Recently IntelliJ suggested to add final to one of a val properties. This particular property was initialized in init {} block. I've tried to find out what is the semantics of final val construct and when should I use it, but Kotlin is all about immutability and how val is equivalent of final in Java and so results were so noisy, that I couldn't find anything.
Example:
final val id: Int // `final` suggested by IDE
init { id = 1 }
What is the meaning and possible usages of similar property? By applying final what limitations it implies beyond immutability, which is already known? Does it have anything to do with inheritance or external access?
IntelliJ stopped sugessting final if property is private.
The example as it is should not suggest adding final as it does nothing in this case. The only place where adding final makes sense in Kotlin is when overriding members. By adding final to an overridden property (or method), you're preventing subclasses from further overriding it.
For example:
open class A {
open val x: Int = 0
}
open class B : A() {
final override val x: Int = 25
}
class C : B() {
override val x: Int = 56 // Error: `x` in `B` is final and cannot be overridden
}
The final keyword isn't really applicable if:
the class you're in isn't open,
the property isn't open,
the property is private.
Related
I'm trying to access the delegate of the property (id) of a class (FooImpl). The problem is, this class implements an interface (Foo), and the property in question overrides a property of this interface. The delegate only exists in the class (not that it could exist in the interface).
The problem is that using the :: operator on a variable of type Foo always returns the property of Foo, not that of the actual instance. The problem in code:
import kotlin.reflect.KProperty
import kotlin.reflect.KProperty0
import kotlin.reflect.jvm.isAccessible
interface Foo {
val id: Int
}
class FooImpl(
id: Int,
) : Foo {
override val id: Int by lazy { id }
}
val <T> KProperty<T>.hasDelegate: Boolean
get() = apply { isAccessible = true }.let { (it as KProperty0<T>).getDelegate() != null }
fun main() {
val foo: Foo = FooImpl(1)
println("foo::id.hasDelegate = ${foo::id.hasDelegate}")
println("(foo as FooImpl)::id.hasDelegate = ${(foo as FooImpl)::id.hasDelegate}")
}
This prints:
foo::id.hasDelegate = false
(foo as FooImpl)::id.hasDelegate = true
But this requires compile-time knowledge of the correct implementation. What I'm looking for is accessing the correct propert without having to specify FooImpl there.
The information is present at runtime because the least (!) intrusive workaround I have found so far is adding fun idProp(): KProperty0<*> to Foo and override fun idProp() = ::id to FooImpl and accessing the property using that.
Is there any better way than that?
I came up with this, but I don't know if there's a better way. The problem to work around is that getDelegate() has to return an actual instance of the delegate, so you need an instance of the class to be able to retrieve a delegate instance. It would really be nice if there was a hasDelegate property built in. Your version of hasDelegate will crash from the cast on unbound KProperty1's, which is all we have to work with when the specific class is unknown.
So to retrieve the delegate instance, we need to do search the class instance's member properties by name, which gives us a KProperty with covariant class type of the super-class type. Since it's covariant, we can call a consuming function like getDelegate() without casting to the invariant type. I think this logically should be safe, since we are passing an instance that we know has the matching type for the ::class that we retrieved the property with.
#Suppress("UNCHECKED_CAST")
fun <T: Any> KProperty1<T, *>.isDelegated(instance: T): Boolean =
(instance::class.memberProperties.first { it.name == name } as KProperty1<T, *>).run {
isAccessible = true
getDelegate(instance) != null
}
fun main() {
val foo: Foo = Foo2()
println("foo::id.hasDelegate = ${Foo::id.isDelegated(foo)}")
}
The problem here is that the owner of the property is resolved on compile time, not on runtime. When you do foo::id then foo (so FooImpl) become its bound receiver, but owner is still resolved to Foo. To fix this we wound need to "cast" property to another owner. Unfortunately, I didn't find a straightforward way to do this.
One solution I found is to use foo::class instead of foo::id as it resolves KClass on runtime, not on compile time. Then I came up with almost exactly the same code as #Tenfour04.
But if you don't mind using Kotlin internals that are public and not protected with any annotation, you can use much cleaner solution:
val KProperty0<*>.hasDelegate: Boolean
get() = apply { isAccessible = true }.getDelegate() != null
fun KProperty0<*>.castToRuntimeType(): KProperty0<*> {
require(this is PropertyReference0)
return PropertyReference0Impl(boundReceiver, boundReceiver::class.java, name, signature, 0)
}
fun main() {
val foo: Foo = FooImpl(1)
println(foo::id.castToRuntimeType().hasDelegate) // true
}
We basically create a new instance of KProperty, copying all its data, but changing the owner to the same type as its bound receiver. As a result, we "cast" it to the runtime type. This is much simpler and it is also cleaner because we separated property casting and checking for a delegate.
