We're trying to do some generic processing in kotlin. Basically, for a given class, we want to get the related Builder object. i.a. for any object that extends a GenericObject, we want a Builder of that Object.
interface Builder<T : GenericObject>
object ConcreteBuilder: Builder<ConcreteObject>
We'd need a function that will return ConcreteBuilder from ConcreteObject
Our current implementation is a Map:
val map = mapOf<KClass<out GenericObject>, Builder<out GenericObject>>(
ConcreteObject::class to ConcreteBuilder
)
Then we can get it with:
inline fun <reified T : GenericObject> transform(...): T {
val builder = map[T::class] as Builder<T>
...
However this isn't very nice as:
we need an explicit cast to Builder<T>
the map has no notion of T, a key and a value could be related to different types.
Is there any better way to achieve it?
A wrapper for the map could be:
class BuilderMap {
private val map = mutableMapOf<KClass<out GenericObject>, Builder<out GenericObject>>()
fun <T: GenericObject> put(key: KClass<T>, value: Builder<T>) {
map[key] = value
}
operator fun <T: GenericObject> get(key: KClass<T>): Builder<T> {
return map[key] as Builder<T>
}
}
This hides the ugliness, while not completely removing it.
To use:
val builderMap = BuilderMap()
builderMap.put(ConcreteObject::class, ConcreteBuilder)
builderMap.put(BetonObject::class, BetonBuilder)
// builderMap.put(BetonObject::class, ConcreteBuilder) – will not compile
val builder = builderMap[T::class]
Related
I'm trying to return a property/value from Data class by checking type parameter
Data Class :
data class SystemConfiguration(
val systemName: String,
val fields: List<String>
)
Abstract class :
abstract class ConfigurationLoader<out T> {
abstract val clazz: Class<out T>
private fun getAssociateAttribute(file: T): String{
return when(clazz){
SystemConfiguration::class.java -> file.systemName // Line causing error
// If its another Data class, I should return value from that data class
else -> ""
}
}
open fun loadConfigurations(): Map<String, T>{
val map = Paths.get(configurationFolderPath, filePath).toFile().walkTopDown().filter { it.isFile }.map {
val x = ionSystem.loader.load(it)[0]
ionValueMapper.readValue<List<T>>(x.toString())
}.flatten().associateBy { getAssociateAttribute(it) }
}
}
inline fun <reified T: Any> javaClasstype(): Class<T> {
return T::class.java
}
Sub class
class ServiceConfigurationLoader (
override val clazz: Class<SystemConfiguration> = javaClasstype()
): ConfigurationLoader<SystemConfiguration>()
I'm getting an exception "e: Unresolved reference: systemName". Not able to access values inside data class while we use type parameter
If i use like this(directly mentioning data class name), I'm able to access the values
private fun getAssociateAttribute(file: SystemConfiguration): String{
return when(clazz){
SystemConfiguration::class.java -> file.systemName
else -> ""
}
}
Could someone help me out here to access the value using Type paramater in Kotlin?
Thanks in Advance !!
I have tried using reified keyword as well. Still getting the same issue. I'm expecting to access the value using Type paramater
I created class with generic in kotlin and want to use receive with generic, but I have error when i want to call.recieve type from generic:
Can not use MType as reified type parameter. Use a class instead.
Code:
class APIRoute<EType : IntEntity, MType : Any> {
fun Route.apiRoute() {
post {
val m = call.receive<MType>()
call.respond(f(model))
}
}
}
How to fix it?
You need to provide the expected type to the receive() function. Due to type erasure in Java/Kotlin, the type of MType is unknown at runtime, so it can't be used with receive(). You need to capture the type as KType or KClass object when constructing APIRoute.
KClass is easier to use, however it works with raw classes only, it doesn't support parameterized types. Therefore, we can use it to create e.g. APIRoute<*, String>, but not APIRoute<*, List<String>>. KType supports any type, but is a little harder to handle.
Solution with KClass:
fun main() {
val route = APIRoute<IntEntity, String>(String::class)
}
class APIRoute<EType : IntEntity, MType : Any>(
private val mClass: KClass<MType>
) {
fun Route.apiRoute() {
post {
val m = call.receive(mClass)
call.respond(f(model))
}
}
}
Solution with KType:
fun main() {
val route = APIRoute.create<IntEntity, List<String>>()
}
class APIRoute<EType : IntEntity, MType : Any> #PublishedApi internal constructor(
private val mType: KType
) {
companion object {
#OptIn(ExperimentalStdlibApi::class)
inline fun <EType : IntEntity, reified MType : Any> create(): APIRoute<EType, MType> = APIRoute(typeOf<MType>())
}
fun Route.apiRoute() {
post {
val m = call.receive<MType>(mType)
call.respond(f(model))
}
}
}
I've created a Kotlin equivalent of TypeReference<T> like so:
abstract class TypeReference<T> : Comparable<T> {
val type: Type get() = getGenericType()
val arguments: List<Type> get() = getTypeArguments()
final override fun compareTo(other: T): Int {
return 0
}
private fun getGenericType(): Type {
val superClass = javaClass.genericSuperclass
check(superClass !is Class<*>) {
"TypeReference constructed without actual type information."
