java - Optional orElse Optional in Java

Question

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"

Consider the following pattern:

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);
}

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.

It's implementation would look like this:

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

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.

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.

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.

score 99 · Accepted Answer

これは JDK 9 の一部でありor、Supplier<Optional<T>>. あなたの例は次のようになります。

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

詳細については、Javadocまたは私が書いたこの投稿を参照してください。

score 36 · Accepted Answer

きれいではありませんが、これはうまくいきます：

return serviceA(args)
  .map(Optional::of).orElseGet(() -> serviceB(args))
  .map(Optional::of).orElseGet(() -> serviceC(args))
  .map(Optional::of).orElseGet(() -> serviceD(args));

.map(func).orElseGet(sup)で使用するのに非常に便利なパターンですOptional。「これOptionalに値が含まれている場合vは私func(v)に、そうでない場合は私にsup.get()」という意味です。

この場合、呼び出しserviceA(args)てを取得しOptional<Result>ます。値がOptional含まれている場合vはを取得しますOptional.of(v)が、空の場合はを取得しますserviceB(args)。より多くの代替品で繰り返します。

このパターンの他の用途は次のとおりです。

.map(Stream::of).orElseGet(Stream::empty)
.map(Collections::singleton).orElseGet(Collections::emptySet)

score 29 · Accepted Answer

おそらく、これはあなたが求めているものです: 1 つの Optional または別の値から値を取得する

それ以外の場合は、を参照してくださいOptional.orElseGet。これがあなたが求めていると私が思うものの例です：

result = Optional.ofNullable(serviceA().orElseGet(
                                 () -> serviceB().orElseGet(
                                     () -> serviceC().orElse(null))));

java - Optional orElse Optional in Java

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