How to use Java lambdas in Scala

问题:

I have the following code:

source
    .mapValues(value -> value + " Stream it!!!")
    .print(Printed.toSysOut());

as you can see, mapValues expects a lambda expression.

Now, I am using Java library but the application is written in Scala. How to pass Scala lambda to Java code?

I tried the following:

source
  .mapValues(value => value + "hello")
  .print(Printed.toSysOut)

But the compiler complains:

[error]   (x$1: org.apache.kafka.streams.kstream.Printed[String,?0(in value x$1)])Unit <and>
[error]   (x$1: org.apache.kafka.streams.kstream.KeyValueMapper[_ >: String, _ >: ?0(in value x$1), String])Unit <and>
[error]   (x$1: String)Unit
[error]  cannot be applied to (org.apache.kafka.streams.kstream.Printed[Nothing,Nothing])
[error]       .print(Printed.toSysOut)
[error]        ^
[error] two errors found
[error] (compile:compileIncremental) Compilation failed
[error] Total time: 2 s, completed Nov 19, 2017 7:53:44 PM

回答1:

The error message lists the types of arguments that print supports. One of them is:

org.apache.kafka.streams.kstream.Printed[String,?0(in value x$1)]

From the error message you can see that you're providing Printed.toSysOut with a type of:

org.apache.kafka.streams.kstream.Printed[Nothing,Nothing]

According to the Kafka 1 javadoc (Printed was not present in Kafka 1.1), toSysOut is defined as:

public static <K,V> Printed<K,V> toSysOut()

So the answer problem is that Scala is inferring K and V with types of Nothing. You need to provide the types explicitly.

The following will probably work:

source
  .mapValues[String](value -> value + " Stream it!!!")
  .print(Printed.toSysOut[String,String])

回答2:

It depends on your version of Scala.

In 2.12 Scala functions can be used in places where Java functions are expected and vice versa.

App1.java

import java.util.function.Function;

public class App1 {
    public static void method(Function<String, String> function) {
        System.out.println(function.apply("a"));
    }

    public static void main(String[] args) {
        App.method1((String s) -> s.toUpperCase());
    }
}

App.scala

object App {
  def main(args: Array[String]): Unit = {
    App1.method((s: String) => s.toUpperCase)
  }

  def method1(function: String => String): Unit = {
    println(function("xyz"))
  }
}

In 2.11 you can use scala-java8-compat

libraryDependencies += "org.scala-lang.modules" %% "scala-java8-compat" % "0.8.0"

App1.java

import java.util.function.Function;
import static scala.compat.java8.JFunction.func;

public class App1 {
    public static void method(Function<String, String> function) {
        System.out.println(function.apply("a"));
    }

    public static void main(String[] args) {
        App.method1(func((String s) -> s.toUpperCase()));
    }
}

App.scala

import scala.compat.java8.FunctionConverters._

object App {
  def main(args: Array[String]): Unit = {
    App1.method(((s: String) => s.toUpperCase).asJava)
  }

  def method1(function: String => String): Unit = {
    println(function("xyz"))
  }
}

Alternatively in 2.11 in Scala you can define implicit converters between java.util.function.Function and scala.Function1.

So if you use 2.11 try

source
  .mapValues((value => value + "hello").asJava)
  .print(Printed.toSysOut) 

or

source
  .mapValues(((value: String) => value + "hello").asJava)
  .print(Printed.toSysOut[String, String])