java - 流收集与 map 收集

Question

Stream 中有一些方法，特别是在处理可以这样或那样写的数值时。 (同样的问题适用于 average())

那么哪种方法更可取:

DoubleSummaryStatistics result;

result = stream()
        .collect( Collectors.summarizingDouble( weighter::weight ) );

对比

result = stream()
        .mapToDouble( weighter::weight )
        .summaryStatistics();

为什么？

(如我所见，第一个的优点是每个元素只“访问”一次，而第二个具有更清晰的语义，但至少访问每个元素两次。但这是否重要/正确？)

Answer 1

在性能方面，似乎第二种方法(映射然后总结)比第一种方法(使用收集器)更快:

Benchmark                         (n)  Mode  Samples     Score     Error  Units
c.a.p.SO26775395.collector         10  avgt       10     0.110 ±   0.004  us/op
c.a.p.SO26775395.collector       1000  avgt       10     9.134 ±   0.310  us/op
c.a.p.SO26775395.collector    1000000  avgt       10  9091.649 ± 274.113  us/op
c.a.p.SO26775395.summary           10  avgt       10     0.110 ±   0.003  us/op
c.a.p.SO26775395.summary         1000  avgt       10     5.593 ±   0.234  us/op
c.a.p.SO26775395.summary      1000000  avgt       10  5598.776 ± 153.314  us/op

基准代码:

@State(Scope.Thread)
@BenchmarkMode(Mode.AverageTime)
public class SO26775395 {

  @Param({"10", "1000", "1000000"}) int n;
  List<Weighter> weights;

  @Setup public void setup() {
    weights = new Random().doubles(n)
            .mapToObj(Weighter::new)
            .collect(toList());
  }

  @Benchmark public DoubleSummaryStatistics collector() {
    return weights.stream().collect(Collectors.summarizingDouble(Weighter::w));
  }

  @Benchmark public DoubleSummaryStatistics summary() {
    return weights.stream().mapToDouble(Weighter::w).summaryStatistics();
  }

  public static class Weighter {
    private final double w;
    public Weighter(double w) { this.w = w; }
    public double w() { return w; }
  }

}