How to call Kotlin function with function as parameter in Java Code?

Question

I have a kotlin method as follows:

fun methodWithCallBackAsParameter(callback:() -> Unit) {
    //Do Somejob
    callback()
}

In kotlin we simply use this method with following syntax:

methodWithCallBackAsParameter {
    //Some Instructions here.
}

Now I want to use this Kotlin method in a Java class but I am not able to figure out how.

I tried returning Unit.INSTANCE; it now says, Lambda expressions are not supported at language level 7. — Prajeet Shrestha, Nov 16 '18 at 13:19
Possible duplicate of [Is it possible to use Java 8 for Android development?](https://stackoverflow.com/questions/23318109/is-it-possible-to-use-java-8-for-android-development) — DaveyDaveDave, Nov 16 '18 at 13:58
If you are using Android Studio 3.0+ then you just need to enable Java 8 language features by copy-pasting a block of code into your build.gradle, see https://developer.android.com/studio/write/java8-support — EpicPandaForce, Nov 16 '18 at 14:02

score 5 · Answer 1 · answered Nov 16 '18 at 13:19

5

Just return Unit.INSTANCE in the end of the lambda expression.

answered Nov 16 '18 at 13:19

nyarian

4,085
1
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I tried returning Unit.INSTANCE; it now says, Lambda expressions are not supported at language level 7. – Prajeet Shrestha Nov 16 '18 at 13:19
Either set language level to 1.8 or transform the lambda into anonymous class. This error occurred because lambdas are supported from Java 1.8, and your project uses Java 1.7. – nyarian Nov 16 '18 at 13:21

gmetax · Answer 2 · 2018-11-16T14:19:03.993

if you want to use lambda in android you have to change your app to use Java 1.8 instead of 1.7 use can do that by it from gradle

android {
  ...
  // Configure only for each module that uses Java 8
  // language features (either in its source code or
  // through dependencies).
  compileOptions {
    sourceCompatibility JavaVersion.VERSION_1_8
    targetCompatibility JavaVersion.VERSION_1_8
  }
}

for more look this documentation https://developer.android.com/studio/write/java8-support

score 0 · Answer 3 · edited May 08 '21 at 20:01

0

I believe you want to do this.

    public class Program {

    public static void main(String[] args) {
        FunctionKt.methodWithCallBackAsParameter(() -> {
            System.out.printf("do something");
            return null;
        });
    }
}

In this example, the function methodWithCallBackAsParameter was defined in the function.kt file.

edited May 08 '21 at 20:01

blackgreen

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answered May 08 '21 at 11:58

Lincoln Pires

1
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score 0 · Answer 4 · answered Sep 14 '21 at 13:01

You can do something like this:

Supposing the kotlin method is

fun handleResult(
    data: Intent?,
    callbackSuccess: (String) -> Unit,
    callbackError: (String) -> Unit
)

Then when you call this from Java you call it like this (either by matching the signature or doing some logic inside)

    handleResult(data, this::success, this::failure);

    handleResult(data, (statusCode) -> success(statusCode), (errorMessage) -> failure(errorMessage));


    private Unit failure(String errorMessage) {
        // do something java here
        return Unit.INSTANCE;
    }

    private Unit success(String code) {
        // do something java here
        return Unit.INSTANCE;
    }

score -1 · Answer 5 · edited May 08 '21 at 20:01

-1

When use in Java: callback: () -> Unit = new Function0<>()

edited May 08 '21 at 20:01

blackgreen

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answered May 08 '21 at 09:38

pan he

64
7

How to call Kotlin function with function as parameter in Java Code?

5 Answers5