连接两个流的 NullSafe 方法是什么？

Question

我有两个 整数流 guestsTravelWith 和 guests，我正在将它们连接起来，但是 当任何一个流为 Null 时抛出错误。是否有 nullsafe 方法来连接两个流？或者使用 if 条件是我唯一的希望？

Stream<Integer> guests = code+some_method();
Stream<Integer> guestsTravelWith = code+some_method();
Stream.concat(guestsTravelWith, guests)

Answer 1

如果您正在使用 Java 9 使用 Strem.ofNullable

Returns a sequential Stream containing a single element, if non-null, otherwise returns an empty Stream.

Answer 2

一点都不好看，但是：

Stream.ofNullable(guestsTravelWith).orElse(Stream.empty()).flatMap(Function.identity())

或者你知道，“不太有趣”的方式：

guests == null ? Stream.empty() : guests;

您应该重新考虑 return null Stream 开始的方法，这是一个糟糕的想法。

Answer 3

为什么不过滤掉 null 个流？

Stream<Integer> s1 = null;
Stream<Integer> s2 = Stream.of(1, 2, null, 4);
    
Stream.of(s1, s2)
      .filter(Objects::nonNull) // Stream<Stream>
      .flatMap(s -> s) // or Function.identity()
      .forEach(System.out::println);

根据 Holger 的评论更新
Stream.concat 应替换为 Stream.of 和 null-safe Stream::flatMap:

<R> Stream<R> flatMap(Function<? super T,? extends Stream<? extends R>> mapper)
Returns a stream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element. Each mapped stream is closed after its contents have been placed into this stream. (If a mapped stream is null an empty stream is used, instead.)

    Stream<Integer> s1 = null;
    Stream<Integer> s2 = Stream.of(1, 2, null, 4);

    Stream.of(s1, s2)
          .flatMap(s -> s) // or Function.identity()
          .forEach(System.out::println);

输出：

1
2
null
4

连接两个流的 NullSafe 方法是什么？

What is NullSafe way to concatenate two Streams?

java

java-8

java-stream