用`_`定义java.util.Set的读取

Question

Using the Play Framework's JSON library, I'm trying to define a Reads[Obj] for:

Obj.java

public class Obj {
  private Long id;
  private String description;
  private Set<Group> groups;

  public Obj(Long id, String description, Set<Group> groups) { 
    // set fields 
  }
}

However, when I use _.asJava , I get missing parameter type compile-time error.

import play.api.libs.json._
import play.api.libs.functional.syntax._
import scala.collection.JavaConverters._

      implicit val ObjReads: Reads[Obj] = (
          (JsPath \ "id").read[Long] and
          (JsPath \ "description").read[String] and
          (JsPath \ "groups").read[Set[Group]] and
        )(new Obj(_,_,_.asJava))

Note that I need to call asJava to convert from scala.collection.immutable.Set to java.util.Set .

Without naming each parameter and its type, how can I succinctly call asJava on the third element?

Answer 1

You can't always always just throw in underscores instead of names because the compiler doesn't always know how you want to resolve them.

def f(a: String, b: String) = f"$a~$b"

Vector(1,2,3).foldLeft("0")(f(_, _.toString))           // ERROR
Vector(1,2,3).foldLeft("0")((z,x) => f(z, x.toString))  // fine

Just giving names to the parameters is an easy way to fix it.