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!
Related
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
}
}
So I have a class with a generic type
class GenericClass<T> {
// At some point in the class I have variable item of type T
val name: String = item.name
}
I know for sure that the type T of GenericClass will be used with a class that has the "name" property. But of course at the line I got a "Unresolved reference name". Android Studio generated me this code via "Create extension property T.name"
private val <T> T.name: String
get() {}
I don't really know what to put in the bracket {} after the get. I tried return name but I got a recursive property error.
Any ideas ?
Thanks
If you know that every type T has property name you can declare it implicitly:
// GenericClass.kt
class GenericClass<T : HasName> {
// At some point in the class I have variable item of type T
val name: String = item.name
}
// HasName.kt
// Create new interface with "name" property
interface HasName {
val name: String
}
But also you must implement this new interface for all classes that can be used as T.
I know for sure that the type T of GenericClass will be used with a class that has the "name" property.
Then you need to explicitly declare that. By default, T extends Any?. You need to narrow down possible types of T by declaring some interface, like
interface Named {
val name : String
}
and passing T : Named as a generic paramteter. Also you need to make all classes, you're going to pass as a generic parameter, to implement that interface. By the way, GenericClass<T : Named> class itself could be declared as implementing that interface:
class GenericClass<T : Named> : Named {
override val name: String = item.name
}
I'm not sure if this is a limitation, a bug or just bad use of GSON. I need to have a hierarchy of Kotlin objects (parent with various subtypes) and I need to deserialize them with GSON. The deserialized object has correct subtype but its field enumField is actually null.
First I thought this is because the field is passed to the "super" constructor but then I found out that "super" works well for string, just enum is broken.
See this example:
import com.google.gson.Gson
import com.google.gson.GsonBuilder
import com.google.gson.typeadapters.RuntimeTypeAdapterFactory
open class Parent(val stringField: String,
val enumField: EnumField) {
enum class EnumField {
SUBTYPE1,
SUBTYPE2,
SUBTYPE3
}
}
class Subtype1() : Parent("s1", EnumField.SUBTYPE1)
class Subtype2(stringField: String) : Parent(stringField, EnumField.SUBTYPE2)
class Subtype3(stringField: String, type: EnumField) : Parent(stringField, type)
val subtypeRAF = RuntimeTypeAdapterFactory.of(Parent::class.java, "enumField")
.registerSubtype(Subtype1::class.java, Parent.EnumField.SUBTYPE1.name)
.registerSubtype(Subtype2::class.java, Parent.EnumField.SUBTYPE2.name)
.registerSubtype(Subtype3::class.java, Parent.EnumField.SUBTYPE3.name)
fun main() {
val gson = GsonBuilder()
.registerTypeAdapterFactory(subtypeRAF)
.create()
serializeAndDeserialize(gson, Subtype1()) // this works (but not suitable)
serializeAndDeserialize(gson, Subtype2("s2")) // broken
serializeAndDeserialize(gson, Subtype3("s3", Parent.EnumField.SUBTYPE3)) // broken
}
private fun serializeAndDeserialize(gson: Gson, obj: Parent) {
println("-----------------------------------------")
val json = gson.toJson(obj)
println(json)
val obj = gson.fromJson(json, Parent::class.java)
println("stringField=${obj.stringField}, enumField=${obj.enumField}")
}
Any ideas how to achieve to deserialization of enumField?
(deps: com.google.code.gson:gson:2.8.5, org.danilopianini:gson-extras:0.2.1)
P.S.: Note that I have to use RuntimeAdapterFactory because I have subtypes with different set of fields (I did not do it in the example so it is easier to understand).
Gson requires constructors without arguments to work properly (see deep-dive into Gson code below). Gson constructs raw objects and then use reflection to populate fields with values.
So if you just add some argument-less dummy constructors to your classes that miss them, like this:
class Subtype1() : Parent("s1", EnumField.SUBTYPE1)
class Subtype2(stringField: String) : Parent(stringField, EnumField.SUBTYPE2) {
constructor() : this("")
}
class Subtype3(stringField: String, type: EnumField) : Parent(stringField, type) {
constructor() : this("", EnumField.SUBTYPE3)
}
you will get the expected output:
-----------------------------------------
{"stringField":"s1","enumField":"SUBTYPE1"}
stringField=s1, enumField=SUBTYPE1
-----------------------------------------
{"stringField":"s2","enumField":"SUBTYPE2"}
stringField=s2, enumField=SUBTYPE2
-----------------------------------------
{"stringField":"s3","enumField":"SUBTYPE3"}
stringField=s3, enumField=SUBTYPE3
Gson deep-dive
If you want to investigate the internals of Gson, a tip is to add an init { } block to Subtype1 since it works and then set a breakpoint there. After it is hit you can move up the call stack, step through code, set more breakpoints etc, to reveal the details of how Gson constructs objects.
By using this method, you can find the Gson internal class com.google.gson.internal.ConstructorConstructor and its method newDefaultConstructor(Class<? super T>) that has code like this (I have simplified for brevity):
final Constructor<? super T> constructor = rawType.getDeclaredConstructor(); // rawType is e.g. 'class Subtype3'
Object[] args = null;
return (T) constructor.newInstance(args);
i.e. it tries to construct an object via a constructor without arguments. In your case for Subtype2 and Subtype3, the code will result in a caught exception:
} catch (NoSuchMethodException e) { // java.lang.NoSuchMethodException: Subtype3.<init>()
return null; // set breakpoint here to see
}
i.e. your original code fails since Gson can't find constructors without arguments for Subtype2 and Subtype3.
In simple cases, the problem with missing argument-less constructors is worked around with the newUnsafeAllocator(Type, final Class<? super T>)-method in ConstructorConstructor, but with RuntimeTypeAdapterFactory that does not work correctly.
I may be missing something in what you're trying to achieve, but is it necessary to use the RuntimeTypeAdapterFactory? If we take out the line where we register that in the Gson builder, so that it reads
val gson = GsonBuilder()
.create()
Then the output returns the enum we would expect, which looks to be serialising / deserialising correctly. I.e. the output is:
-----------------------------------------
{"stringField":"s1","enumField":"SUBTYPE1"}
stringField=s1, enumField=SUBTYPE1
-----------------------------------------
{"stringField":"s2","enumField":"SUBTYPE2"}
stringField=s2, enumField=SUBTYPE2
-----------------------------------------
{"stringField":"s3","enumField":"SUBTYPE3"}
stringField=s3, enumField=SUBTYPE3
It also may be an idea to implement Serializable in Parent. i.e.
open class Parent(val stringField: String, val enumField: EnumField) : Serializable {
enum class EnumField {
SUBTYPE1,
SUBTYPE2,
SUBTYPE3
}
}
Try adding #SerializedName annotation to each enum.
enum class EnumField {
#SerializedName("subtype1")
SUBTYPE1,
#SerializedName("subtype2")
SUBTYPE2,
#SerializedName("subtype3")
SUBTYPE3
}
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.
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>() }