Benchmarking of toArray

ToArrayBench.java

@State(Scope.Benchmark)
public class ToArrayBench {

    ArrayList<Byte> list;

    @Setup
    public void setup() throws Throwable
    {
        list = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            list.add((byte) i);
        }
    }

    @Benchmark
    public void zero_sized_array()
    {
        list.toArray(new Byte[0]);
    }

    @Benchmark
    public void simple_toArray()
    {
        list.toArray();
    }

    @Benchmark
    public void sized_array_from_list()
    {
        list.toArray(new Byte[list.size()]);
    }

    @Benchmark
    public void sized_array_fixed_size()
    {
        list.toArray(new Byte[100]);
    }

    @Benchmark
    public void defensive_copy()
    {
        new ArrayList<>(list);
    }

    public static void main(String[] args) throws RunnerException, IOException
    {
        Options opt = new OptionsBuilder().include(".*" + ToArrayBench.class.getSimpleName() + ".*")
                .warmupIterations(20)
                .warmupTime(TimeValue.seconds(1))
                .measurementIterations(20)
                .timeUnit(TimeUnit.MILLISECONDS)
                .forks(1)
                // .addProfiler(LinuxPerfProfiler.class)
                .build();

        new Runner(opt).run();
    }
}

No idea either, can someone please elaborate on that ?

Updated:

@State(Scope.Benchmark)
public class ToArrayBench {

    private static final int SIZE = 100;

    ArrayList<Byte> list;

    @Setup
    public void setup() throws Throwable
    {
        list = new ArrayList<>();
        for (int i = 0; i < SIZE; i++) {
            list.add((byte) i);
        }
    }

    @Benchmark
    public void zero_sized_array(Blackhole bh)
    {
        bh.consume(list.toArray(new Byte[0]));
    }

    @Benchmark
    public void simple_toArray(Blackhole bh)
    {
        bh.consume(list.toArray());
    }

    @Benchmark
    public void sized_array_from_list(Blackhole bh)
    {
        bh.consume(list.toArray(new Byte[list.size()]));
    }

    @Benchmark
    public void sized_array_fixed_size(Blackhole bh)
    {
        bh.consume(list.toArray(new Byte[SIZE]));
    }

    @Benchmark
    public void defensive_copy(Blackhole bh)
    {
        bh.consume(new ArrayList<>(list));
    }

    public static void main(String[] args) throws RunnerException, IOException
    {
        Options opt = new OptionsBuilder().include(".*" + ToArrayBench.class.getSimpleName() + ".*")
                .warmupIterations(20)
                .warmupTime(TimeValue.seconds(1))
                .measurementIterations(20)
                .timeUnit(TimeUnit.MILLISECONDS)
                .forks(1)
//                .addProfiler(LinuxPerfAsmProfiler.class)
                .build();

        new Runner(opt).run();
    }
}

Benchmark	Mode	Cnt	Score	Error	Units
ToArrayBench.defensive_copy	thrpt	200	16 714 192	± 129515,217	ops/s
ToArrayBench.simple_toArray	thrpt	200	17 918 950	± 102801,298	ops/s
ToArrayBench.sized_array_fixed_size	thrpt	200	5 799 136	± 65921,564	ops/s
ToArrayBench.sized_array_from_list	thrpt	200	5 643 162	± 85215,009	ops/s
ToArrayBench.zero_sized_array	thrpt	200	6 529 068	± 78960,062	ops/s

Routinely, I will chew on people who can't use perfasm profiler, but this is not your fault it wasn't helping here. ;) Only in JMH 1.5+ (released yesterday) perfasm can decode the VM stubs, and VM stubs are the crucial piece of info to untangle this. See: http://cr.openjdk.java.net/~shade/scratch/ToArrayBench.java