How to use java8 lambda and function in scala

Question

I have a code like this:

public class A {

    private String id;
    //constructor, getter, setter

}

In java I can use this:

public class C {

    @Test
    public void t() {
        List<A> list = Arrays.asList(new A("1"));
        list.sort(Comparator.comparing(A::getId));
        Map<String, List<A>> map = list.stream().collect(Collectors.groupingBy(A::getId));
    }
}

Here is scala(2.12) test:

class B {

  @Test
  def t(): Unit = {
    val list = util.Arrays.asList(new A("1"))
    list.sort(Comparator.comparing(a => a.getId))
    list.stream().collect(Collectors.groupingBy(a => a.getId))
  }

}

But in scala test, list.sort(Comparator.comparing(a => a.getId)) will get two errors:

Error:(21, 26) inferred type arguments [com.test.A,?0] do not conform to method comparing's type parameter bounds [T,U <: Comparable[_ >: U]]
Error:(21, 38) type mismatch; found : java.util.function.Function[com.test.A,String] required: java.util.function.Function[_ >: T, _ <: U]

and list.stream().collect(Collectors.groupingBy(a => a.getId)) will get this error:

Error:(22, 49) missing parameter type

How can I use it?

I only consider completing it by means of java.util.List, but how to convert to java.util.Map<K, java.util.List> after groupBy. — 西门吹雪
I hope you have heard about javaConvertors in Decorate package there you will get a method asJava so I will get converted into the java Map — Raman Mishra
Yes, I know, but I feel that it is not convenient to make multiple conversions. list -> asScala -> asJava ... — 西门吹雪
1. Provide exact error messages; 2. Are you using Scala 2.12? 3. Are you using Scala List or Java one in Scala? — Alexey Romanov

Dmytro Mitin Dmytro Mitin · Accepted Answer · 2019-04-02T09:42:19

Try

import scala.compat.java8.FunctionConverters._

val list = util.Arrays.asList(new A("1"))
list.sort(Comparator.comparing[A, String](((a: A) => a.getId).asJava))
list.stream().collect(Collectors.groupingBy[A, String](a => a.getId))

libraryDependencies += "org.scala-lang.modules" %% "scala-java8-compat" % "0.9.0"

How to use Java lambdas in Scala

How to use java8 lambda and function in scala

2 Answers