC, the default capacity is 10, so in your test case, you are getting hit
with a reallocation each time. What is the metrics if you do not comment
out STRINGS.length to the ArrayList ctor?

--ewh


On Sat, May 31, 2025, 9:16 AM esaulpaugh (via GitHub) <[email protected]>
wrote:

>
> esaulpaugh commented on PR #212:
> URL: https://github.com/apache/ant/pull/212#issuecomment-2925232549
>
>    I have not faced a performance issue but I discovered this trick while
> optimizing a String parser in one of my projects. It performs better in
> microbenchmarks compared to building an `ArrayList`. I suggested it to gson
> as well: https://github.com/google/gson/pull/2734
>
>    On corretto-11.0.27 aarch64:
>
>    ```
>    Benchmark     Mode  Cnt   Score    Error  Units
>    Measure.arr   avgt   14  17.527 ±  0.435  ns/op
>    Measure.list  avgt   14  44.013 ± 27.030  ns/op
>    ```
>
>    On graalvm-jdk-24+36.1 (24.2.0) aarch64:
>
>    ```
>    Benchmark     Mode  Cnt   Score   Error  Units
>    Measure.arr   avgt   14   9.795 ± 0.173  ns/op
>    Measure.list  avgt   14  41.321 ± 3.544  ns/op
>    ```
>
>    ```java
>    @State(Scope.Benchmark)
>    @Fork(2)
>    @BenchmarkMode(Mode.AverageTime)
>    @Warmup(iterations = 7, time = 500, timeUnit = TimeUnit.MILLISECONDS)
>    @Measurement(iterations = 7, time = 500, timeUnit =
> TimeUnit.MILLISECONDS)
>    @OutputTimeUnit(TimeUnit.NANOSECONDS)
>    public class Measure {
>
>      private static final String[] STRINGS = { null, "", "00", "\0",
> "...", null, "H", "ab", "+", "UUU", null, null, "5\t5" };
>
>      @Benchmark
>      public void list(Blackhole blackhole) {
>          ArrayList<String> list = new ArrayList<>(/* STRINGS.length */);
>          for (String s : STRINGS) {
>              if (s != null) {
>                  list.add(s);
>              }
>          }
>          blackhole.consume(list.toArray(new String[0]));
>      }
>
>      @Benchmark
>      public void arr(Blackhole blackhole) {
>          String[] arr = new String[STRINGS.length];
>          int i = 0;
>          for (String s : STRINGS) {
>              if (s != null) {
>                  arr[i++] = s;
>              }
>          }
>          blackhole.consume(Arrays.copyOf(arr, i));
>      }
>    }
>    ```
>
>    It's unlikely that this change alone would make a noticeable difference
> to users, but small changes can add up eventually, in my experience.
>
>
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