问题

I have created an interface:

interface ProgressListener {
    fun transferred(bytesUploaded: Long)
}

but can use it only as anonymous class, not lambda

dataManager.createAndSubmitSendIt(title, message,
object : ProgressListener {
    override fun transferred(bytesUploaded: Long) {
        System.out.println(bytesUploaded.toString())
    }
})

I think it should be a possibility to replace it by lambda:

dataManager.createAndSubmitSendIt(title, message, {System.out.println(it.toString())})

But I am getting error: Type mismatch; required - ProgressListener, found - () -> Unit?

What am I doing wrong?

回答1:

As @zsmb13 said, SAM conversions are only supported for Java interfaces.

You could create an extension function to make it work though:

// Assuming the type of dataManager is DataManager.
fun DataManager.createAndSubmitSendIt(title: String, 
                                      message: String, 
                                      progressListener: (Long) -> Unit) {
    createAndSubmitSendIt(title, message,
        object : ProgressListener {
            override fun transferred(bytesUploaded: Long) {
                progressListener(bytesUploaded)
            }
        })
}

回答2:

Kotlin only supports SAM conversions for Java interfaces.

... note that this feature works only for Java interop; since Kotlin has proper function types, automatic conversion of functions into implementations of Kotlin interfaces is unnecessary and therefore unsupported.

-- Official documentation

If you want to use a lambda in the parameter, make your function take a function parameter instead of an interface. (For now at least. Supporting SAM conversions for Kotlin interfaces is an ongoing discussion, it was one of the possible future features at the Kotlin 1.1 live stream.)

回答3:

A little late to the party: instead of making an interface, you let the compile create one by taking a function directly instead of an interface in your datamanager, like this:

fun createAndSubmitSendIt(title: String, message: String, transferred: (Long) -> Unit) {
    val answer = TODO("whatever you need to do")
    transferred(answer)
}

and then you just use it like how you want it! If I remember correctly, what the kotlin/jvm compiler do is the same as making an interface.

Hope it helps!

回答4:

Another solution would be by declaring a typealias, injecting it somewhere and invoking it. Here the example:

internal typealias WhateverListener = (String) -> Unit

and then we inject that typealias to our class:

class Gallery constructor(private val whateverListener: WhateverListener) {

    ...

    galleryItemClickListener.invoke("hello")

    ...
}

so we have our lambda:

val gallery = Gallery { appNavigator.openVideoPlayer(it) }

Credits to my colleague Joel Pedraza, who showed me the trick while trying to find a solution <3.

回答5:

There is no single ultimate solution for this problem if you are aiming best access experience from both Kotlin and Java.

If Kotlin developers had not thought SAM conversion for Kotlin interfaces is unnecessary, "Kotlin Interface" method would be ultimate solution.

https://kotlinlang.org/docs/reference/java-interop.html#sam-conversions
Also note that this feature works only for Java interop; since Kotlin has proper function types, automatic conversion of functions into implementations of Kotlin interfaces is unnecessary and therefore unsupported.

Select the best solution for your use case.

Kotlin Function Type

• Kotlin API: Perfect
• Kotlin Access: Perfect
• Java Access:
  • Auto generated parameter type like Function1 (not a big problem for Java 8 lambda)
  • Verbose return Unit.INSTANCE; instead of void return.

class KotlinApi {
    fun demo(listener: (response: String) -> Unit) {
       listener("response")
    }
}

fun kotlinConsumer() {
    KotlinApi().demo { success ->
        println(success)
    }
}


public void javaConsumer() {
    new KotlinApi().demo(s -> {
        System.out.println(s);
        return Unit.INSTANCE;
    });
}

Kotlin Interface

• Kotlin API: Additional interface definition.
• Kotlin Access: Too verbose
• Java Access: Perfect

class KotlinApi {
    interface Listener {
        fun onResponse(response: String)
    }

    fun demo(listener: Listener) {
       listener.onResponse("response")
    }
}

fun kotlinConsumer() {
    KotlinApi().demo(object : KotlinApi.Listener {
        override fun onResponse(response: String) {
            println(response)
        }
    })
}

//If Kotlin had supported SAM conversion for Kotlin interfaces. :(
//fun kotlinConsumer() {
//    KotlinApi().demo {
//        println(it)
//    }
//}

public void javaConsumer() {
    new KotlinApi().demo(s -> {
        System.out.println(s);
    });
}

Java Interface

• Kotlin API: Mixed Java code.
• Kotlin Access: A little verbose
• Java Access: Perfect

class KotlinApi {
    fun demo(listener: Listener) {
        listener.onResponse("response")
    }
}

public interface Listener {
    void onResponse(String response);
}

//Semi SAM conversion
fun kotlinConsumer() {
    KotlinApi().demo(Listener {
        println(it)
    })
}

public void javaConsumer() {
    new KotlinApi().demo(s -> {
        System.out.println(s);
    });
}

Multiple Methods

• Kotlin API: Multiple method implementations
• Kotlin Access: Perfect if correct method is used. Auto completion suggests verbose method also.
• Java Access: Perfect. Auto completion does not suggest function type method because of JvmSynthetic annotation

class KotlinApi {
    interface Listener {
        fun onResponse(response: String)
    }

    fun demo(listener: Listener) {
        demo {
            listener.onResponse(it)
        }
    }

    @JvmSynthetic //Prevents JVM to use this method
    fun demo(listener: (String) -> Unit) {
        listener("response")
    }
}

fun kotlinConsumer() {
    KotlinApi().demo {
        println(it)
    }
}

public void javaConsumer() {
    new KotlinApi().demo(s -> {
        System.out.println(s);
    });
}

Java API

• Kotlin API: There is no Kotlin API, all API code is Java
• Kotlin access: Perfect
• Java access: Perfect

public class JavaApi {
    public void demo(Listener listener) {
        listener.onResponse("response");
    }

    public interface Listener {
        void onResponse(String response);
    }

}

//Full SAM conversion
fun kotlinConsumer() {
    JavaApi().demo {
        println(it)
    }
}

public void javaConsumer() {
    new JavaApi().demo(s -> {
        System.out.println(s);
    });
}

来源：https://stackoverflow.com/questions/43469241/passing-lambda-instead-of-interface