可选orElse Java中的可选项 [英] Optional orElse Optional in Java

问题描述

我一直在使用新的可选类型在Java 8中，我遇到了一个似乎是功能上不支持的常见操作：orElseOptional

I've been working with the new Optional type in Java 8, and I've come across what seems like a common operation that isn't supported functionally: an "orElseOptional"

考虑以下模式：

Optional<Result> resultFromServiceA = serviceA(args);
if (resultFromServiceA.isPresent) return result;
else {
    Optional<Result> resultFromServiceB = serviceB(args);
    if (resultFromServiceB.isPresent) return resultFromServiceB;
    else return serviceC(args);
}

这种模式有很多种形式，但归结为想要 orElse在一个可选项上，它接受一个生成一个新的可选项的函数，仅当当前的函数不存在时调用。

There are many forms of this pattern, but it boils down to wanting an "orElse" on an optional that takes a function producing a new optional, called only if the current one does not exist.

它的实现看起来像这样：

It's implementation would look like this:

public Optional<T> orElse(Supplier<Optional<? extends T>> otherSupplier) {
    return value != null ? this : other.get();
}

我很好奇是否有这样的方法不存在的原因，如果我只是以一种无意的方式使用Optional，以及人们如何处理这种情况的其他方式。

I'm curious if there's a reason such a method doesn't exist, if I'm just using Optional in an unintended way, and what other ways people have come up with to deal with this case.

我应该说我认为解决方案涉及自定义实用程序类/方法并不优雅，因为使用我的代码的人不一定知道它们存在。

I should say that I think that solutions involving custom utility classes/methods aren't elegant because people working with my code won't necessarily know they exist.

另外，如果有人知道，这样的方法是不是包含在JDK 9中，我可以在哪里提出这样的方法？对我来说这似乎是一个非常明显的遗漏。

Also, if anyone knows, will such a method be included in JDK 9, and where might I propose such a method? This seems like a pretty glaring omission to the API to me.

推荐答案

这是JDK 9的一部分，形式为或，需要供应商< Optional< T>> 。那么你的例子是：

This is part of JDK 9 in the form of or, which takes a Supplier<Optional<T>>. Your example would then be:

return serviceA(args)
    .or(() -> serviceB(args))
    .or(() -> serviceC(args));

详见 Javadoc 或这篇文章我写的。

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