When I don't remember which to use I always refer to this list . I've always felt this functional interface business is just bad design when compared with C# simple Action/Function. Another victim of type erasure. More on reddit.com

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The Java virtual machine doesn't have function types. The Kotlin function type compiles, quite literally, into an interface class. Go into your editor and create a class that inherits from ()->Unit like in the example. Intellij will say you need to implement a member from the interface "Kotlin.Function0". That interface is the "real code" and the function type syntax is sugar on top of that. Many Kotlin features are syntax sugar because the JVM literally doesn't support them natively. More on reddit.com