}
return (superClass as ParameterizedType).actualTypeArguments[0]
}
private fun getTypeArguments(): List<Type> {
val type = getGenericType()
return if (type is ParameterizedType) {
type.actualTypeArguments.toList()
} else emptyList()
}
}
In order to obtain Class<*> of the generic type and its arguments, I've also created the following extension function (and this is where I believe the problem lies, since this is where the stack trace fails).
fun Type.toClass(): Class<*> = when (this) {
is ParameterizedType -> rawType.toClass()
is Class<*> -> this
else -> Class.forName(typeName)
}
I'm unit testing this like so:
#Test
fun `TypeReference should correctly identify the List of BigDecimal type`() {
// Arrange
val expected = List::class.java
val expectedParameter1 = BigDecimal::class.java
val typeReference = object : TypeReference<List<BigDecimal>>() {}
// Act
val actual = typeReference.type.toClass()
val actualParameter1 = typeReference.arguments[0].toClass()
// Assert
assertEquals(expected, actual)
assertEquals(expectedParameter1, actualParameter1)
}
The problem I think, lies in the extension function else -> Class.forName(typeName) as it throws:
java.lang.ClassNotFoundException: ? extends java.math.BigDecimal
Is there a better way to obtain the Class<*> of a Type, even when they're generic type parameters?
You need to add is WildcardType -> ... branch to your when-expression to handle types like ? extends java.math.BigDecimal (Kotlin equivalent is out java.math.BigDecimal), ?(Kotlin equivalent is *), ? super Integer(Kotlin equivalent is in java.math.Integer):
fun Type.toClass(): Class<*> = when (this) {
is ParameterizedType -> rawType.toClass()
is Class<*> -> this
is WildcardType -> upperBounds.singleOrNull()?.toClass() ?: Any::class.java
else -> Class.forName(typeName)
}
Note that in this implementation single upper bound types will be resolved as its upper bound, but all other wildcard types (including multiple upper bounds types) will be resolved as Class<Object>
https://github.com/pluses/ktypes
val typeReference = object : TypeReference<List<BigDecimal>>() {}
val superType = typeReference::class.createType().findSuperType(TypeReference::class)!!
println(superType.arguments.first())// List<java.math.BigDecimal>
println(superType.arguments.first().type?.arguments?.first())// java.math.BigDecimal
I´m working on a code with generics and when I use an in I got a TypeMismatch when compiling.
The code is the following:
open class A
class B:A()
data class DataContainer(val a:String,
val b:A)
interface Repo<T:A>{
fun setParam(param:T)
fun getParam():T
}
abstract class RepoImp<T:A>:Repo<T>{
private lateinit var parameter:T
override fun setParam(param: T) {
parameter = param
}
override fun getParam(): T {
return parameter
}
}
class BRepo:RepoImp<B>()
class Repo2(val repo: Repo<in A>){
fun process(b:DataContainer){
repo.setParam(b.b)
}
}
val repoB = BRepo()
val repo2 = Repo2(repoB)// Here I got: Type mismatch: inferred type is BRepo but Repo<in A> was expected
I also tried changing the attribute repo from Repo2 to Repo<*>
Since BRepo is a Repo<B>, it is not a Repo<in A>, (but it would satisfy Repo<out A>).
In other words, a Repo<in A> must be able to accept setParam(A()), but BRepo.setParam() can only accept a B or subclass of B.
Or to put it another way, BRepo is a Repo<B>, which is a tighter restriction on the type than Repo<A> when it comes to writing values (but looser restriction when reading values).
The reason class Repo2(val repo: Repo<*>) doesn't work is that Repo<*> is essentially a Repo<in Nothing/out A>. You can't call setParam() on a Repo<*> with any kind of object.
There's a design flaw in your code that you can't fix simply by changing Repo2's constructor signature. As it stands now, Repo2 needs to be able write A's to the object you pass to it, and a BRepo by definition does not support writing A's, only B's. You will need to make at least one of your class's definitions more flexible about types.
It might be easier to understand the covariance limitation with more common classes:
val stringList: MutableList<String> = ArrayList()
var anyList: MutableList<in Any> = ArrayList()
anyList.add(5) // ok
anyList = stringList // Compiler error.
// You wouldn't be able to call add(5) on an ArrayList<String>
Basically MutableList<String> is not a MutableList<in Any> the same way Repo<B> is not a Repo<in A>.
