java lambda-如何遍历可选列表/可选流 [英] java lambda - how to traverse optional list/stream of optionals

问题描述

具有Optional的可选列表，例如:

Having an Optional List of Optional's like:

Optional<List<Optional<String>>> optionalList = Optional.of(
    Arrays.asList(
        Optional.empty(),
        Optional.of("ONE"),
        Optional.of("TWO")));

如何遍历optionalList以打印出字符串的ONE和TWO吗?

How to traverse optionalList to print out the string's ONE and TWO ?

拥有可选的可选流怎么样?

What about having an Optional Stream of Optionals?

Optional<Stream<Optional<String>>> optionalStream = Optional.of(
    Stream.of(
        Optional.empty(),
        Optional.of("ONE"),
        Optional.of("TWO")));

更新:感谢您的回答，为optionalStream(非嵌套)提供了解决方案:

Update: Thanks for answers, solution for optionalStream (non nested):

optionalStream
    .orElseGet(Stream::empty)
    .filter(Optional::isPresent)
    .map(Optional::get)
    .forEach(System.out::println);

推荐答案

首先，检查Optional是否存在.如果是，则流式处理列表并过滤非空列表，然后打印每个列表.

First, check if the Optional is present. If yes, then stream the list and filter the non-empty ones and print each of them.

optionalList.ifPresent(list -> list.stream()
            .filter(Optional::isPresent)
            .map(Optional::get)
            .forEach(System.out::println));

对于流情况也差不多

optionalStream.ifPresent(stream -> stream
            .filter(Optional::isPresent)
            .map(Optional::get)
            .forEach(System.out::println));

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