I know a little java and am currently studying kotlin. I can't quite figure out getters. I have a class and some function.
class Client(val personalInfo: PersonalInfo?){} //class
fun sendMessageToClient(client: Client?) {
val personalInfo: PersonalInfo? = client?.personalInfo
//...
}
As far as I understand, getter is called in the code client?.personalInfo. Or is it a class field, since private is not explicitly specified anywhere?
Next, I want to add some logic to getter, but I get an error that such a signature already exists.
class Client(val personalInfo: PersonalInfo?){
fun getPersonalInfo():PersonalInfo?{
print(personalInfo)
return personalInfo
}
}
If I specify that the field is private, the error disappears class Client(private val personalInfo: PersonalInfo?), but but the code client?.personalInfowill not work
I tried to rewrite the code, but I can't figure out how to specify val and pass it a value from the constructor
class Client(personalInfo: PersonalInfo?) {
val personalInfo = //??
get() {
print("personal info $personalInfo")
return personalInfo
}
}
Is it possible to somehow add print to the getter and still use client?.personalInfo?
You were almost there. When creating custom getters in kotlin you must use the keyword field when you want the value of the associated property to be used (you can read more about this in re reference documentation at https://kotlinlang.org/docs/properties.html#backing-fields or at https://www.baeldung.com/kotlin/getters-setters#1-accessing-the-backing-field):
Every property we define is backed by a field that can only be accessed within its get() and set() methods using the special field keyword. The field keyword is used to access or modify the property’s value. This allows us to define custom logic within the get() and set() methods.
Having written this you just need to change your code a little bit as follows:
class Client(personalInfo: String?) {
val personalInfo: String? = personalInfo
get() {
print("personal info $field")
return field
}
}
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.
I have the following code setup;
abstract class GenericQuestionEditor() {
protected abstract var data: GenericQuestionData
}
but then when I create EditorSimple() it throws an error when I try to set data to DataSimple(), why?
class EditorSimple(): GenericQuestionEditor() {
override var data = DataSimple()
}
my GenericQeustionData and DataSimple() are setup like this;
abstract class GenericQuestionData {}
class DataSimple: GenericQuestionData() {}
it doesn't complain if I create this function in GenericQuestionEditor()
fun test() {
data = DataSimple()
}
Why do I get an error on data in EditorSimple()? It should recognize it as a subtype and it should be allowed as I understand.
I feel like the answer is found in the kotlin documentation but i'm not sure how to configure it in this case since they are not passed values or part of a collection.
You need to specify the type explicitly:
class EditorSimple(): GenericQuestionEditor() {
override var data: GenericQuestionData = DataSimple()
}
Without the type annotation, the type of data would be inferred to be DataSimple, which doesn't match the type of its super class' data. Even though the types are related, you can't override writable a property with a subtype. Imagine if I did:
class SomeOtherData: GenericQuestionData()
val editor: GenericQuestionEditor = EditorSimple()
editor.data = SomeOtherData() // data is of type GenericQuestionData, so I should be able to do this
But, editor actually has a EditorSimple, which can only store DataSimple objects in data!
for example , I want to change all setters this way:
this.a = StringUtils.trim(a);
If it's a java bean, I can do this by modifying the code generating template of the ide. But Intellij seems not support to atomically add getter/setter for kotlin data class.
Is there a way to do this?
There is not a way to do this as of Kotlin 1.1.
A Kotlin data class, for the most part, is a class "to do nothing but hold data".
I think the closest you can get is to validate your data upon class initialization and make your data class properties read-only values. e.g.:
data class Data(val a: String) {
init {
require(a == a.trim())
}
}
The following won't throw an exception:
val a = Data("ab")
val b = a.copy(a = "abc")
While the following will:
val c = a.copy(a = "abc ")
It looks like if you declare the property as private, you can create your own getter/setters for accessing it. This example works for me.
fun main(args: Array<String>) {
var t = test("foo")
t.setHello("bar")
println(t)
}
data class test(private var hello: String) {
fun setHello(blah: String) {
this.hello = blah
}
}
But you will still have an issue when the property is passed in to the constructor. You will probably need to rethink how you are doing this, either declaring the field private and trimming it in the getter, or not using a data class for this instance.
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>() }