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

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

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

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.

You can do:

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

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"))

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.

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)

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));