Java .forEach(list::add) vs .collect(Collectors.toList())

Question

What's faster?

List<E> bar = new ArrayList<>();
pan.stream() /* other functions */.forEach(bar::add);

or

List<E> bar = pan.stream() /* other functions */.collect(Collectors.toList());

Answer 1

I've tested these two scenarios for a list of size 1 mln. Overall there is almost no difference for a sequential stream but there is a difference for a parallel stream:

Benchmark                    Mode  Cnt  Score    Error   Units
Performance.collect          avgt  200  0.022 ±  0.001   s/op
Performance.forEach          avgt  200  0.021 ±  0.001   s/op
Performance.collectParallel  avgt  200  0.124 ±  0.004   s/op
Performance.forEachParallel  avgt  200  0.131 ±  0.001   s/op

In my opinion you shouldn't make a list using forEach because it breaks function purity rule and also collect is more efficient while using with a parallel stream.

@Benchmark @BenchmarkMode(Mode.AverageTime)
public void collect(Blackhole blackhole) {
    Stream<Double> stream = Stream.iterate(0.0, e -> Math.random());
    List<Double> list = stream.limit(1000000).collect(Collectors.toList());
    blackhole.consume(list);
}

@Benchmark @BenchmarkMode(Mode.AverageTime)
public void forEach(Blackhole blackhole) {
    Stream<Double> stream1 = Stream.iterate(0.0, e -> Math.random());
    List<Double> list = new ArrayList<>();
    stream1.limit(1000000).forEach(e -> list.add(e));
    blackhole.consume(list);
}

@Benchmark @BenchmarkMode(Mode.AverageTime)
public void collectParallel(Blackhole blackhole) {
    Stream<Double> stream = Stream.iterate(0.0, e -> Math.random());
    List<Double> list = stream.parallel().limit(1000000).collect(Collectors.toList());
    blackhole.consume(list);
}

@Benchmark @BenchmarkMode(Mode.AverageTime)
public void forEachParallel(Blackhole blackhole) {
    Stream<Double> stream1 = Stream.iterate(0.0, e -> Math.random());
    List<Double> list = Collections.synchronizedList(new ArrayList<>());
    stream1.parallel().limit(1000000).forEach(e -> list.add(e));
    blackhole.consume(list);
}