Exercise: Function references
This is a continuation of the exercise "Function types and literals". This time, your task is to implement the following classes:
FunctionReference- that defines propertiesprintNum,tripleandproduceNamewith explicit function types, and define their values using references to functions from the Kotlin standard library or fromNameclass.FunctionMemberReferencethat defines propertiesprintNum,triple,produceNameandlongestOfwith explicit function types, and define their values using references to methods from its own body.BoundedFunctionReferencethat defines propertiesprintNum,triple,produceNameandlongestOfwith explicit function types, and define their values using references to methodsclassicobject.
This problem can either be solved in the below playground or you can clone kotlin-exercises project and solve it locally. In the project, you can find code template for this exercise in functional/base/References.kt. You can find there starting code and unit tests.
Once you are done with the exercise, you can check your solution here.
Playground
import org.junit.Test
import kotlin.reflect.KClass
import kotlin.reflect.typeOf
import kotlin.test.assertEquals
import kotlin.test.assertNotNull
class FunctionsClassic {
fun add(num1: Int, num2: Int): Int = num1 + num2
fun printNum(num: Int) {
print(num)
}
fun triple(num: Int): Int = num * 3
fun produceName(name: String): Name = Name(name)
fun longestOf(
str1: String,
str2: String,
str3: String,
): String =
maxOf(str1, str2, str3, compareBy { it.length })
}
data class Name(val name: String)
class FunctionReference {
val add: (Int, Int) -> Int = Int::plus
// TODO: Implement printNum, triple and produceName properties using function references
// to functions from the Kotlin stdlib or from the Name class
// See add property for example
}
class FunctionMemberReference {
val add: (Int, Int) -> Int = this::add
// TODO: Implement printNum, triple, produceName and longestOf properties using function references
// to functions from the current class
// See add property for example
private fun add(num1: Int, num2: Int): Int = num1 + num2
private fun printNum(num: Int) {
print(num)
}
private fun triple(num: Int): Int = num * 3
private fun produceName(name: String): Name = Name(name)
private fun longestOf(
str1: String,
str2: String,
str3: String
): String =
maxOf(str1, str2, str3, compareBy { it.length })
}
class BoundedFunctionReference {
private val classic = FunctionsClassic()
val add: (Int, Int) -> Int = classic::add
// TODO: Implement printNum, triple, produceName and longestOf properties using function references
// to functions from the `classic` object
// See add property for example
}
class ReferencesTest {
@Test
fun `FunctionReference has correct property signatures`() {
checkPropertySignatures(FunctionReference::class, expectLongestOf = false)
}
@Test
fun `FunctionReference has correct property behavior`() {
checkPropertyBehavior(FunctionReference(), expectLongestOf = false)
}
@Test
fun `FunctionMemberReference has correct property signatures`() {
checkPropertySignatures(FunctionMemberReference::class)
}
@Test
fun `FunctionMemberReference has correct property behavior`() {
checkPropertyBehavior(FunctionMemberReference())
}
@Test
fun `BoundedFunctionReference has correct property signatures`() {
checkPropertySignatures(BoundedFunctionReference::class)
}
@Test
fun `BoundedFunctionReference has correct property behavior`() {
checkPropertyBehavior(BoundedFunctionReference())
}
private fun checkPropertySignatures(
classToCheck: KClass<*>,
expectLongestOf: Boolean = true,
) {
val c = classToCheck.members
val properties = mutableMapOf(
"add" to typeOf<(Int, Int) -> Int>(),
"printNum" to typeOf<(Int) -> Unit>(),
"triple" to typeOf<(Int) -> Int>(),
"produceName" to typeOf<(String) -> Name>(),
)
if (expectLongestOf) {
properties += "longestOf" to typeOf<(String, String, String) -> String>()
}
for ((propertyName, propertyType) in properties) {
val propertyReference = c.find { it.name == propertyName }
assertNotNull(propertyReference) { "Property $propertyName is missing" }
assertEquals(propertyType, propertyReference.returnType, "Property $propertyName has wrong type")
}
}
@Suppress("UNCHECKED_CAST")
private fun <T: Any> checkPropertyBehavior(
instance: T,
expectLongestOf: Boolean = true,
) {
val members = instance::class.members
val add = members.find { it.name == "add" }!!
assertEquals(3, (add.call(instance) as (Int, Int) -> Int)(1, 2))
assertEquals(12, (add.call(instance) as (Int, Int) -> Int)(4, 8))
val printNum = members.find { it.name == "printNum" }!!
(printNum.call(instance) as (Int) -> Unit)(42)
val triple = members.find { it.name == "triple" }!!
assertEquals(9, (triple.call(instance) as (Int) -> Int)(3))
assertEquals(15, (triple.call(instance) as (Int) -> Int)(5))
val produceName = members.find { it.name == "produceName" }!!
assertEquals(Name("John"), (produceName.call(instance) as (String) -> Name)("John"))
assertEquals(Name("Jane"), (produceName.call(instance) as (String) -> Name)("Jane"))
if (expectLongestOf) {
val longestOf = members.find { it.name == "longestOf" }!!
assertEquals("abc", (longestOf.call(instance) as (String, String, String) -> String)("a", "ab", "abc"))
assertEquals("xyz", (longestOf.call(instance) as (String, String, String) -> String)("x", "xy", "xyz"))
}
}
}
