How to invoke scala method containing functional parmeter in java?

Question

env:

java:1.8.0_201
scala:2.11.8
IDE: IntelliJ IDEA 2018.3.5 (Ultimate Edition) (Build #IU-183.5912.21)
Intellij Scala plugin:V2018.3.6

I have a class method like this

class ScalaA {
  def cal(op: () => String): String = {
    op.apply()
  }
}

And I want to invoke method cal, my first try is like this

public class JavaB {

  public static void main(String[] args) {
    new ScalaA().cal(() -> "Hello");

  }
}

But an compile error occured:

Error:(9, 22) java: incompatible types: scala.Function0 is not a functional interface
    multiple non-overriding abstract methods found in interface scala.Function0

Then I try to invoke method cal like this:

public static void main(String[] args) {
    new ScalaA().cal(new Function0<String>() {
      @Override
      public String apply() {
        return "aa";
      }
    });
  }

But there is still a compile error

Error:(9, 46) java: is not abstract and does not override abstract method apply$mcV$sp() in scala.Function0

It seems method apply$mcV$sp() doesn't implemented, but I don't see a method named apply$mcV$sp() in scala.Function0

trait Function0[@specialized(Specializable.Primitives) +R] extends AnyRef { self =>
  /** Apply the body of this function to the arguments.
   *  @return   the result of function application.
   */
  def apply(): R

  override def toString() = "<function0>"
}

, what should I do to solve this problem, could you give me some suggesitions, thanks in advance!