The Repo2 class expect to consume only type A, use Repo2<T : A>(val repo: Repo<in T>)
open class A
class B : A()
class C : A()
class D : A()
class BRepo : RepoImp<B>()
class CRepo : RepoImp<C>()
class DRepo : RepoImp<D>()
interface Repo<T : A> {
fun setParam(param: T)
fun getParam(): T
}
abstract class RepoImp<T : A> : Repo<T> {
private lateinit var parameter: T
override fun setParam(param: T) {
parameter = param
}
override fun getParam(): T {
return parameter
}
}
class Repo2<T : A>(val repo: Repo<in T>) {
fun process(b: DataContainer<T>) {
repo.setParam(b.b)
}
}
data class DataContainer<T : A>(
val a: String,
val b: T
)
fun main() {
val repoB = BRepo()
val repoC = CRepo()
val repoD = DRepo()
val repo2 = Repo2(repoB)
val repo3 = Repo2(repoC)
val repo4 = Repo2(repoD)
repo2.process(DataContainer("Process B type", B()))
repo3.process(DataContainer("Process C type", C()))
repo4.process(DataContainer("Process D type", D()))
println(repo2.repo.getParam())
println(repo3.repo.getParam())
println(repo4.repo.getParam())
}
In javascript we can do something like this
function putritanjungsari(data){
console.log(data.name)
}
let data = {
name:"putri",
div:"m4th"
}
putritanjungsari(data)
In kotlin, i'am creating a function that accept an object as parameter then read it's properties later, how to do that in kotlin that targeting JVM?
If I understood your question correct, you are trying to have a variable that associates keys with some value or undefined(null in kt) if none are found. You are searching for a Map
If you don't know what types you want, you can make a map of type Any? So
Map<String, Any?>
Which is also nullable
Map<String, Any>
If you don't want nullables
Your code for example:
fun putritanjungsari(data: Map<String, Any?>){
print(data["name"])
}
val data: Map<String, Any?> =mapOf(
"name" to "putri",
"div" to "m4th"
)
putritanjungsari(data)
Note that you can't add new keys or edit any data here, the default map is immutable. There is MutableMap (which is implemented the same, only it has a method to put new data)
You can apply the property design pattern to solve your problem.
Here is its implementation in Kotlin:
interface DynamicProperty<T> {
fun cast(value: Any?): T
fun default(): T
companion object {
inline fun <reified T> fromDefaultSupplier(crossinline default: () -> T) =
object : DynamicProperty<T> {
override fun cast(value: Any?): T = value as T
override fun default(): T = default()
}
inline operator fun <reified T> invoke(default: T) = fromDefaultSupplier { default }
inline fun <reified T> required() = fromDefaultSupplier<T> {
throw IllegalStateException("DynamicProperty isn't initialized")
}
inline fun <reified T> nullable() = DynamicProperty<T?>(null)
}
}
operator fun <T> DynamicProperty<T>.invoke(value: T) = DynamicPropertyValue(this, value)
data class DynamicPropertyValue<T>(val property: DynamicProperty<T>, val value: T)
class DynamicObject(vararg properties: DynamicPropertyValue<*>) {
private val properties = HashMap<DynamicProperty<*>, Any?>().apply {
properties.forEach { put(it.property, it.value) }
}
operator fun <T> get(property: DynamicProperty<T>) =
if (properties.containsKey(property)) property.cast(properties[property])
else property.default()
operator fun <T> set(property: DynamicProperty<T>, value: T) = properties.put(property, value)
operator fun <T> DynamicProperty<T>.minus(value: T) = set(this, value)
}
fun dynamicObj(init: DynamicObject.() -> Unit) = DynamicObject().apply(init)
You can define your properties these ways:
val NAME = DynamicProperty.required<String>() // throws exceptions on usage before initialization
val DIV = DynamicProperty.nullable<String>() // has nullable type String?
val IS_ENABLED = DynamicProperty(true) // true by default
Now you can use them:
fun printObjName(obj: DynamicObject) {
println(obj[NAME])
}
val data = dynamicObj {
NAME - "putri"
DIV - "m4th"
}
printObjName(data)
// throws exception because name isn't initialized
printObjName(DynamicObject(DIV("m4th"), IS_ENABLED(false)))
Reasons to use DynamicObject instead of Map<String, Any?>:
Type-safety (NAME - 3 and NAME(true) will not compile)
No casting is required on properties usage
You can define what the program should do when a property isn't initialized
Kotlin is statically typed language, so it required a param type to be precisely defined or unambiguously inferred (Groovy, for instance, addresses the case by at least two ways). But for JS interoperability Kotlin offers dynamic type.
Meanwhile, in your particular case you can type data structure to kt's Map and do not argue with strict typing.
You have to use Any and after that, you have to cast your object, like this
private fun putritanjungsari(data : Any){
if(data is Mydata){
var data = data as? Mydata
data.name
}
}
Just for the sake of inspiration. In Kotlin, you can create ad hoc objects:
val adHoc = object {
var x = 1
var y = 2
}
println(adHoc.x + adHoc.y)