从一个Optional或另一个中获取价值 [英] Get value from one Optional or another
问题描述
我有两个 java.util.Optional
个实例,我希望得到一个 Optional
:
I have two java.util.Optional
instances and I want to get an Optional
that either:
- 如果它有值,则具有第一个可选值的值。
- 具有值的值第二个可选,如果它有一个值。
- 为空且两个都没有可选的值。
有没有直接的方法来做到这一点,即是否已经有一些API可以做到这一点?
Is there a straight-forward way to do that, i.e. is there already some API to do that?
以下表达式会做到这一点,但我不得不提第一个可选的两次:
The following expressions will do that, but I have to mention the first optional twice:
firstOptional.isPresent() ? firstOptional : secondOptional
这正是 com.google.common.base。 Optional.or()
可以,但Java 8的API中不存在该方法。
This is exactly what com.google.common.base.Optional.or()
does, but that method is not present in Java 8's API.
aioobe 接受的答案列出了一些替代方法来克服这个遗漏可选
API的权利,其中必须计算这样的值(其中回答我的问题)。我现在选择在我的代码库中添加一个实用工具函数:
The accepted answer by aioobe lists a few alternative approaches to overcome this omission of the Optional
API right where such a value has to be computed (which answers my question). I've now opted to add a utility function to my codebase:
public static <T> Optional<T> or(Optional<T> a, Optional<T> b) {
if (a.isPresent())
return a;
else
return b;
}
推荐答案
更新:从Java 9开始(变更集12885 ),你可以做到
Update: As of Java 9 (changeset 12885) you can do
firstOptional.or(() -> secondOptional);
对于Java 8,请继续阅读......
For Java 8, read on...
如果您想避免两次提及 firstOptional
,您可能需要使用
If you want to avoid mentioning firstOptional
twice, you'd probably have to go with something like
firstOptional.map(Optional::of).orElse(secondOptional);
或
Optional.ofNullable(firstOptional.orElse(secondOptional.orElse(null)));
但最可读的变体可能只是做
But the most readable variant is probably to simply do
Optional<...> opt = firstOptional.isPresent() ? firstOptional
: secondOptional.isPresent() ? secondOptional
: Optional.empty();
如果有人偶然遇到这个问题,但有<选项的em> list ,我建议像
Optional<...> opt = optionals.stream()
.filter(Optional::isPresent)
.findFirst()
.orElse(Optional.empty());
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