Mixing Kotlin and Java code

Question

I want to use a Kotlin Android library (FotoApparat) in a Java Android project.

In a Kotlin code base the whenAvailble function gets a kotlin callback as a param, which will be called when the async operation is done.

val photoResult = fotoapparat.takePicture()

// Asynchronously saves photo to file
photoResult.saveToFile(someFile)

// Asynchronously converts photo to bitmap and returns the result on the main thread
photoResult
    .toBitmap()
    .whenAvailable { bitmapPhoto ->
            val imageView = (ImageView) findViewById(R.id.result)

            imageView.setImageBitmap(bitmapPhoto.bitmap)
            imageView.setRotation(-bitmapPhoto.rotationDegrees)
    }

whenAvailable code can be found here

The equivalent java implementation would be: (Previously the library was written in java)

fotoApparat.takePicture().
                    toPendingResult().
                    whenAvailable( /* some sort of call back */);

How do I provide the whenAvailable callback from a Java code?

In previous versions of the lib, there was a Java pending result callback class which no longer available.

i dont get your question: you already posted java code, so what do you need actually? — pskink
– pskink, Commented Apr 16, 2018 at 10:40
If you use Java 8 or higher, you can use a lambda or a method reference, otherwise you need to provide an instance of your callback (e.g., an anonymous class). By the way your original question before the edit contained a possible answer, as said by @pskink — user2340612
– user2340612, Commented Apr 16, 2018 at 10:45
short answer: yes you can use a Kotlin library and access it from a Java class — Ege Kuzubasioglu
– Ege Kuzubasioglu, Commented Apr 16, 2018 at 10:46
@user2340612 the code before the edit, does not work, it used to work for v1.x - if you can get it to work, i'd love to see how. — Aviran
– Aviran, Commented Apr 16, 2018 at 11:27

Community · Accepted Answer · 2020-06-20 09:12:55Z

5

Abreslav said:

Unit is a type (unlike void) and has a value (unlike Void). This enable uniform treatment of Unit when it comes to generic classes. I.e. we don’t need another two types: a function that returns something and a function that returns void. It’s all one type: a function that returns something tat may be Unit.

Introducing void would pose many problems in areas like type inference, compositionality of any sort of functions, etc

So, unlike in Kotlin, in Java you need to return this value explicitly.

Your lambda needs to be:

fotoApparat.takePicture().
                toPendingResult().
                whenAvailable(bitmapPhoto -> {
        ImageView imageView = (ImageView) findViewById(R.id.result);
        return Unit.INSTANCE;
});

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Apr 16, 2018 at 12:22

tieskedh

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Comments

ice1000 · Accepted Answer · 2018-04-16 11:11:31Z

0

Yes you can use something like this:

fotoApparat.takePicture().
                toPendingResult().
                whenAvailable(bitmapPhoto -> {
        ImageView imageView = (ImageView) findViewById(R.id.result);
        // rest of your code
});

answered Apr 16, 2018 at 11:11

ice1000

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3 Comments

Aviran Over a year ago

maybe it's something with my project settings, but that code has a syntax error. "missing return statement" on the last line.

user2340612 Over a year ago

Is the error shown in the callback of whenAvailable, or in the enclosing method? From the source code you linked, the callback passed to whenAvailable returns Unit (i.e., void), so the error you see shouldn't be linked to that

Aviran Over a year ago

@user2340612 the error shown in the callback provided to whenAvailable

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