Java 8 多级 flatMap 最佳实现

Question

我有这两个代码示例:

代码A:

Stream<String> aStream = firstLevelList.stream()
                            .flatMap(firstLevelElement -> firstLevelElement.getSecondLevelList().stream()
                                .flatMap(secondLevelElement -> secondLevelElement.getThirdLevelList().stream()
                                    .map(thirdLevelElement -> thirdLevelElement.toString())));

代码 B:

Stream<String> aStream = firstLevelList.stream()
                            .flatMap(firstLevelElement -> firstLevelElement.getSecondLevelList().stream())
                            .flatMap(secondLevelElement -> secondLevelElement.getThirdLevelList().stream())
                            .map(thirdLevelElement -> thirdLevelElement.toString());

两者的结果完全一样，哪个是最好的实现？为什么？

Answer 1

选项 1

以下读起来很好，感觉更常见:

firstLevelList.stream()
        .flatMap(firstLevelElement -> firstLevelElement.getSecondLevelList().stream())
        .flatMap(secondLevelElement -> secondLevelElement.getThirdLevelList().stream())
        .map(Object::toString);

它避免了 @Holger 指出的嵌套，这对读者很重要。

选项 2

或者，如果它与其余代码基样式不冲突，我们可以使用不同的缩进进行嵌套:

firstLevelList.stream().flatMap(
        firstLevelElement -> firstLevelElement.getSecondLevelList().stream().flatMap(
                secondLevelElement -> secondLevelElement.getThirdLevelList().stream().map(Object::toString)
        )
);

选项 3

如果仍然感觉太神秘，我们可以提取变量或方法来命名不同的流:

firstLevelList.stream().flatMap(
        firstLevelElement -> firstLevelElement.getSecondLevelList().stream().flatMap(
                secondLevelElement -> getThirdLevelStream(secondLevelElement)
        )
);

Stream<String> getThirdLevelStream(SecondLevelElement secondLevelElement) {
    return secondLevelElement.getThirdLevelList().stream().map(Object::toString);
}

选项 4

我们现在可以将这些方法移动到关于 Law of Demeter 的元素类中:

firstLevelList.stream().flatMap(FirstLevelElement::getSecondLevelStream);

class FirstLevelElement {

    Stream<String> getSecondLevelStream() {
        return this.getSecondLevelList().stream().flatMap(SecondLevelElement::getThirdLevelStream);
    }

    Collection<SecondLevelElement> getSecondLevelList() {...}
}

class SecondLevelElement {

    Stream<String> getThirdLevelStream() {
        return this.getThirdLevelList().stream().map(Object::toString);
    }

    Collection<ThirdLevelElement> getThirdLevelList() {...}
}

如果在代码中的其他地方重用流方法，这将增加一些值(value)。

结论

对于这些选项中哪一个更好，可以提出不同的论点。总的来说，它们都很好。