CODEWITHSUNDEEP
javajava-8java-streamoption-type
slartidan
slartidan

Reputation: 21576

How to convert an Optional<T> into a Stream<T>?

I want to prepend a stream with an Optional. Since Stream.concat can only concatinate Streams I have this question:

How do I convert an Optional<T> into a Stream<T>?

Example:

Optional<String> optional = Optional.of("Hello");
Stream<String> texts = optional.stream(); // not working

Upvotes: 77

Views: 31073

Answers (7)

JustAnotherCoder
JustAnotherCoder

Reputation: 621

It can depend of how you want to convert empty optional to stream element. If you want to interpret it as "nothing" (or "no element"):

Stream<String> texts = optional.stream(); // since JDK 9
Stream<String> texts = optional.map(Stream::of).orElseGet(Stream::empty); // JDK 8

But if you want to interpret it as null:

Stream<String> texts = Stream.of(optional.oreElse(null));

Upvotes: 1

slartidan
slartidan

Reputation: 21576

If restricted with Java-8, you can do this:

Stream<String> texts = optional.map(Stream::of).orElseGet(Stream::empty);

Upvotes: 150

Tagir Valeev
Tagir Valeev

Reputation: 100209

In Java-9 the missing stream() method is added, so this code works:

Stream<String> texts = optional.stream();

See JDK-8050820. Download Java-9 here.

Upvotes: 83

G&#225;bor Lipt&#225;k
G&#225;bor Lipt&#225;k

Reputation: 9776

A nice library from one of my ex collegues is Streamify. A lot of collectors, creating streams from practicly everything.

https://github.com/sourcy/streamify

Creating a stream form an optional in streamify:

Streamify.stream(optional)

Upvotes: 0

JohnnyLambada
JohnnyLambada

Reputation: 12826

If you're on an older version of Java (lookin' at you, Android) and are using the aNNiMON Lightweight Stream API, you can do something along the lines of the following:

    final List<String> flintstones = new ArrayList<String>(){{
        add("Fred");
        add("Wilma");
        add("Pebbles");
    }};

    final List<String> another = Optional.ofNullable(flintstones)
            .map(Stream::of)
            .orElseGet(Stream::empty)
            .toList();

This example just makes a copy of the list.

Upvotes: 1

Utku &#214;zdemir
Utku &#214;zdemir

Reputation: 7725

I can recommend Guava's Streams.stream(optional) method if you are not on Java 9. A simple example:

Streams.stream(Optional.of("Hello"))

Also possible to static import Streams.stream, so you can just write

stream(Optional.of("Hello"))

Upvotes: 8

Konstantin Yovkov
Konstantin Yovkov

Reputation: 62864

You can do:

Stream<String> texts = optional.isPresent() ? Stream.of(optional.get()) : Stream.empty();

Upvotes: 8

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