Java8 ContainingType::methodName references

[pragp...@gmail.com]

Java 8 supports method references using the double colon :: operator.
https://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html

There are four types of these:
-ContainingClass::staticMethodName (Reference to a static method)
-containingObject::instanceMethodName (Reference to an instance method of a particular object)
-ContainingType::methodName (Reference to an instance method of an arbitrary object of a particular type)
-ClassName::new (Reference to a constructor)

In Scala, for example this one is supported: containingObject::instanceMethodName
by using:
containingObject.instanceMethodName _

But what suprises me is that Scala doesn't seem to support ContainingType::methodName.
ContainingType::methodName can be handy when for example you need to dynamically create objects and perform reflections to call getter and setter method in a strong typed way. Which can be handy in UI code and object construction/binding code.

Other languages also support this:
-Java: https://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html
-Kotlin: https://kotlinlang.org/docs/reference/reflection.html
-Groovy: http://groovy-lang.org/metaprogramming.html (ExpandoMetaClass)
-C#: http://joelabrahamsson.com/getting-property-and-method-names-using-static-reflection-in-c/

It would be nice if something like this would be possible in Scala:

  case class Person(name: String)

  def getMethodName(....): String = {
    // ....
  }
 
  getMethodName(Person::name) // returns "name" or "Person.name"

Am I missing something?

[Sébastien Doeraene]

Hi,

You can get what you want with:

val f: (Person, Person) => Int = _ instanceMethodName _

or, in the absence of an expected type helping the inference of the type of _,

val f = (_: Person) instanceMethodName (_: Person)

Cheers,

Sébastien

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[pragp...@gmail.com]

Hi, thank you for your reply.

I just tried this:

class Person {
  def firstname() = {
    "Scala"
  }
}

val f: (Person, Person) => Int = _ firstname _
val g = (_: Person) firstname (_: Person)

But this result in this compilation error:

Error:(9, 23) too many arguments for method firstname: ()String
  val g = (_: Person) firstname (_: Person)
                      ^
Error:(8, 38) too many arguments for method firstname: ()String
  val f: (Person, Person) => Int = _ firstname _
                                     ^

Cheers,
Prag

[Sébastien Doeraene]

For a method without parameter, it becomes:

(_: Person).firstname

or

_.firstname

Sébastien

[pragp...@gmail.com]

Ah, that would be a solution for the syntax.
The second part of the question is how to reflect on this function body to get the property name back as a string.

Example:

class Person {
  def age() = {
    Random.nextInt
  }
}

def getMethodName(property: Person => Int): String = {
    // How to reflect the property name itself out of the anonymous function
}

val property: Person => Int = (_: Person).age

println(getMethodName(property)) // should print "Person.age"

This that possible with Scala reflections?

[Jasper-M]

As far as I can see Java doesn't make it possible either to reflect on method references to get the method name. (http://stackoverflow.com/questions/19845213/how-to-get-the-methodinfo-of-a-java-8-method-reference)

I think you can use the same trick as they do in Java if you really want to do this: http://benjiweber.co.uk/blog/2013/12/28/typesafe-database-interaction-with-java-8/

Best,

Jasper

Op vrijdag 4 maart 2016 23:46:18 UTC+1 schreef pragp...@gmail.com: