How to Efficiently Convert List of Objects to Optional in Java Streams?

Becoming Concise with Java 8's Optional and Stream::flatMap

When working with Java 8 streams, transforming a List to Optional and extracting the first Other value efficiently can be a challenge. While flatMap typically requires a return stream, the absence of stream() for Optional complicates matters.

Java 16 Solution

Java 16 introduces Stream.mapMulti, enabling a more concise approach:

<code class="java">Optional<Other> result = things.stream()
    .map(this::resolve)
    .<Other>mapMulti(Optional::ifPresent)
    .findFirst();</code>

Java 9 Solution

JDK 9 adds Optional.stream, simplifying the task:

<code class="java">Optional<Other> result = things.stream()
    .map(this::resolve)
    .flatMap(Optional::stream)
    .findFirst();</code>

Java 8 Solution

In Java 8, the following approach can be taken:

<code class="java">Optional<Other> result = things.stream()
    .map(this::resolve)
    .flatMap(o -> o.isPresent() ? Stream.of(o.get()) : Stream.empty())
    .findFirst();</code>

Using a helper function to convert Optional to Stream:

<code class="java">static <T> Stream<T> streamopt(Optional<T> opt) {
    if (opt.isPresent())
        return Stream.of(opt.get());
    else
        return Stream.empty();
}

Optional<Other> result = things.stream()
    .flatMap(t -> streamopt(resolve(t)))
    .findFirst();</code>

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