I want to be able to say make an instance of this class and give a type then the code can instantiate a new instance of that class.
fun maker(type: Class<Animal>): Animal {
if(type == Class<Dog>) {
return Dog()
}
else if (type == Class<Cat>) {}
...
}
What is a good way to do this?
If they all have zero-argument constructors, you can do:
fun maker(type: Class<Animal>): Animal {
return type.newInstance()
}
You can make it return the type that was passed in for a little more versatility:
fun <T: Animal> maker(type: Class<T>): T {
return type.newInstance()
}
Correct version following your example (not sure if best approach overall):
fun <T: Animal> maker(type: Class<T>): T? {
return when (type) {
Cat::class.java -> Cat() as T
Dog::class.java -> Dog() as T
else -> null
}
}
And then to create objects:
val a = maker(Cat::class.java)
val b = maker(Dog::class.java)
(Updated) I am not an expert in Kotlin but you can do something like this :
import kotlin.reflect.KClass
import kotlin.reflect.full.createInstance
class A {
fun greet() {
println("Hello A");
}
}
class B{
fun greet() {
println("Hello B");
}
}
fun <T : Any> maker(clazz: KClass<T>): T {
return clazz.createInstance();
}
val aObj = maker<A>(A::class);
aObj.greet();
val bObj = maker<B>(B::class);
bObj.greet();
Output:
Hello A
Hello B
I hope now it makes sense you just need to pass the class to the method and it returns an object.
As you will be using Animal as a parent class so you can replace Any => Animal
fun <T : Animal> maker(clazz: KClass<T>): T {
return clazz.createInstance();
}
If the function can be inline you can also use reified type
inline fun<reified T: Animal> make() = T::class.createInstance()
...
val dog = make<Dog>()
Please notice that to use createInstance() the class must have no-arg constructor or IllegalArgumentException will be thrown
Related
In the PagingWithNetworkSample, in the RedditActivity.kt on the line 68 is a function that contains another function call followed by square brackets and the class type (line 78):
private fun getViewModel(): SubRedditViewModel {
return ViewModelProviders.of(this, object : ViewModelProvider.Factory {
override fun <T : ViewModel?> create(modelClass: Class<T>): T {
val repoTypeParam = intent.getIntExtra(KEY_REPOSITORY_TYPE, 0)
val repoType = RedditPostRepository.Type.values()[repoTypeParam]
val repo = ServiceLocator.instance(this#RedditActivity)
.getRepository(repoType)
#Suppress("UNCHECKED_CAST")
return SubRedditViewModel(repo) as T
}
})[SubRedditViewModel::class.java]
}
What does this exactly do? Automatically cast to that type? (it's not an array/list to suppose it calls get)
Can you bring an example where this is useful?
That code might look strange, but it's really just a way of calling get(). This would be just as valid, but slightly more verbose:
private fun getViewModel(): SubRedditViewModel {
return ViewModelProviders.of(this, object : ViewModelProvider.Factory {
override fun <T : ViewModel?> create(modelClass: Class<T>): T {
// ...
}
}).get(SubRedditViewModel::class.java)
}
I want to return T in the function.
I have an interface class IRepository.kt
interface IRepository
{
fun <T>Save(model:T)
fun <T>Delete(model:T)
fun <T>Get(id:Long):T
}
I want to implement in Repolmpl.kt
class Repolmpl:IRepository
{
override fun <T>Delete(model:T)
{
println("$model : Save}")
}
override fun <T>Get(id:Long):T
{
return T //ERROR here I want to return T...
}
override fun <T> Save(model: T)
{
println("$model : Delete")
}
}
I saw some similar questions online but I just can't find the right solution.
A generic type T is basically just a template. You cannot return it but have to replace it with an actual type first. Make the interface itself generic, not its methods. When implementing, specify T:
interface IRepository<T> {
fun save(model: T)
fun delete(model: T)
fun get(id: Long): T
}
class Repolmpl: IRepository<String>
{
override fun delete(model: String) {}
override fun get(id: Long): String {}
override fun save(model: String) {}
}
You cannot just return T. T is type here, and it is like return String.
You have to return instance of T. So, sth like:
class Repo {
val data = mapOf<Long, Any>()
// ...
fun <T> get(id: Long): T {
return data[id] as T // Get data from somewhere and then cast it to expected type
}
}
I have code which looks something like,
data class X{
fun getSomething(){
var y: Y()
//How can I write this
//this=y.doSomething()
}
}
class Y{
fun doSomething(): X{
//...
return x }
}
I want to assign this to an object that I am returning from some other method in some other class.
You cannot assign anything to this and also, data classes should be immutable. Just reassign your reference:
data class X(val x: String) {
fun getSomething() = Y().doSomething()
}
class Y {
fun doSomething(): X {
return X("fromY")
}
}
fun main(args: Array<String>) {
val second = X("first").getSomething()
}
The following is a very simple illustration of what I'm trying to do:
interface Event {
fun value(): Int
}
class Event1: Event {
override fun value() = 1
}
class Event2: Event {
override fun value() = 2
}
interface EventConsumer<T> where T: Event {
fun consume(event: T)
}
class Event1Consumer: EventConsumer<Event1> {
override fun consume(event: Event1) {
println(event.value())
}
}
class Event2Consumer: EventConsumer<Event2> {
override fun consume(event: Event2) {
println(event.value())
}
}
class EventManager {
private val consumers: Map<KClass<*>, EventConsumer<*>> = mapOf(
Event1::class to Event1Consumer(),
Event2::class to Event2Consumer()
)
fun consume(event: Event) {
val consumer = consumers[event::class]
consumer?.consume(event)
}
}
The final method call (consumer.consume()) is giving me a compiler error
Out-projected type 'EventConsumer<*>?' prohibits the use of 'public
abstract fun consume(event: T): Unit defined in EventConsumer'
I know that Kotlin is a lot more strict about generics than Java which is probably why it doesn't work, but how would I implement something like this properly?
Since you are building the consumers map, it would be safe to make an unchecked cast to the correct generic EventConsumer type:
fun <T: Event> consume(event: T) {
val consumer = consumers[event::class] as? EventConsumer<T>
consumer?.consume(event)
}
I have got the following Java interfaces:
interface Action1<T> {
void call(T t);
}
interface Test<T> {
void test(Action1<? super T> action)
}
And the following Kotlin class:
interface A {
fun go()
}
abstract class Main {
abstract fun a(): Test<out A>
fun main() {
a().test(Action1 { it.go() })
a().test { it.go() }
}
}
Now in the function main, the first statement compiles, but IntelliJ gives a warning that the SAM-constructor can be replaced with a lambda.
This would result in the second statement.
However, this second statement does not compile, because it has type Any?, not A. Removing the out modifier makes it compile again.
Why does this happen?
The use case of this is when the implementing class of Main needs to return Test<B> for the function a(), where B implements A:
class B : A {
override fun go() {
TODO()
}
}
class MainImp : Main() {
override fun a(): Test<out A> {
val value: Test<B> = object : Test<B> {
override fun test(action: Action1<in B>?) {
TODO()
}
};
return value
}
}
It is a compiler bug. You can track it here: https://youtrack.jetbrains.com/issue/KT-12238.