Im kinda new to Kotlin and I was wondering how I could make a static method.
Test.foo() //I want to do this from somewhere else in the program
open class Test() {
private giorgor: String? = null
fun foo(value:String) {
giorgor = value
}
}
I need to change the value of giorgor from somewhere else in the code and I thought I could use a static method to do that but I dont know how. Test also needs to be an open class
The equivalent of a static method in Kotlin is a method in the companion object, e.g.:
class Test() {
// …
companion object {
fun foo() = // …
}
}
You can access it in the obvious way:
fun main() {
Test.foo()
}
However, the bits of code I've elided make me suspect that this isn't exactly what you want: your example has foo() referring to a property of Test — and because each instance of Test has its own version of that property, it would need to refer to an specific instance. (And if you had an instance, there would be no need of the companion class — you could just make it a regular method of Test.)
In a comment, you refer to a Minecraft plug-in — can you explain what requirements that has?
BTW, a simpler alternative to the companion object is just a top-level method:
fun foo() = // …
From the same file, you can call it as foo(); from elsewhere, you'd call it as <Filename>Kt.foo() (since the compiler generates a dummy classname for top-level functions and properties, based on the filename). However, the same issues apply to that too.
If I'm understanding you correctly, you want an object instead of a class
object Test {
var giorgor: String = "jiorgor"
fun foo() = println(giorgor)
}
fun main() {
Test.foo()
}
One way to do this is by making the class open or abstract and adding this
companion object Default: Test()
This makes it act like every method of Test() is inside the companion object Default.
If you wanted, you could also override an open method and make it have a different output for when it is used statically:
fun main() {
val test = Test()
test.foo() //Output: "jiorgor"
Test.foo() //Output: "static jiorgor"
}
public open class Test() {
var giorgor: String = "jiorgor"
open fun foo() = println(giorgor)
companion object Default : Test() {
override fun foo() = println("static jiorgor")
}
}
Related
I want to call a function in the 'static' way
i heard there isn't an equivalent for static in Kotlin
so I'm looking into a way to make it work.
fun main() {
var ob1 : Ob1 = Ob1()
ob1.try1()
}
class Ob1{
fun try1() = println("try 1S")
}
If I use java i would just name it static than call the function directly.
Thanks on advance..
You can define function outside class or simply define it in a companion object:
class Ob1{
companion object{
fun try1() = println("try 1S")
}
}
fun main() {
Ob1.try1()
}
Yo can read more in official documentation
So what I want to achieve is that to have the top-level variable set some time later in the main function, but I don't want to make it a lateinit var which certainly breaks the Extension variable functionality.
For instance this code doesn't work since extension variables don't support lateinit modifier:
lateinit var Dispatchers.Konvironment: MainCoroutineDispatcher
private set
fun main() {
...
Dispatchers.Konvironment = ArbitraryMainDispatcher(Thread.currentThread()) { queue.add(it) }
...
}
So what I finally came up with is to use a dummy variable and implement the getter of the val variable.
val Dispatchers.Konvironment: MainCoroutineDispatcher
get() = dispatcher
private lateinit var dispatcher: MainCoroutineDispatcher
fun main() {
...
dispatcher = ArbitraryMainDispatcher(Thread.currentThread()) { queue.add(it) }
...
}
But it is certainly not clean way to do that. It looks ugly (ish) creating multiple variable in the top-level structure is not very clean architecture.
So is there any possible clean workarounds? Sort of like lazy initialization, by some delegates or something.
Well, partially answering your question:
var Dispatchers.Konvironment: MainCoroutineDispatcher
get() = dispatcher
private set(value) {
dispatcher = value
}
private lateinit var dispatcher: MainCoroutineDispatcher
fun main() {
...
Dispatchers.Konvironment = ArbitraryMainDispatcher(Thread.currentThread()) { queue.add(it) }
...
}
will give you the desired way of assigning the value. There is no way to get rid of this additional lazyinit variable, though.
