Greater order capabilities in Kotlin — Underneath the hood. | by Shangeeth Sivan | Jan, 2023 | Rank Tech

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Let’s take a look at a easy operate with a operate sort parameter in Kotlin. For simplicity, I’ve created this technique that accepts two parameters:

  • title of the occasion chain sort
  • completionListener of the Sort operate that returns Unit.
enjoyable insertEvent(eventName: String, completionListener: () -> Unit) 
eventRepository.insert(eventName) // Think about this a delayed occasion

Let’s convert it to Kotlin bytecode and decompile it to the Java model.

Instruments -> Kotlin -> Present Bytecode -> Decompile (The right way to Hyperlink)

public closing void insertEvent(@NotNull String eventName, @NotNull Function0 completionListener) 
Intrinsics.checkNotNullParameter(eventName, "eventName");
Intrinsics.checkNotNullParameter(completionListener, "completionListener");

Let’s break down every parameter:

  • title of the occasion – the information sort continues to be a string and is annotated with the @NonNull annotation in Java; this parameter appears simple.
  • completionListener – the information sort seems to be bizarre, it is referred to as Function0 ?? 🤔

What’s Function0? For those who do a management click on on Function0, you’ll navigate to Features.kt

Let’s have a look at what Features.kt incorporates:

public interface Function0<out R> : Perform<R> 
public operator enjoyable invoke(): R

public interface Function1<in P1, out R> : Perform<R>
public operator enjoyable invoke(p1: P1): R

public interface Function2<in P1, in P2, out R> : Perform<R>
public operator enjoyable invoke(p1: P1, p2: P2): R


function0 it is nothing greater than an interface with technique name that has 0 parameters.

function1 is one other interface with name technique which has 1 parameter

function2 is one other interface with name technique which has 2 parameters
and proceed to Function22

Underneath that hood, our larger order operate completionListener has been outmoded by Function0 by Kotlin. If a bit about generics, it is best to have the ability to determine what P1, P2, … and R are in Features.kt

P1, P2, … are the generic sorts for the parameters of completionListener
R is the generic return sort for the return sort of the operate in our case completionListener return sort

For those who nonetheless cannot determine what is going on on, attempt calling the identical operate you created in Kotlin from a Java file, you may determine the way it works.

Since Kotlin is designed with Java interoperability in thoughts, current Java code might be referred to as from Kotlin naturally, and Kotlin code can be used from Java with out difficulty.

//Calling the identical technique from Java world

public static void major(String[] args)
// Calling the identical insertEvent from java world
eventHelper.insertEvent("take a look at", new Function0<Unit>()
public Unit invoke()
return null;


As you may see above, we now have to create an nameless interior class for the Function0 interface to get the callback into the Java world.

Okay, let’s modify completionListener Y add a parameter:

// eliminated the eventName param
enjoyable insertEvent(completionListener: (take a look at: String) -> Unit)

That is the generated code:

public closing void insertEvent(@NotNull Function1 completionListener) 
Intrinsics.checkNotNullParameter(completionListener, "completionListener");

function0 is now changed with function1 to help the String parameter.

I hope this weblog has helped you perceive how larger order capabilities work underneath the hood.

Some further data:
Okay, will Kotlin help any variety of parameters in our larger order capabilities? I bought this query as a result of I seen one thing unusual in Features.kt, there are operate interfaces as much as Function22. What’s going to occur if I’ve greater than 22 parameters? Will Kotlin throw a compile error? We’ll see…

Let’s modify our completionListener have 23 parameters. 😅

// I've eliminated the string param and changed invoke technique with a println.
enjoyable insertEvent(
completionListener: (
test1: String, test2: String, test3: String, test4: String, test5: String, test6: String, test7: String, test8: String, test9: String, test10: String, test11: String, test12: String, test13: String, test14: String, test15: String, test16: String, test17: String, test18: String, test19: String, test20: String, test21: String, test22: String, test23: String,
) -> Unit
println("Simply Printing dont wish to invoke it :sweat:")

Let’s decompile…

public closing void insertEvent(@NotNull FunctionN completionListener) 
Intrinsics.checkNotNullParameter(completionListener, "completionListener");
String var2 = "Simply Printing dont wish to invoke it :sweat:";

Wow, now there’s one thing referred to as FunctionN. I will allow you to discover what that’s.

I hope the article nearly Greater order capabilities in Kotlin — Underneath the hood. | by Shangeeth Sivan | Jan, 2023 provides acuteness to you and is helpful for tallying to your data

Higher order functions in Kotlin — Under the hood. | by Shangeeth Sivan | Jan, 2023