java 如何将列表转换为 Optional<List>？
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How to convert a List to Optional<List>?
提问by Pavel Klindziuk
How can I convert a Listto Optional<List>?
如何将 a 转换List为Optional<List>？
The following code produces a compilation error :
以下代码产生编译错误：
public Collection<Optional<UserMeal>> getAll() {
Comparator comparator = new SortedByDate();
List<UserMeal> mealList = new ArrayList<>(repository.values());
Collections.sort(mealList,comparator);
Collections.reverse(mealList);
**List<Optional<UserMeal>> resultList = Optional.of(mealList);**
return resultList;
}
回答by Eran
Optional.of(mealList)returns an Optional<List<UserMeal>>, not a List<Optional<UserMeal>>.
Optional.of(mealList)返回一个Optional<List<UserMeal>>，而不是一个List<Optional<UserMeal>>。
To get the desired List<Optional<UserMeal>>, you should wrap each element of the Listwith an Optional:
为了得到想要的List<Optional<UserMeal>>，你应该换行的每个元素List有Optional：
List<Optional<UserMeal>> resultList =
mealList.stream()
.map(Optional::ofNullable)
.collect(Collectors.toList());
Note I used Optional::ofNullableand not Optional::of, since the latter would produce a NullPointerExceptionif your input Listcontains any null elements.
注意我使用了Optional::ofNullableand not Optional::of，因为NullPointerException如果您的输入List包含任何空元素，后者会产生 a 。
回答by shijin raj
Simple conversion of mealList to Optional
mealList 到 Optional 的简单转换
Optional.ofNullable(mealList)
Optional.ofNullable(mealList)
