映射嵌套可选？

Question

我有点运行遇到了 Java 8 "Optional" 容器的乏味问题。我无法将可选映射到 "bubble up" 另一个可选。

假设我有一个 RussianNestingDoll class

  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));
        }
    }
}

最好不要使用嵌套的可选值，而是只使用可选值 "bubble up"。这样我就可以调用 "isPresent()" 和 "get()" 一次，而不是同时调用它们两次。有什么办法可以做到这一点？

Answer 1

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 return 为空 Optional。

Answer 2

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);

万一以后想用娃娃：

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

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

映射嵌套可选？

Mapping a Nested Optional?

java

lambda

java-8

guava

optional