Unfortunately, I think Kotlin reflection API is still missing a lot of features. There should be hasDelegate() function, so we don't have to provide receivers, which is not really needed to check if property is delegated. It should be possible to cast KProperty to another type. It should be possible to create bound properties with some API call. But first of all, it should be possible to do something like: Foo::id(foo), so create KProperty of the runtime type of foo. And so on.
guys, I am learning kotlin. From https://kotlinlang.org/docs/interfaces.html#properties-in-interfaces it says:
Properties declared in interfaces can't have backing fields, and
therefore accessors declared in interfaces can't reference them.
(I think the pronoun "them" at the end of quoted sentence should refer to "properties" rather than "fields". )
However the following code works. It seems that we can refer to properties. Why is print(prop) highlighted as red then?
interface MyInterface {
val prop: Int // abstract
val propertyWithImplementation: String
get() = "foo"
fun foo() {
print(prop) // this is highlighted red but it works. what's does the author want to say?
}
}
class Child : MyInterface {
override val prop: Int = 29
}
fun main() {
val c = Child()
c.foo()
}
Besides, I noticed that in the above example foo is not accessor. So I tried following example and it works too:
interface User {
val email: String
val nickname: String
get() = email.substringBefore('#') // aren't we referring to a property in accessor? why does this work then?
}
So what does the author want to say in here? what does "them" refer to?
"Them" in this sentence means "fields".
Property is basically a getter (setter) and it could be optionally backed by a field. For technical reasons interfaces can't hold fields, so properties in interfaces have to be "fieldless". Property has to be either abstract or its implementation can only use e.g. other properties/functions, but it can't store/read any data directly. Note that referencing other properties does not break above rule, because, as I said, property is mainly a getter/setter, not a field.
print(prop) is highlighted as red, because... well, this is how automatic highlighter colored it... :-)
I have recently reviewed some kotlin codes, All nullable field initialized as null.
What is the difference between val x : String? = null and val x : String?
Should we initialize the nullable fields as null?
Everything, even nullable variables and primitives, need to be initialized in Kotlin. You can, as tynn mentioned, mark them as abstract if you require overriding. If you have an interface, however, you don't have to initialize them. This won't compile:
class Whatever {
private var x: String?
}
but this will:
interface IWhatever {
protected var x: String?
}
This too:
abstract class Whatever {
protected abstract var x: String?
}
If it's declared in a method, you don't have to initialize it directly, as long as it's initialized before it's accessed. This is the exactly same as in Java, if you're familiar with it.
If you don't initialize it in the constructor, you need to use lateinit. Or, if you have a val, you can override get:
val something: String?
get() = "Some fallback. This doesn't need initialization because the getter is overridden, but if you use a different field here, you naturally need to initialize that"
As I opened with, even nullable variables need to be initialized. This is the way Kotlin is designed, and there's no way around that. So yes, you need to explicitly initialize the String as null, if you don't initialize it with something else right away.
A property must be initialized. Therefore you have to do the initialization var x : String? = null. Not assigning a value is only the declaration of the property and thus you'd have to make it abstract abstract val x : String?.
Alternatively you can use lateinit, also on non-nullable types. But this has the effect, that it's not null, but uninitialized lateinit var x : String.
val x : String? will create an uninitialized variable or property, depending on where it's defined. If it's in a class (rather than a function), it creates a property, and you cannot create an uninitalized property unless it's abstract. For example take this code:
class MyClass {
val x : String?
}
This won't compile. You'll get Property must be initialized or be abstract.
This code, however, will compile
class MyClass {
fun test() {
val x : String?
}
}
However it's a bit pointless as you will not be able to refer to that variable: as soon as you do you'll get Variable 'x' must be initialized.
So yes, generally when defining a nullable member you should initialize it (e.g. with a value of null), unless it's abstract, in which case the overriding class should initialize it.
A previous question shows how to put a static initializer inside a class using its companion object. I'm trying to find a way to add a static initializer at the package level, but it seems packages have no companion object.
// compiler error: Modifier 'companion' is not applicable inside 'file'
companion object { init { println("Loaded!") } }
fun main(args: Array<String>) { println("run!") }
I've tried other variations that might've made sense (init on its own, static), and I know as a workaround I can use a throwaway val as in
val static_init = {
println("ugly workaround")
}()
but is there a clean, official way to achieve the same result?
Edit: As #mfulton26's answer mentions, there is no such thing as a package-level function really in the JVM. Behind the scenes, the kotlin compiler is wrapping any free functions, including main in a class. I'm trying to add a static initializer to that class -- the class being generated by kotlin for the free functions declared in the file.
