How to convert an Optional into a Stream?

Question

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

Answer 1

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.

Answer 2

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

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

Answer 3

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.

Answer 4

You can do:

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

Answer 5

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"))

Answer 6

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)