Consider following Kotlin-Code:
class Foo(input: Int) {
private var someField: Int = input
get() = -field
set(value) {
field = -value
}
fun bar() {
println(someField)
}
}
fun main() {
Foo(1).bar()
}
This prints -1 in the console which means that inside method bar() someField references the attribute and not the corresponding getter. Is there a way that allows me to use the get()-method as if I was referencing this field from outside?
Perhaps you could track the "raw" value separately from the negative value? Something like this:
class Foo(input: Int) {
private var _someField: Int = input
var someField: Int
get() = -_someField
set(value) {
_someField = -value
}
fun bar() {
println(someField)
}
}
Now the class internals can reference _someField to deal directly with the raw value, while outside clients can only "see" someField.
Related
I have a class, A, that needs to be marked as dirty anytime one of its properties is changed.
After reviewing the Kotlin docs, I know I need a delegate. So far I have:
abstract class CanBeDirty {
var isDirty = false
}
class A(
// properties getting set in constructor here
) : CanBeDirty {
var property1: String by DirtyDelegate()
var property2: Int by DirtyDelegate()
var property3: CustomObject by DirtyDelegate()
}
class DirtyDelegate() {
operator fun getValue(thisRef: CanBeDirty, property: KProperty<*>): Resource {
return valueOfTheProperty
}
operator fun setValue(thisRef: CanBeDirty, property: KProperty<*>, value: Any?) {
if (property != value) {
thisRef.isDirty = true
//set the value
}
else {
//don't set the value
}
}
}
I believe the lack of setting has something to do with vetoable() but the examples I see in Kotlin documentation don't really show me how to do this with a fully formed class Delegate (and I'm just not that up to speed on Kotlin syntax, honestly).
Your delegate class needs its own property to store the value it will return. And if you don't want to deal with uninitialized values, it should also have a constructor parameter for the initial value. You don't have to implement ReadWriteProperty, but it allows the IDE to autogenerate the correct signature for the two operator functions.
class DirtyDelegate<T>(initialValue: T): ReadWriteProperty<CanBeDirty, T> {
private var _value = initialValue
override fun getValue(thisRef: CanBeDirty, property: KProperty<*>): T {
return _value
}
override fun setValue(thisRef: CanBeDirty, property: KProperty<*>, value: T) {
if (_value != value) {
_value = value
thisRef.isDirty = true
}
}
}
Since this takes an initial value parameter, you have to pass it to the constructor:
class A: CanBeDirty() {
var property1: String by DirtyDelegate("")
var property2: Int by DirtyDelegate(0)
var property3: CustomObject by DirtyDelegate(CustomObject())
}
If you wanted to set an initial value based on something passed to the constructor, you could do:
class B(initialName: String): CanBeDirty() {
var name by DirtyDelegate(initialName)
}
I am new to kotlin. I wonder if this is possible
I wish to create a function that will change the value of the properties of the object and return the object itself. The main benefit is that I can chain this setter.
class Person {
var name:String? = null
var age:Int? = null
fun setter(propName:String, value:Any): Person{
return this.apply {
try {
// the line below caused error
this[propName] = value
} catch(e:Exception){
println(e.printStackTrace())
}
}
}
}
//usage
var person = Person(null,null)
person
.setter(name, "Baby")
.setter(age, 20)
But I get error "unknown references"
This question is marked as duplicate, however the possible duplicate question specifically want to change the property of "name", but I wish to change anyProperty that is pass from the function to object. Can't seem to connect the dot between two questions. #Moira Kindly provide answer that explain it. thankyou
Why not just simplify your answer to
fun setter(propName: String, value: Any): Person {
val property = this::class.memberProperties.find { it.name == propName }
when (property) {
is KMutableProperty<*> ->
property.setter.call(this, value)
null ->
// no such property
else ->
// immutable property
}
}
Java reflection isn't needed, its only effect is to stop non-trivial properties from being supported.
Also, if you call it operator fun set instead of fun setter, the
this[propName] = value
syntax can be used to call it.
After googling around, I think I can provide an answer, but relying on java instead of kotlin purely. It will be great if someone can provide a better answer in kotlin.
class Person(
var name: String,
val age: Int
){
fun setter(propName: String, value: Any): Person{
var isFieldExistAndNotFinal = false
try{
val field = this.javaClass.getDeclaredField(propName)
val isFieldFinal = (field.getModifiers() and java.lang.reflect.Modifier.FINAL == java.lang.reflect.Modifier.FINAL)
if(!isFieldFinal) {
// not final
isFieldExistAndNotFinal = true
}
// final variable cannot be changed
else throw ( Exception("field '$propName' is constant, in ${this.toString()}"))
} catch (e: Exception) {
// object does not have property
println("$e in ${this.toString()}")
}
if(isFieldExistAndNotFinal){
val property = this::class.memberProperties.find { it.name == propName }
if (property is KMutableProperty<*>) {
property.setter.call(this, value)
}
}
return this;
}
}
usage like this
person
.setter(propName = "age", value = 30.00)
.setter(propName = "asdf", value = "asdf")
.setter(propName = "name", value = "A Vidy")
You have error because when you do this[propName] = value you are trying to use this as a list, but it is not a list, it is a Person and it doesn't overload the [] operator.
