We don't think that Kotlin itself needs Optional because it has strong null-safe type system that effectively eliminates need in such a wrapper. However there are Java APIs and libraries like RxJava 2 and RxJava 3 which don't accept null values and language-level nullability cannot help with that.

We also think that in many cases you can use sealed classes to express absent values, however in simple cases like passing String? through RxJava stream Optional is a more convenient solution.

The goal of this implementation is to be convenient to use and fit Kotlin's null-safe type system, which resulted in:

• Only two functions: toOptional() and toNullable().
  • Mimics Kotlin functions like toInt() and toBoolean().
• Some and None are declared as top level types.
  • No need to write Optional.Some or Optional.None.
• No functions like map(), getOrElse(), filter(), etc.
  • Use toNullable() and Kotlin stdlib functions like let(), takeIf() instead.

Usage

Create

val some = Some(value)
val none = None // It's an object!

Convert

// T? → Optional<T>
// If value is null — you'll get None, otherwise you'll get Some(value).
val o = value.toOptional()

// Optional<T> → T?
// If optional is None — you'll get null, otherwise you'll get non-null T value.
val t = optional.toNullable()

Leverage Kotlin Features

Fallback from None (like java.util.Optional.getOrElse())

val f = optional.toNullable() ?: "fallback"

Smart Cast

when (optional) {
    is Some -> println(optional.value)
    is None -> println("Nope!")
}

Destructuring

// If Optional is None — you'll get null, otherwise you'll get non-null T value.
val (value) = optional

Java Interop

Use the static Optional.toOptional() to wrap an instance of T into Optional<T>.

RxJava Extensions

val values = Observable.just(Some("a"), None, Some("b"))

// Filter Some values.
values
    .filterSome()
    .test()
    .assertValues("a", "b")

// Filter None values.
values
    .filterNone()
    .test()
    .assertValues(Unit) // filterNone() maps None to Unit.

Reactor Extensions

val values = Flux.just(Some("a"), None, Some("b"))

// Filter Some values.
values.filterSome()

// Filter None values.
values.filterNone()

Download

Koptional is available on jcenter. All the releases and changelogs can be found on Releases Page.

Optional

implementation "com.gojuno.koptional:koptional:$koptional_version"

RxJava 2 Extensions

implementation "com.gojuno.koptional:koptional-rxjava2-extensions:$koptional_version"

RxJava 3 Extensions

implementation "com.gojuno.koptional:koptional-rxjava3-extensions:$koptional_version"

Reactor Extensions

implementation "com.gojuno.koptional:koptional-reactor-extensions:$koptional_version"

License

Copyright 2017 Juno, Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.