[SillyUsername]

I have seen it for millions of add/del operations, an analytics framework actually for a big American games company (first guess and you'll probably say it), which is where I originally did the analysis about 10 years ago.

I've also written a a video processor around that time too that was bottle necked using ArrayLists - typically a decode, store and read once op. It was at this point I looked at other collections, other list implementations and blocking deques (ArrayList was the wrong collection type to use, but I'd been in a rush for MVP) and ultimately came across https://github.com/conversant/disruptor and used that instead.

The ArrayList Vs Linkedlist was a real eye opener for me in two different systems this same behaviour was replicated when using ArrayLists like queues or incorrect sizing of the buffer increments as load increases.

[stopthe]

Of course, deletion is a whole different story. I was talking about addition in isolation.

Anyway, I felt I had to run the benchmarks myself.

  @Benchmark
  @Fork(1)
  @BenchmarkMode(Mode.Throughput)
  @OutputTimeUnit(TimeUnit.SECONDS)
  public Object arrayListPreallocAddMillionNulls() {
    ArrayList<Object> arrList = new ArrayList<>(1048576);
    for (int i = 0; i <= 1_000_000; i++) {
      arrList.add(null);
    }
    return arrList;
  }

  @Benchmark
  @Fork(1)
  @BenchmarkMode(Mode.Throughput)
  @OutputTimeUnit(TimeUnit.SECONDS)
  public Object arrayListAddMillionNulls() {
    ArrayList<Object> arrList = new ArrayList<>();
    for (int i = 0; i <= 1_000_000; i++) {
      arrList.add(null);
    }
    return arrList;
  }

  @Benchmark
  @Fork(1)
  @BenchmarkMode(Mode.Throughput)
  @OutputTimeUnit(TimeUnit.SECONDS)
  public Object linkedListAddMillionNulls() {
    LinkedList<Object> linkList = new LinkedList<>();
    for (int i = 0; i <= 1_000_000; i++) {
      linkList.add(null);
    }
    return linkList;
  }

And as I expected, on JDK 8 ArrayList with an appropriate initial capacity was faster than LinkedList. Admittedly not an order of magnitude difference, only 1.7x.

  JDK8
  Benchmark                                      Mode  Cnt    Score    Error  Units
  MyBenchmark.arrayListAddMillionNulls          thrpt    5  229.950 ±  9.994  ops/s
  MyBenchmark.arrayListPreallocAddMillionNulls  thrpt    5  344.116 ±  7.070  ops/s
  MyBenchmark.linkedListAddMillionNulls         thrpt    5  199.446 ± 15.910  ops/s

But! On JDK 17 the situation is completely upside-down:

  JDK17
  Benchmark                                      Mode  Cnt    Score    Error  Units
  MyBenchmark.arrayListAddMillionNulls          thrpt    5   90.462 ± 18.576  ops/s
  MyBenchmark.arrayListPreallocAddMillionNulls  thrpt    5  214.079 ± 15.505  ops/s
  MyBenchmark.linkedListAddMillionNulls         thrpt    5  216.796 ± 19.392  ops/s

I wonder why ArrayList with default initial capacity got so much worse. Worth investigating further.

[SillyUsername]

Thanks for taking the time to test.

This helps prove my point that adds (and deletes) are generally faster by default when not pre sizing, or removing.

Typically (in my experience) ArrayLists are used without thought to sizing, often because initial capacity and amount to resize, cannot be determined sensibly or consistently.

If in your example you were also to resize the lists, (perhaps adding then dropping those in the Fibonacci sequence?), it would help prove my statement further.

Certainly not worth the -2 points I got from making the statement, but hey you can "please some people some of the time..." :D