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

Question

什么更快？

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

要么

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

Answer 1

我已經針對大小為100萬的列表測試了這兩種方案。 總體而言，順序流幾乎沒有區別，但並行流卻有區別：

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

我認為您不應該使用forEach列出列表，因為它違反了函數純度規則，並且在與並行流一起使用時收集效率更高。

@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);
}