RFR: 8283346: Optimize observable ArrayList creation in FXCollections [v5]

[Marius Hanl]

> This simple PR optimizes the observable `ArrayList` creation by using the ArrayList constructor/array size so that the underlying array will be initialized at the correct size which will speed up the creation as the array does not need to grow as a result of the `addAll` call.
> 
> I also added tests which will succeed before and after to verify that nothing got broken by this change.
> Also I made a benchmark test. Results:
> 
> | Benchmark | Mode| Cnt | Score | Error | Units
> | ------------- | ------------- | ------------- | ------------- | ------------- | ------------- |
> | ListBenchmark OLD  | thrpt | 25 | 722,842 | ± 26,93 | ops/s
> | ListBenchmark NEW | thrpt  | 25 | 29262,274 | ± 2088,712 | ops/s
> 
> Edit: I also made a synthetic benchmark by measuring the same code below 100 times with `System.nanoTime`.
> ListBenchmark OLD (avg): 21-23ms
> ListBenchmark NEW (avg): 2 ms
> 
> <details><summary>Benchmark code</summary>
> 
> 
> import javafx.collections.FXCollections;
> import javafx.collections.ObservableList;
> import org.openjdk.jmh.annotations.Benchmark;
> import org.openjdk.jmh.annotations.Scope;
> import org.openjdk.jmh.annotations.Setup;
> import org.openjdk.jmh.annotations.State;
> import org.openjdk.jmh.annotations.TearDown;
> 
> import java.util.ArrayList;
> import java.util.List;
> 
> @State(Scope.Benchmark)
> public class ListBenchmark {
> 
>     List<String> strings;
> 
>     @Setup
>     public void setup() {
>         strings = new ArrayList<>();
>         for(int i = 0; i< 100000;i++) {
>             strings.add("abc: " + i);
>         }
>     }
> 
>     @TearDown
>     public void tearDown() {
>         strings = null;
>     }
> 
>     @Benchmark
>     public ObservableList<String> init() {
>         return FXCollections.observableArrayList(strings);
>     }
> }
> 
> 
> </details>

Marius Hanl has updated the pull request incrementally with two additional commits since the last revision:

 - 8283346: Added null to the test data
 - 8283346: Refactoring: Using the ArrayList constructor directly

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/758/files
  - new: https://git.openjdk.org/jfx/pull/758/files/3f288403..28fbd344

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=758&range=04
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=758&range=03-04

  Stats: 6 lines in 2 files changed: 0 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/jfx/pull/758.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/758/head:pull/758

PR: https://git.openjdk.org/jfx/pull/758