Currently there is no way to add code to the static constructor generated for Kotlin file classes, only top-level property initializers are getting there. This sounds like a feature request, so now there is an issue to track this: KT-13486 Package-level 'init' blocks
Another workaround is to place initialization in top-level private/internal object and reference that object in those functions that depend on the effect of that initialization. Objects are initialized lazily, when they are referenced first time.
fun dependsOnState(arg: Int) = State.run {
arg + value
}
private object State {
val value: Int
init {
value = 42
println("State was initialized")
}
}
As you mentioned, you need a property with something that would run on initialisation:
val x = run {
println("The package class has loaded")
}
I got around it by using a Backing Property on the top-level, under the Kotlin file. Kotlin Docs: Backing Properties
private var _table: Map<String, Int>? = null
public val table: Map<String, Int>
get() {
if (_table == null) {
_table = HashMap() // Type parameters are inferred
// .... some other initialising code here
}
return _table ?: throw AssertionError("Set to null by another thread")
}
It's easy to write extension methods in Kotlin:
class A { }
class B {
fun A.newFunction() { ... }
}
But is there some way to create extension variable? Like:
class B {
var A.someCounter: Int = 0
}
You can create an extension property with overridden getter and setter:
var A.someProperty: Int
get() = /* return something */
set(value) { /* do something */ }
But you cannot create an extension property with a backing field because you cannot add a field to an existing class.
No - the documentation explains this:
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 instances of this class.
and
Note that, since extensions do not actually insert members into classes, there’s no efficient way for an extension property to have a backing field. This is why initializers are not allowed for extension properties. Their behavior can only be defined by explicitly providing getters/setters.
Thinking about extension functions/properties as just syntactic sugar for calling a static function and passing in a value hopefully makes this clear.
However, if you really, really want to do something like this...
As stated above regarding efficiency, an additional backing field added directly to the class is the best way to store data non-derivable from existing non-private members from the class. However, if you don't control the implementation of the class and are dead-set on creating a new property that can store new data, it can be done in a way that is not abysmally inefficient by using separate external tables. Use a separate map that keys on object instances of this class with values that map directly to the value you want to add then define an extension getter and/or setter for this property which uses your external table to store the data associated with each instance.
val externalMap = mutableMapOf<ExistingClass, Int>()
var ExistingClass.newExtensionProperty : Int
get() = externalMap[this] ?: 0
set(value:Int) { externalMap[this] = value }
The additional map lookups will cost you - and you need to consider memory leaks, or using appropriately GC-aware types, but it does work.
There's no way to add extension properties with backing fields to classes, because extensions do not actually modify a class.
You can only define an extension property with custom getter (and setter for var) or a delegated property.
However, if you need to define an extension property which would behave as if it had a backing field, delegated properties come in handy.
The idea is to create a property delegate that would store the object-to-value mapping:
using the identity, not equals()/hashCode(), to actually store values for each object, like IdentityHashMap does;
not preventing the key objects from being garbage collected (using weak references), like WeakHashMap does.
Unfortunately, there is no WeakIdentityHashMap in JDK, so you have to implement your own (or take a complete implementation).
Then, based on this mapping you can create a delegate class satisfying the property delegates requirements. Here's an example non-thread-safe implementation:
class FieldProperty<R, T : Any>(
val initializer: (R) -> T = { throw IllegalStateException("Not initialized.") }
) {
private val map = WeakIdentityHashMap<R, T>()
operator fun getValue(thisRef: R, property: KProperty<*>): T =
map[thisRef] ?: setValue(thisRef, property, initializer(thisRef))
operator fun setValue(thisRef: R, property: KProperty<*>, value: T): T {
map[thisRef] = value
return value
}
}
Usage example:
var Int.tag: String by FieldProperty { "$it" }
fun main(args: Array<String>) {
val x = 0
println(x.tag) // 0
val z = 1
println(z.tag) // 1
x.tag = "my tag"
z.tag = x.tag
println(z.tag) // my tag
}
When defined inside a class, the mapping can be stored independently for instances of the class or in a shared delegate object:
private val bATag = FieldProperty<Int, String> { "$it" }
class B() {
var A.someCounter: Int by FieldProperty { 0 } // independent for each instance of B
var A.tag: String by bATag // shared between the instances, but usable only inside B
}
Also, please note that identity is not guaranteed for Java's primitive types due to boxing.
And I suspect the performance of this solution to be significantly worse than that of regular fields, most probably close to normal Map, but that needs further testing.
For nullable properties support and thread-safe implementation please refer to here.
You can't add a field, but you can add a property, that delegates to other properties/methods of the object to implement its accessor(s). For example suppose you want to add a secondsSinceEpoch property to the java.util.Date class, you can write
var Date.secondsSinceEpoch: Long
get() = this.time / 1000
set(value) {
this.time = value * 1000
}
If you are extending View you can do it quite easily like this...
This is example how I create some my custom class Event property in EditText class extension:
Define id for key :
<?xml version="1.0" encoding="utf-8"?>
<resources>
<item name="EditTextEventOnClearTagKey" type="id" />
</resources>
Define one reusable extension like this:
fun <T : Any> View.tagProperty(#IdRes key: Int, onCreate: () -> T): T {
#Suppress("UNCHECKED_CAST")
var value = getTag(key) as? T
if (value.isNull) {
value = onCreate()
setTag(key, value)
}
return value!!
}
Use it in wherever View extension you need:
val EditText.eventClear get() = tagProperty(R.id.EditTextEventOnClearTagKey) { event<Unit>() }