相关疑难解决方法(0)

JDK 正在引入一个Stream.toList()带有JDK-8180352的 API 。这是一个基准代码，我试图将其性能与现有的进行比较Collectors.toList：

@BenchmarkMode(Mode.All)
@Fork(1)
@State(Scope.Thread)
@Warmup(iterations = 20, time = 1, batchSize = 10000)
@Measurement(iterations = 20, time = 1, batchSize = 10000)
public class CollectorsVsStreamToList {

    @Benchmark
    public List<Integer> viaCollectors() {
        return IntStream.range(1, 1000).boxed().collect(Collectors.toList());
    }

    @Benchmark
    public List<Integer> viaStream() {
        return IntStream.range(1, 1000).boxed().toList();
    }
}

结果总结如下：

Benchmark                                                       Mode  Cnt   Score    Error  Units
CollectorsVsStreamToList.viaCollectors                         thrpt   20  17.321 ±  0.583  ops/s
CollectorsVsStreamToList.viaStream                             thrpt   20  23.879 ±  1.682  ops/s
CollectorsVsStreamToList.viaCollectors                          avgt   20   0.057 ± …

toList() 的实现明确返回一个 ArrayList，它确实保证了可变性：

public static <T>
Collector<T, ?, List<T>> toList() {
    return new CollectorImpl<>(ArrayList::new, List::add,
                               (left, right) -> { left.addAll(right); return left; },
                               CH_ID);
}

但是 toList 的 JavaDoc 是这样的：

/**
 * Returns a {@code Collector} that accumulates the input elements into a
 * new {@code List}. There are no guarantees on the type, mutability,
 * serializability, or thread-safety of the {@code List} returned; if more
 * control over the returned {@code List} is required, use {@link #toCollection(Supplier)}.
 *
 * …