https://pl.kotl.in/WJxo0DujU (below is the code in the link)
open class A() { }
class B(): A() {
fun pew2() { }
}
fun <a: A> a.pew() = apply { }
fun main() {
val b = B()
b.pew().pew2()
}
Is there a way to have the function pew() in class A (not in A's companion object) and still be able to type b.pew().pew2() (not b.apply { pew().pew2() })?
You could create the pew() method in A and have the method return the instance itself:
open class A() {
fun pew() = this
}
class B(): A() {
fun pew2() { }
}
fun main() {
val b = (B().pew() as B).pew2()
}
You can override the function in B and narrow the return type.
open class A() {
open fun pew(): A {
//...
return this
}
}
class B: A() {
fun pew2() { }
override fun pew(): B {
super.pew()
return this
}
}
Kotlin doesn't have a self type that would make these kinds of chainable functions easier to implement, but the apply function makes chainable functions that return the same object unnecessary.
Related
Whats the difference between having a function on a companion object and having the function at package level?
class Foo {
companion object {
fun bar() = println("bar")
}
}
fun bar() = println("bar")
You can call both pretty similar
class B {
fun print() {
Foo.bar()
}
fun print2() {
bar()
}
}
class ModelFactory {
fun setA() : ModelFactory {
// blabla...
}
fun setB() : ModelFactory {
// blabla...
}
fun setC() : ModelFactory {
// blabla...
}
fun build() : Model {
// An error occurs if any of setA, setB, and setC is not called.
}
}
//example
fun successTest() {
ModelFactory().setA().setB().setC().build() // No error occurs at compile time
}
fun failTest() {
ModelFactory().setA().build() // An error occurs at compile time because setB and setC are not called.
}
It's awkward grammatically, but I think it's been expressed what I want.
I have already implemented an error-raising runtime for this requirement, but I want to check this at compile time.
If possible, I think I should use annotations. But is this really possible at compile time?
With Kotlin, I have been avoiding builder pattern, as we can always specify default values for non-mandatory fields.
If you still want to use a builder pattern, you can use Step builder pattern that expects all mandatory fields to be set before creating the object. Note that each setter method returns the reference of next setter interface. You can have multiple Step builders based on the combination of mandatory fields.
class Model(val a: String = "", val b: String = "", val c: String = "")
class StepBuilder {
companion object {
fun builder(): AStep = Steps()
}
interface AStep {
fun setA(a: String): BStep
}
interface BStep {
fun setB(b: String): CStep
}
interface CStep {
fun setC(c: String): BuildStep
}
interface BuildStep {
//fun setOptionalField(x: String): BuildStep
fun build(): Model
}
class Steps : AStep, BStep, CStep, BuildStep {
private lateinit var a: String
private lateinit var b: String
private lateinit var c: String
override fun setA(a: String): BStep {
this.a = a
return this
}
override fun setB(b: String): CStep {
this.b = b
return this
}
override fun setC(c: String): BuildStep {
this.c = c
return this
}
override fun build() = Model(a, b , c)
}
}
fun main() {
// cannot build until you call all three setters
val model = StepBuilder.builder().setA("A").setB("B").setC("C").build()
}
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()
}
Is there any way to get a reference to the delegated object in Kotlin?
Here's an example:
interface A {
fun test()
}
class B: A {
override fun test() {
println("test")
}
}
class C: A by B() {
override fun test() {
// ??? how to get a reference to B's test() method?
}
}
There's currently no way to do that directly. You can achieve that by storing it in a property declared in the primary constructor as follows:
class C private constructor(
private val bDelegate: B
) : A by bDelegate {
constructor() : this(B())
/* Use bDelegate */
}
Another workaround to hotkey's answer might be to include a delegate value
on the A interface...
interface A {
val delegate: A
fun test()
}
class B: A {
override val delegate get() = this
override fun test() {
println("test")
}
}
class C: A by B() {
override fun test() {
delegate.test()
}
}
... this is useful for when zero-arg constructors are required by a framework, e.g. Android Activities
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.