Kotlin function reference

Question

Let records be stream/collection and extract function which transforms data form an element of such collection.

Is there a way in Kotlin to

Answer 1

• If extract is a value (local variable, property, parameter) of a functional type (T) -> R or T.() -> R for some T and R, then you can pass it directly to map:

  records.map(extract)
  

  ^Example:

  val upperCaseReverse: (String) -> String = { it.toUpperCase().reversed() }
  
  listOf("abc", "xyz").map(upperCaseReverse) // [CBA, ZYX]
  

• If extract is a top-level single argument function or a local single argument function, you can make a function reference as ::extract and pass it to map:

  records.map(::extract)
  

  ^Example:

  fun rotate(s: String) = s.drop(1) + s.first()
  
  listOf("abc", "xyz").map(::rotate) // [bca, yzx]
  

• If it is a member or an extension function of a class SomeClass accepting no arguments or a property of SomeClass, you can use it as SomeClass::extract. In this case, records should contain items of SomeType, which will be used as a receiver for extract.

  records.map(SomeClass::extract)
  

  ^Example:

  fun Int.rem2() = this % 2
  
  listOf("abc", "defg").map(String::length).map(Int::rem2) // [1, 0]
  

• Since Kotlin 1.1, if extract is a member or an extension function of a class SomeClass accepting one argument, you can make a bound callable reference with some receiver foo:

  records.map(foo::extract)
  records.map(this::extract) // to call on `this` receiver
  

  ^Example:

  listOf("abc", "xyz").map("prefix"::plus) // [prefixabc, prefixxyz]
  

^{(runnable demo with all the code samples above)}