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.
Related
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!
The Kotlin documentation describes cloning only in accessing Java and in enum class. In latter case clone is just throwing an exception.
So, how would I / should I clone arbitrary Kotlin object?
Should I just use clone() as in Java?
For a data class, you can use the compiler-generated copy() method. Note that it will perform a shallow copy.
To create a copy of a collection, use the toList() or toSet() methods, depending on the collection type you need. These methods always create a new copy of a collection; they also perform a shallow copy.
For other classes, there is no Kotlin-specific cloning solution. You can use .clone() if it suits your requirements, or build a different solution if it doesn't.
You can use Gson library to convert the original object to a String and then convert back that String to an actual Object type, and you'll have a clone. Although this is not the intended usage of the Gson library which is actually used to convert between JSON and other object types, but I have devised this method to solve the cloning problem in many of my Kotlin based Android applications.
See my example. Put this function in the class/model of which you want to create a clone. In my example I'm cloning an Animal type object so I'll put it in the Animal class
class Animal{
fun clone(): Animal
{
val stringAnimal = Gson().toJson(this, Animal::class.java)
return Gson().fromJson<Animal>(stringAnimal, Animal::class.java)
}
}
Then use it like this:
val originalAnimal = Animal()
val clonedAnimal = originalAnimal.clone()
A Kotlin data class is easy to clone using .copy()
All values will be shallow copied, be sure to handle any list/array contents carefully.
A useful feature of .copy() is the ability to change any of the values at copy time. With this class:
data class MyData(
val count: Int,
val peanuts: Int?,
val name: String
)
val data = MyData(1, null, "Monkey")
You could set values for any of the properties
val copy = data.copy(peanuts = 100, name = "Elephant")
The result in copy would have values (1, 100, "Elephant")
If the class you are trying to clone does not implement Cloneable or is not a data class and is a part of an outside library, you can create an extension method that returns a new instance. For example:
class Person {
var id: String? = null
var name: String? = null
}
fun Person.clone(): Person {
val person = Person()
person.id = id
person.name = name
return person
}
It requires to implement Cloneable for your class then override clone() as a public like:
public override fun clone(): Any {<your_clone_code>}
https://discuss.kotlinlang.org/t/how-to-use-cloneable/2364/3
fun <T : Any> clone (obj: T): T {
if (!obj::class.isData) {
println(obj)
throw Error("clone is only supported for data classes")
}
val copy = obj::class.memberFunctions.first { it.name == "copy" }
val instanceParam = copy.instanceParameter!!
return copy.callBy(mapOf(
instanceParam to obj
)) as T
}
I've voted for #yole for nice answer, but other ways if you don't (or can't) use data class. You can write helper method like this:
object ModelHelper {
inline fun <reified T : Serializable> mergeFields(from: T, to: T) {
from::class.java.declaredFields.forEach { field ->
val isLocked = field.isAccessible
field.isAccessible = true
field.set(to, field.get(from))
field.isAccessible = isLocked
}
}
}
So you can "copy" instance A into B by:
val bInstance = AClassType()
ModelHelper.mergeFields(aInstance, bInstance)
Sometimes, I use this way to merge data from many instances into one object which value available (not null).
Here is a consistent solution that works for any object type:
Kotlin's Array data structure provides a clone() method that can be used to clone the contents of the array:
val a = arrayOf(1)
//Prints one object reference
println(a)
//Prints a different object reference
println(a.clone())
As of Kotlin 1.3, the clone method has been supported on all major targets, so it should be usable across platforms.
It's also possible to clone an object using kotlinx.serialization
import kotlinx.serialization.Serializable
import kotlinx.serialization.json.Json
import kotlinx.serialization.json.JsonConfiguration
#Serializable
class A
{
val name: String = "Cloneable class A"
fun clone(): A {
val json = Json(JsonConfiguration.Stable)
val jsonStr = json.stringify(serializer(), this)
return json.parse(serializer(), jsonStr)
}
}
Collection copying functions, such as toList(), toMutableList(), toSet() and others, create a snapshot of a collection at a specific moment. Their result is a new collection of the same elements. If you add or remove elements from the original collection, this won't affect the copies. Copies may be changed independently of the source as well.
val alice = Person("Alice")
val sourceList = mutableListOf(alice, Person("Bob"))
val copyList = sourceList.toList()
sourceList.add(Person("Charles"))
alice.name = "Alicia"
println("First item's name is: ${sourceList[0].name} in source and ${copyList[0].name} in copy")
println("List size is: ${sourceList.size} in source and ${copyList.size} in copy")
First item's name is: Alicia in source and Alicia in copy
List size is: 3 in source and 2 in copy
Kotlin Official Document
Sample Screenshot
i'm new in kotlin and i want to know if we can transform a content value at initialisation : with this example :
#Document
data class Category(
#Id val id: Id? = null,
val label: String
)
Category is a document (entity for mongodb) and when i'm instanciating this object, i want to transform label property in uppercase. How can i do that to stay idiomatic with the language ? The point is to keep the immutable properties of the val keyword.
val categ = Category(label = "Test")
println(categ.label) // --> TEST
Thanks.
You can encapsulate the "upperCasing" into a factory:
data class Category constructor(val label: String) {
init {
if (label != label.toUpperCase()) {
throw IllegalStateException("Label must be uppercase")
}
}
companion object {
fun createInstance(str: String) = Category(str.toUpperCase())
}
}
The init block ensures, that clients don't create unwanted instances with non-upper labels (which should be documented).
Create an instance like this:
val instance = Category.createInstance("xy")
You might want to make explicit that you do transformations if the parameter is not upper case already by naming the factory accordingly, e.g. withTransformedLabel or simply add some documentation ;-)
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>() }