Java 8 是否缺少处理可变流的 Stream.concat？

Question

目前我们在 Java 8 中有以下Stream.concat：

public static <T> Stream<T> concat(Stream<? extends T> a, Stream<? extends T> b);

我很惊讶为什么没有版本采用Stream<? extends T> 的可变参数？

目前我的代码如下：

Stream<Integer> resultStream = Stream.concat(stream1, Stream.concat(stream2, Stream.of(element)))
        .filter(x -> x != 0)
        .filter(x -> x != 1)
        .filter(x -> x != 2);

如果此签名的可变参数可用：

public static <T> Stream<T> concat(Stream<? extends T>... streams);

然后我可以更清楚地写成：

Stream<Integer> resultStream = Stream.concat(
                stream1,
                stream2,
                Stream.of(element)
        )
        .filter(x -> x != 0)
        .filter(x -> x != 1)
        .filter(x -> x != 2);

没有各种嵌套的Stream.concat 调用。

或者还有其他没有提供的原因吗？
我想不出这样的原因，因为我们现在无论如何都要完成可变参数调用的工作。

Answer 1

就flatMap吧：

public static void main(final String[] args) throws Exception {
    final Stream<String> stream1 = /*some stream*/
    final Stream<String> stream2 = /*some stream*/
    final Stream<String> stream3 = /*some stream*/
    final Stream<String> stream4 = /*some stream*/
    final Stream<String> stream5 = /*some stream*/

    final Stream<String> stream = Stream.of(stream1, stream2, stream3, stream4, stream5).flatMap(Function.identity());
}

在你的例子中：

Stream<Integer> resultStream = Stream.of(stream1, stream2, Stream.of(element))
        .flatMap(identity())
        .filter(x -> x != 0)
        .filter(x -> x != 1)
        .filter(x -> x != 2);

Answer 2

收集自a message in the thread linked by @RohitJain:

Stream.of(s1, s2, s3, ...)
  /* .parallel() if you want*/
  .reduce(Stream::concat)
  .orElseGet(Stream::empty);

Answer 3

在 Google Guava v21.0+ 中有：

com.google.common.collect.Streams#concat方法

Answer 4

所以，聚合答案 of.flatMap() 与 of.reduce().orElseGet() ：of.flatMap 无法处理无限流，而 of.reduce() - 可以。请看下面的测试示例：

@RunWith(JUnit4.class)
public class StreamConcatTest {

@Rule
public Timeout globalTimeout = Timeout.seconds(3);

private static final Random randomSupplier = new Random(System.currentTimeMillis());

private Stream<Stream<Integer>> mergedStream;

@Before
public void setUp() throws Exception {
    Stream<Integer> infinite = Stream.concat(
            Stream.of(1, 2, 3, 4, 5),
            Stream.generate(randomSupplier::nextInt)
    );

    Stream<Integer> finite1 = Stream.of(100, 101, 102, 103);
    Stream<Integer> finite2 = Stream.of(222, 333, 444, 555);

    mergedStream = Stream.of(infinite, finite1, finite2);
}

@Test
public void of_flatMap_FAILS_BY_TIMEOUT() throws Exception {
    Stream<Integer> streamToTest = mergedStream
            .flatMap(i -> i);

    assertThat(streamToTest
                    .skip(3)
                    .findFirst() // this should break infinite stream, but can't
                    .orElse(-1),
            is(4));
}

@Test
public void of_reduce_SUCCESS() throws Exception {
    Stream<Integer> streamToTest = mergedStream
            .reduce(Stream::concat)
            .orElseGet(Stream::empty);

    assertThat(streamToTest
                    .skip(3)
                    .findFirst() // this really breaks infinite stream
                    .orElse(-1),
            is(4));
}
}