Extensions are nothing more than just some Kotlin syntax sugar for static methods which take an instance of the extended class as one of the arguments, and perform some action. If you're familiar with Java then, for example, these extensions:
// Extensions.kt
fun Foo.extendedAction() {
println(this)
}
var Foo.extendedBar: Bar
get() = this.bar
set(value) {
this.bar = value
}
are under the hood these methods in Java:
public class ExtensionsKt {
public static final void extendedAction(Foo foo) {
System.out.println(foo);
}
public static final Bar getExtendedBar(Foo foo) {
return foo.getBar();
}
public static final Bar setExtendedBar(Foo foo, Bar bar) {
foo.setBar(bar);
}
}
The conclusion which maybe drawn from the above is that extensions don't actually add anything to the extended classes' signatures, they simply decorate them with additional functionality. Or, as put in the docs:
Extensions do not actually modify classes they extend. By defining an extension, you do not insert new members into a class, but merely make new functions callable with the dot-notation on variables of this type.
So you can see, unless dispatcher somehow already exists within Dispatchers, you can't do what you want without providing an external, "backing" variable which value can be actually referenced by the extension.
In the following code I would like to set a reference to the class instance so that static functions can return a reference to it:
open class TestRunner {
init {
instance = this
}
companion object {
private lateinit var instance: TestRunner
fun addTestSetups(vararg testSetups: () -> TestSetup): TestRunner {
for (setup in testSetups) {
testsSetups.add(setup)
}
return instance
}
}
}
But setting instance = this is not allowed. How can I return an instance of the class from a function while keeping the class as a singleton?
If I get you right, you want something like this:
abstract class TestRunner {
companion object : TestRunner()
}
This seems to work. Instead of keeping a variable that holds a reference to the class, simply referencing the name of the class is sufficient. However, to return an instance of the class from functions, the return type must be Companion:
open class TestRunner {
companion object {
fun addTestSetups(vararg testSetups: () -> TestSetup): Companion {
for (setup in testSetups) {
testsSetups.add(setup)
}
return TestRunner
}
}
}
This is not a true singleton because you can still create a new instance if you did this:
val testRunner = TestRunner()
However, if you never create an instance but only refer to the functions statically, it does behave like a singleton and the state of any private variables inside the companion object will still be maintained.
Update:
I came across this code on the Android developer site that shows an example of a class that is setup as a singleton:
class StockLiveData(symbol: String) : LiveData<BigDecimal>() {
private val stockManager: StockManager = StockManager(symbol)
private val listener = { price: BigDecimal ->
value = price
}
override fun onActive() {
stockManager.requestPriceUpdates(listener)
}
override fun onInactive() {
stockManager.removeUpdates(listener)
}
companion object {
private lateinit var sInstance: StockLiveData
#MainThread
fun get(symbol: String): StockLiveData {
sInstance = if (::sInstance.isInitialized) sInstance else StockLiveData(symbol)
return sInstance
}
}
}
But it should be pointed out that this example requires functions that need to return an instance to first check if the instance variable is set and if not, create a new instance. I'm not sure what the point of that is since to call the function you already have an instance. So why bother create a new instance? Doesn't seem to make any sense.
object in Kotlin is the singleton, not the class its defined within. A companion object has the extra convenience of allowing you to call it by the name of that outer class. But it otherwise shares no hierarchy with it.
To make your class subclassable, you can't define the functions in the companion object. But you can make the class abstract so it can't be instantiated unless subclassed. Then make your companion object extend the abstract class so it will have all those functions available.
abstract class TestRunner{
open fun addTestSetups(vararg testSetups: () -> TestSetup): TestRunner{
//...
return this
}
companion object: TestRunner()
}
Usage:
TestRunner.addTestSetups(someTestSetup)
Note that your singleton is not an instance of TestRunner. It is a singleton instance of a subclass of TestRunner. But since you define no extra functions and override nothing, it behaves exactly like a TestRunner.
If you want a subclass:
abstract class ExtendedTestRunner: TestRunner() {
fun someOtherFunction() {}
companion object: ExtendedTestRunner()
}
The companions are not being subclassed, but their abstract parents can be.
How to use method references to refer to super class methods?
In Java 8 you can do SubClass.super::method.
What would be the syntax in Kotlin?
Looking forward to your response!
Conclusion
Thanks to Bernard Rocha!
