What's the Java equivalent of this Kotlin declaration?

Question

var doOnAppCouldNotBeFrozen: ((Context) -> Unit)? = null

I think this is a "function type" but I'm not sure. Is it a method or a variable? I know that Unit means void so then is it a method that returns void?

The only way I can understand what it means is if I can see how it's written in Java (without the question mark after Unit).

It's the definition of a variable, holding a lambda of type `(Context) -> Unit` (a function that takes in a `Context` and returns a `Unit`), and it is nullable. — Turing85, Apr 03 '21 at 14:46
Well then maybe if your experience is all your questions getting downvoted, you should read [ask]? I think the comment from @Turing85 explains it perfectly. — Henry Twist, Apr 03 '21 at 14:49
There is no 1:1 relation since Java has no concept of non-nullableness. If we ignore this, the equivalent would be `Function doOnAppCouldNotBeFrozen = null;` or `Consumer doOnAppCouldNotBeFrozen = null;` — Turing85, Apr 03 '21 at 14:49
@Turing85 I'm confused now. Is it a variable or a function? I've never seen that kind of statement in java before — Farou Faroud, Apr 03 '21 at 14:54
It is a variable. If you have never seen the concept, I recommend reading a tutorial on lambdas, e.g. [this one by oracle](https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html), as well as the [documentation of the `java.util.function`-package](https://docs.oracle.com/en/java/javase/15/docs/api/java.base/java/util/function/package-summary.html). — Turing85, Apr 03 '21 at 14:55
I didn't make my suggestion to prevent you from getting downvoted. I made the suggestion because if this question is to be useful for future readers than they must be able to find it and your original title is not really searchable. — takendarkk, Apr 03 '21 at 14:57
@FarouFaroud *"Is it a variable or a function?"* It's kind of a variable *to* a function. — Andreas, Apr 03 '21 at 15:02
@Andreas Functions are not first-class citizens in Java. It is actually a variable holding a reference to a concrete interface-implementation. This may or may not be a lambda expression or a method reference, both of which, as far as I know, get compiled-down to an anonymous interface implementations. — Turing85, Apr 03 '21 at 15:05
@Turing85 "Kind of" means "not really, but abstractly it behaves very close to that". --- And no, lambdas and method references never become anonymous classes. A lambda becomes a hidden (synthetic) method and is then a method reference to that hidden method. — Andreas, Apr 03 '21 at 15:25
@Turing85 in Android studio trying to use the `accept` or `apply` methods after defining those interfaces always says they require min API 24 — Farou Faroud, Apr 03 '21 at 16:17
@FarouFaroud Lambdas and the `java.util.function`-package were introduced in Java 8. — Turing85, Apr 03 '21 at 16:24
@FarouFaroud Please read: [Can I ask only one question per post?](https://meta.stackexchange.com/questions/222735/can-i-ask-only-one-question-per-post) — Turing85, Apr 03 '21 at 16:26
@Turing85 To be pedantic, a lambda isn’t a type of object. It is one of many possible syntaxes used to define a function. The variable here is not of a lambda type. You could assign any function with matching type, not just ones defined by lambdas. — Tenfour04, Apr 03 '21 at 20:57
@Tenfour04 For Kotlin, I take you word for it :) (I am not as firm in Kotlin's spec as I am in Java's). For Java, the last information I had was that lambdas are "just" syntactic sugar for anonymous interface implementations. — Turing85, Apr 03 '21 at 21:58

score 2 · Answer 1 · edited Apr 03 '21 at 20:54

2

The java equivalent of the kotlin code is:

Consumer<Context> doOnAppCouldNotBeFrozen = null;

If you want to learn more about the Consumer, I can recommend the documentation.

EDIT: Using the java stream below api 24 requires desugaring to be enabled:

If you're building your app using Android Gradle plugin 4.0.0 or higher, the plugin extends support for using a number of Java 8 language APIs without requiring a minimum API level for your app.

edited Apr 03 '21 at 20:54

Tenfour04

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answered Apr 03 '21 at 14:51

flaxel

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Ok so I have to use `doOnAppCouldNotBeFrozen.accept(context)` to use it later? Android Studio says the accept() method requires API 24 – Farou Faroud Apr 03 '21 at 16:11
Yes that is correct. And I found [this post](https://stackoverflow.com/questions/39515035/is-it-possible-to-use-the-java-8-stream-api-on-android-api-24). So it is not possible to use the java stream below api 24. – flaxel Apr 03 '21 at 16:27

Kakyire · Answer 2 · 2021-04-03T15:21:57.620

-2

To get Java equivalent of every Kotlin code

Select Tools > Kotlin > Show Kotlin Bytecode

Screenshot 2021-04-03 at 15 01 49

Then click DECOMPILE

Screenshot 2021-04-03 at 15 08 03

This is Java equivalent in Android Studio

   @Nullable
   private static Function1 doOnAppCouldNotBeFrozen;

   @Nullable
   public static final Function1 getDoOnAppCouldNotBeFrozen() {
      return doOnAppCouldNotBeFrozen;
   }

   public static final void setDoOnAppCouldNotBeFrozen(@Nullable Function1 var0) {
      doOnAppCouldNotBeFrozen = var0;
   }

edited Apr 03 '21 at 15:21

answered Apr 03 '21 at 15:08

Kakyire

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You may want to mention which IDE you are using. – Turing85 Apr 03 '21 at 15:10
I am using Android Studio – Kakyire Apr 03 '21 at 15:11
2

Maybe you want to [edit] your answer and include this information? – Turing85 Apr 03 '21 at 15:12
While we are on the topic: You may want to include the definition of `Function1` aswell. – Turing85 Apr 03 '21 at 15:15
The `Function1` was generated by the IDE – Kakyire Apr 03 '21 at 15:23
I doubt that. If it appears in the bytecode, it should have been generated by the compiler. Anyway, the definition of `Function1` is where the actual magic happens. – Turing85 Apr 03 '21 at 15:24
Actually the ``Function1`` is the `Type`. – Kakyire Apr 03 '21 at 15:25
1

@Turing85 `Function1` is likely a (static) nested interface generated by Kotlin. Since it is a different type, the UI likely didn't show it, it only showed the bytecode of the main type in the source file. Limit of the IDE. – Andreas Apr 03 '21 at 15:34
@KakyireLastBorn yes, but the type needs an implementation, that is where the "mapping" from the kotlin-syntax to the JVM-code (an, in last instance, the Java-code) happens. – Turing85 Apr 03 '21 at 15:54
@Andreas I know. But the definition of `Function1` is what reveals the "mapping" from the kotlin-syntax to the JVM-code. – Turing85 Apr 03 '21 at 15:56

What's the Java equivalent of this Kotlin declaration?

2 Answers2