Call Kotlin suspend function in Java class

For coroutines 1.3.0 use this:

BuildersKt.launch(GlobalScope.INSTANCE,
                Dispatchers.getMain(),//context to be ran on
                CoroutineStart.DEFAULT,
                (coroutineScope, continuation) -> suspendFunction(arguments)
        );

For java < 8:

BuildersKt.launch(
        GlobalScope.INSTANCE,
        Dispatchers.getMain(),//context to be ran on
        CoroutineStart.DEFAULT,
        new Function2<CoroutineScope, Continuation<? super Unit>, Unit/*or your return type here*/>() {
            @Override
            public Unit/*or your return type here*/ invoke(CoroutineScope coroutineScope, Continuation<? super Unit> continuation) {
                //do what you want
                return Unit.INSTANCE; //or something with the defined type
            }
        }
);

My gradle file:

implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:1.3.50"
implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:1.3.0"
implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:1.3.0"

Kotlin uses static classes for extension functions, launch is an extension function, so it is defined in BuildersKt. The first parameter is the target of the extension function, the rest are the parameters from the extension functions.

If you dont want to use org.jetbrains.kotlinx:kotlinx-coroutines-jdk8, I have a new idea.

Write below code in your kotlin project.

    @JvmOverloads
    fun <R> getContinuation(onFinished: BiConsumer<R?, Throwable?>, dispatcher: CoroutineDispatcher = Dispatchers.Default): Continuation<R> {
        return object : Continuation<R> {
            override val context: CoroutineContext
                get() = dispatcher

            override fun resumeWith(result: Result<R>) {
                onFinished.accept(result.getOrNull(), result.exceptionOrNull())
            }
        }
    }

I write it in my Coroutines class

Then you can call your suspend function like:

            Coroutines coroutines = new Coroutines();
            UserUtils.INSTANCE.login("user", "pass", coroutines.getContinuation(
                    (tokenResult, throwable) -> {
                        System.out.println("Coroutines finished");
                        System.out.println("Result: " + tokenResult);
                        System.out.println("Exception: " + throwable);
                    }
            ));

login() function is a suspend function.
suspend fun login(username: String, password: String): TokenResult

For your code, you can:

doSomething(getContinuation((result, throwable) -> { 
       //TODO
}));

Besides, you may want to run your callback code in different thread (e.g. Main thread), just use launch(Dispathers.Main) to wrap resumeWith()

First, add org.jetbrains.kotlinx:kotlinx-coroutines-jdk8 module to your dependencies. In your Kotlin file define the following async function that corresponds to Java style of writing async APIs:

fun doSomethingAsync(): CompletableFuture<List<MyClass>> =
    GlobalScope.future { doSomething() }

Now use doSomethingAsync from Java in the same way as you are using other asynchronous APIs in the Java world.