The syntax is SubClass::method.
But be careful. In my case the subclass was a generic class. Don't forget to declare it as those:
MySubMap<K, V>::method.
EDIT
It still doesn't work in Kotlin.
Hers's an example in Java 8 of a method reference to a super class method:
public abstract class SuperClass {
void method() {
System.out.println("superclass method()");
}
}
public class SubClass extends SuperClass {
#Override
void method() {
Runnable superMethodL = () -> super.method();
Runnable superMethodMR = SubClass.super::method;
}
}
I'm still not able to do the same in Kotlin...
EDIT
This is an example how I tried to achieve it in Kotlin:
open class Bar {
open fun getString(): String = "Hello"
}
class Foo : Bar() {
fun testFunction(action: () -> String): String = action()
override fun getString(): String {
//this will throw an StackOverflow error, since it will continuously call 'Foo.getString()'
return testFunction(this::getString)
}
}
I want to have something like that:
...
override fun getString(): String {
//this should call 'Bar.getString' only once. No StackOverflow error should happen.
return testFunction(super::getString)
}
...
Conclusion
It's not possible to do so in Kotlin yet.
I submitted a feature report. It can be found here: KT-21103 Method Reference to Super Class Method
As the documentation says you use it like in java:
If we need to use a member of a class, or an extension function, it
needs to be qualified. e.g. String::toCharArray gives us an extension
function for type String: String.() -> CharArray.
EDIT
I think you can achieve what you want doing something like this:
open class SuperClass {
companion object {
fun getMyString(): String {
return "Hello"
}
}
}
class SubClass : SuperClass() {
fun getMyAwesomeString(): String {
val reference = SuperClass.Companion
return testFunction(reference::getMyString)
}
private fun testFunction(s: KFunction0<String>): String {
return s.invoke()
}
}
Don't know if it is possible to get the reference to super class's function, but here is an alternative to what you want to achieve:
override fun getString(): String = testFunction { super.getString() }
According to Bernardo's answer, you might have something like this. It doesn't have remarkable changes.
fun methodInActivity() {
runOnUiThread(this::config)
}
fun config(){
}
What is more, in the incoming 1.2 version you can use just
::config
I want to be able to save a class instance to a public static variable but I can't figure out how to do this in Kotlin.
class Foo {
public static Foo instance;
public Foo() {
instance = this;
}
}
Update: since this answer is getting a decent amount of upvotes, I really wanted to say that you shouldn't do the below, but instead just use object Foo { ... }, like Roman rightly points out in the comment.
Previous answer:
The closest thing to Java's static fields is a companion object. You can find the documentation reference for them here: https://kotlinlang.org/docs/reference/object-declarations.html#companion-objects
Your code in Kotlin would look something like this:
class Foo {
companion object {
lateinit var instance: Foo
}
init {
instance = this
}
}
If you want your fields/methods to be exposed as static to Java callers, you can apply the #JvmStatic annotation:
class Foo {
companion object {
#JvmStatic lateinit var instance: Foo
}
init {
instance = this
}
}
It looks that you want to define a singleton object. It is supported in Kotlin as a first-class concept:
object Foo {
...
}
All the boilerplate code with static field and constructor is taken care by the Kotlin automatically. You don't have to write any of that.
From the Kotlin code you can refer to the instance of this object simply as Foo. From the Java code you can referer to the instance of this object as Foo.INSTANCE, because the Kotlin compiler automatically creates the corresponding static field named INSTANCE.
first you create a simple class then after create a block followed by companion object keyword
for example:
class Test{
companion object{
fun getValue(): String{
return "Test String"
}
}
}
you can call this class function using class name dot function name
for example:
// here you will get the function value
Test.getValue()
You can create a companion object for the class, and if you want the field to be static you can use the annotation #JvmStatic. Companion object have access to private members of the class it is companion for.
See below an example:
class User {
private lateinit var name: String
override fun toString() = name
companion object {
#JvmStatic
val instance by lazy {
User().apply { name = "jtonic" }
}
}
}
class CompanionTest {
#Test
fun `test companion object`() {
User.instance.toString() shouldBe "jtonic"
}
}