Kotlin: how to pass an object function as parameter to another? - kotlin

Question

I am trying to learn functional Kotlin and have written this test code:
import java.util.*
data class BorrowerX(val name: String, val maxBooks: Int) {
companion object {
fun getName(br: BorrowerX): String = br.name
fun findBorrowerX(n: String, brs: ArrayList<BorrowerX>): BorrowerX? {
val coll: List<BorrowerX> = brs.filter { BorrowerX.getName(it) == n }
if (coll.isEmpty()) {
return null
} else return coll.first()
}
fun findBorrowerX2(n: String, brs: ArrayList<BorrowerX>, f: (BorrowerX) -> String): BorrowerX? {
val coll: List<BorrowerX> = brs.filter { f(it) == n }
if (coll.isEmpty()) {
return null
} else return coll.first()
}
}
}
In the REPL I can successfully call "findBorrowerX":
import BorrowerX
val br1 = BorrowerX(name = "Borrower1", maxBooks = 1)
val br2 = BorrowerX(name = "Borrower2", maxBooks = 2)
val br3 = BorrowerX(name = "Borrower3", maxBooks = 3)
val brs1 = arrayListOf(br1, br2, br3)
BorrowerX.findBorrowerX("Borrower1", brs1)
BorrowerX(name=Borrower1, maxBooks=1)
BorrowerX.findBorrowerX("Borrower-Bad", brs1)
null
But how do I make the call to "findBorrowerX2":
BorrowerX.findBorrowerX2("Borrower1", brs1, BorrowerX.getName(???))
And pass the iterated BorrowerX to getName??
This looks related, but I'm not sure:
Kotlin: how to pass a function as parameter to another?
Thank you in advance for your help with this!
EDIT:
Here is the equivalent Scala code for what I want to do:
def findBorrowerX2(n: String, brs: List[BorrowerX], f: BorrowerX => String): BorrowerX = {
val coll: List[BorrowerX] = brs.filter(f(_) == n)
if (coll.isEmpty) {
null
} else {
coll.head
}
}
scala> BorrowerX.findBorrowerX2("Borrower3", brs1, BorrowerX.getName(_))
res1: BorrowerX = BorrowerX(Borrower3,3)
scala> BorrowerX.findBorrowerX2("Borrower33", brs1, BorrowerX.getName(_))
res2: BorrowerX = null
Perhaps this is not possible in Kotlin?

Answer 1

You can use :: operator to get a function reference:
BorrowerX.findBorrowerX2("Borrower1", brs1, BorrowerX.Companion::getName)
Here BorrowerX.Companion::getName is a reference to the function getName declared in the companion object (named Companion) of the class BorrowerX. It has the type KFunction1<BorrowerX, String> which is a subtype of the required functional parameter type (BorrowerX) -> String.
It's worth noting that you can use :: operator to get a property reference too:
BorrowerX.findBorrowerX2("Borrower1", brs1, BorrowerX::name)
BorrowerX::name has the type KProperty1<BorrowerX, String> which also is a subtype of (BorrowerX) -> String. When invoked with the specified BorrowerX instance it returns the value of its name property.

Answer 2

As stated in the documentation on lambdas:
BorrowerX.findBorrowerX2("Borrower-Bad", brs1, { it.name })
or when the lambda is the last parameter of the method:
BorrowerX.findBorrowerX2("Borrower-Bad", brs1) { it.name }
Stating types and parameter names explicitly often improves readability:
BorrowerX.findBorrowerX2("Borrower-Bad", brs1) { borrower:BorrowerX -> borrower.name }