[PATCH] Enhancement proposal for java.util.StringJoiner

[Сергей Цыпанов]

Oh, I was unaware of it! Thanks!

13.02.2020, 21:49, "Brent Christian" <brent.christian at oracle.com>:
> As a point of interest, some investigation of updating StringJoiner for
> CompactStrings was done a while back.
>
> See https://bugs.openjdk.java.net/browse/JDK-8148937
>
> -Brent
>
> On 2/3/20 2:38 PM, Сергей Цыпанов wrote:
>>  Hello,
>>
>>  as of JDK14 java.util.StringJoiner still uses char[] as a storage of glued Strings.
>>
>>  This applies for the cases when all joined Strings as well as delimiter, prefix and suffix contain only ASCII symbols.
>>
>>  As a result when StringJoiner.toString() is invoked, byte[] stored in String is inflated in order to fill in char[] and
>>  finally char[] is compressed when constructor of String is called:
>>
>>  String delimiter = this.delimiter;
>>  char[] chars = new char[this.len + addLen];
>>  int k = getChars(this.prefix, chars, 0);
>>  if (size > 0) {
>>       k += getChars(elts[0], chars, k); // inflate byte[] -> char[]
>>
>>       for(int i = 1; i < size; ++i) {
>>           k += getChars(delimiter, chars, k);
>>           k += getChars(elts[i], chars, k);
>>       }
>>  }
>>
>>  k += getChars(this.suffix, chars, k);
>>  return new String(chars); // compress char[] -> byte[]
>>
>>  This can be improved by detecting cases when String.isLatin1() returns true for all involved Strings.
>>
>>  I've prepared a patch along with benchmark proving that this change is correct and brings improvement.
>>  The only concern I have is about String.isLatin1(): as far as String belongs to java.lang and StringJoiner to java.util
>>  package-private String.isLatin1() cannot be directly accessed, we need to make it public for successful compilation.
>>
>>  Another solution is to create an intermediate utility class located in java.lang which delegates the call to String.isLatin1():
>>
>>  package java.lang;
>>
>>  public class StringHelper {
>>       public static boolean isLatin1(String str) {
>>           return str.isLatin1();
>>       }
>>  }
>>
>>  This allows to keep java.lang.String intact and have access to it's package-private method outside of java.lang package.
>>
>>  Below I've added results of benchmarking for specified case (all Strings are Latin1). The other case (at least one String is UTF-8) uses existing code so there will be only a tiny regression due to several if-checks.
>>
>>  With best regards,
>>  Sergey Tsypanov
>>
>>                                             (count) (length) Original Patched Units
>>  stringJoiner 1 1 26.7 ± 1.3 38.2 ± 1.1 ns/op
>>  stringJoiner 1 5 27.4 ± 0.0 40.5 ± 2.2 ns/op
>>  stringJoiner 1 10 29.6 ± 1.9 38.4 ± 1.9 ns/op
>>  stringJoiner 1 100 61.1 ± 6.9 47.6 ± 0.6 ns/op
>>  stringJoiner 5 1 91.1 ± 6.7 83.6 ± 2.0 ns/op
>>  stringJoiner 5 5 96.1 ± 10.7 85.6 ± 1.1 ns/op
>>  stringJoiner 5 10 105.5 ± 14.3 84.7 ± 1.1 ns/op
>>  stringJoiner 5 100 266.6 ± 30.1 139.6 ± 14.0 ns/op
>>  stringJoiner 10 1 190.7 ± 23.0 162.0 ± 2.9 ns/op
>>  stringJoiner 10 5 200.0 ± 16.9 167.5 ± 11.0 ns/op
>>  stringJoiner 10 10 216.4 ± 12.4 164.8 ± 1.7 ns/op
>>  stringJoiner 10 100 545.3 ± 49.7 282.2 ± 12.0 ns/op
>>  stringJoiner 100 1 1467.0 ± 90.3 1302.0 ± 18.5 ns/op
>>  stringJoiner 100 5 1491.8 ± 166.2 1493.0 ± 135.4 ns/op
>>  stringJoiner 100 10 1768.8 ± 160.6 1760.8 ± 111.4 ns/op
>>  stringJoiner 100 100 3654.3 ± 113.1 3120.9 ± 175.9 ns/op
>>
>>  stringJoiner:·gc.alloc.rate.norm 1 1 120.0 ± 0.0 120.0 ± 0.0 B/op
>>  stringJoiner:·gc.alloc.rate.norm 1 5 128.0 ± 0.0 120.0 ± 0.0 B/op
>>  stringJoiner:·gc.alloc.rate.norm 1 10 144.0 ± 0.0 136.0 ± 0.0 B/op
>>  stringJoiner:·gc.alloc.rate.norm 1 100 416.0 ± 0.0 312.0 ± 0.0 B/op
>>  stringJoiner:·gc.alloc.rate.norm 5 1 144.0 ± 0.0 136.0 ± 0.0 B/op
>>  stringJoiner:·gc.alloc.rate.norm 5 5 200.0 ± 0.0 168.0 ± 0.0 B/op
>>  stringJoiner:·gc.alloc.rate.norm 5 10 272.0 ± 0.0 216.0 ± 0.0 B/op
>>  stringJoiner:·gc.alloc.rate.norm 5 100 1632.0 ± 0.0 1128.0 ± 0.0 B/op
>>  stringJoiner:·gc.alloc.rate.norm 10 1 256.0 ± 0.0 232.0 ± 0.0 B/op
>>  stringJoiner:·gc.alloc.rate.norm 10 5 376.0 ± 0.0 312.0 ± 0.0 B/op
>>  stringJoiner:·gc.alloc.rate.norm 10 10 520.0 ± 0.0 408.0 ± 0.0 B/op
>>  stringJoiner:·gc.alloc.rate.norm 10 100 3224.1 ± 0.0 2216.1 ± 0.0 B/op
>>  stringJoiner:·gc.alloc.rate.norm 100 1 1760.2 ± 14.9 1544.2 ± 0.0 B/op
>>  stringJoiner:·gc.alloc.rate.norm 100 5 2960.3 ± 14.9 2344.2 ± 0.0 B/op
>>  stringJoiner:·gc.alloc.rate.norm 100 10 4440.4 ± 0.0 3336.3 ± 0.0 B/op
>>  stringJoiner:·gc.alloc.rate.norm 100 100 31449.3 ± 12.2 21346.7 ± 14.7 B/op