映射嵌套选项？

Question

Java 8“可选”容器遇到了一个乏味的问题。我无法将 Optional 映射到“冒泡”另一个 Optional。

假设我有一个 RussianNestingDoll 类

  public class NestedOptionalTest {

    public static void main(String[] args) { 

        RussianNestingDoll doll  = RussianNestingDoll.createInstance(RussianNestingDoll.createInstance(RussianNestingDoll.createInstance()));

        Optional<Optional<RussianNestingDoll>> thirdDollContents = doll.getInnerDoll().map(d -> d.getInnerDoll());

        if (thirdDollContents.isPresent() && thirdDollContents.get().isPresent()) { 
            System.out.println(thirdDollContents.get().get());
        }
        else { 
            System.out.println("empty");
        }
    }

    private static final class RussianNestingDoll { 
        private final Optional<RussianNestingDoll> innerDoll;

        public Optional<RussianNestingDoll> getInnerDoll() { 
            return innerDoll;
        }

        private RussianNestingDoll(Optional<RussianNestingDoll> innerDoll) { 
            this.innerDoll = innerDoll;
        }
        public static RussianNestingDoll createInstance() { 
            return new RussianNestingDoll(Optional.empty());
        }
        public static RussianNestingDoll createInstance(RussianNestingDoll innerDoll) { 
            return new RussianNestingDoll(Optional.of(innerDoll));
        }
    }
}

最好不必使用嵌套的可选项，而只需使用可选的“冒泡”。这样我就可以只调用一次“isPresent()”和“get()”，而不是同时调用它们两次。有没有办法我可以做到这一点？

Answer 1

我不太确定你想要什么，但你可以像这样重写你的代码：

RussianNestingDoll doll  = RussianNestingDoll.get(RussianNestingDoll.get(RussianNestingDoll.get()));

String content = doll.getInnerDoll()
                 .flatMap(d -> d.getInnerDoll())
                 .map(d -> d.get().toString())
                 .orElse("empty");
System.out.println(content);

如果您以后想使用娃娃：

Optional<RussianNestingDoll> thirdDoll = doll.getInnerDoll()
                 .flatMap(d -> d.getInnerDoll());

if (thirdDoll.isPresent()) {
  System.out.println(thirdDoll.get());
}
else {                 
  System.out.println("empty");
}

Answer 2

你想flatMap吗？

thirdDollContents
    .flatMap(Function.identity()) // un-nest, get back an Optional<RussianNestingDoll>
    .get() // or isPresent()

如果thirdDollContents 是empty，flatMap 将返回一个空的Optional。