What you can do is to add a check for the property that is setted:
class Person {
privavar name:String? = null
var age:Int? = null
fun setter(propName:String, value:Any): Person{
return this.apply {
if (propName == "name" && value is String?) {
it.name = value as String?
} else if (propName == "age" && value is Int?) {
it.age = value as Int?
} else {
// handle unknown property or value has incorrect type
}
}
}
}
Another more dynamic solution without reflection:
class Person {
private var fields: Map<String, Any?> = HashMap()
fun setter(propName:String, value:Any): Person{
return this.apply {
it.fields[propName] = value;
}
}
fun getName() = fields["name"]
}
If you want to get rid of the getters as well then you need to use reflection.
I am aware that in Kotlin classes will have an equals and hashcode created automatically as follows:
data class CSVColumn(private val index: Int, val value: String) {
}
My question is, is there a way to have the implementation just use one of these properties (such as index) without writing the code yourself. What was otherwise a very succinct class now looks like this:
data class CSVColumn(private val index: Int, val value: String) {
override fun equals(other: Any?): Boolean {
if (this === other) {
return true
}
if (javaClass != other?.javaClass) {
return false
}
other as CSVColumn
if (index != other.index) {
return false
}
return true
}
override fun hashCode(): Int {
return index
}
}
In Java with Lombok, I can do something like:
#Value
#EqualsAndHasCode(of="index")
public class CsvColumn {
private final int index;
private final String value;
}
Would be cool if there were a way to tell Kotlin something similar.
From the Data Classes documentation you get:
Note that the compiler only uses the properties defined inside the primary constructor for the automatically generated functions. To exclude a property from the generated implementations, declare it inside the class body
So you have to implement equals() and hashCode() manually or with the help of a Kotlin Compiler Plugin.
You can't do something like this for data classes, they always generate equals and hashCode the same way, there's no way to provide them such hints or options.
However, they only include properties that are in the primary constructor, so you could do this for them to only include index:
data class CSVColumn(private val index: Int, value: String) {
val value: String = value
}
... except you can't have parameters in the primary constructor that aren't properties when you're using data classes.
So you'd have to somehow introduce a secondary constructor that takes two parameters, like this:
class CSVColumn private constructor(private val index: Int) {
var value: String = ""
constructor(index: Int, value: String) : this(index) {
this.value = value
}
}
... but now your value property has to be a var for the secondary constructor to be able to set its value.
All this to say that it's probably not worth trying to work around it. If you need an non-default implementation for equals and hashCode, data classes can't help you, and you'll need to implement and maintain them manually.
Edit: as #tynn pointed out, a private setter could be a solution so that your value isn't mutable from outside the class:
class CSVColumn private constructor(private val index: Int) {
var value: String = ""
private set
constructor(index: Int, value: String) : this(index) {
this.value = value
}
}
I wrote a little utility called "stem", which allows you to select which properties to consider for equality and hashing. The resulting code is as small as it can get with manual equals()/hashCode() implementation:
class CSVColumn(private val index: Int, val value: String) {
private val stem = Stem(this, { index })
override fun equals(other: Any?) = stem.eq(other)
override fun hashCode() = stem.hc()
}
You can see its implementation here.
I guess that we have to write equals()/hashCode() manually for now.
https://discuss.kotlinlang.org/t/automatically-generate-equals-hashcode-methods/3779
It is not supported and is planning to be, IMHO.
I guess that we have to write equals()/hashCode() manually for now. https://discuss.kotlinlang.org/t/automatically-generate-equals-hashcode-methods/3779
It is not supported and is planning to be, IMHO.
Below are some reference which may be helpful.
https://discuss.kotlinlang.org/t/how-does-kotlin-implement-equals-and-hashcode/940
https://kotlinlang.org/docs/reference/data-classes.html
https://medium.com/#appmattus/effective-kotlin-item-11-always-override-hashcode-when-you-override-equals-608a090aeaed
See the following performance optimized way (with the use of value classes and inlining) of implementing a generic equals/hashcode for any Kotlin class:
#file:Suppress("EXPERIMENTAL_FEATURE_WARNING")
package org.beatkit.common
import kotlin.jvm.JvmInline
#Suppress("NOTHING_TO_INLINE")
#JvmInline
value class HashCode(val value: Int = 0) {
inline fun combineHash(hash: Int): HashCode = HashCode(31 * value + hash)
inline fun combine(obj: Any?): HashCode = combineHash(obj.hashCode())
}
#Suppress("NOTHING_TO_INLINE")
#JvmInline
value class Equals(val value: Boolean = true) {
inline fun combineEquals(equalsImpl: () -> Boolean): Equals = if (!value) this else Equals(equalsImpl())
inline fun <A : Any> combine(lhs: A?, rhs: A?): Equals = combineEquals { lhs == rhs }
}
#Suppress("NOTHING_TO_INLINE")
object Objects {
inline fun hashCode(builder: HashCode.() -> HashCode): Int = builder(HashCode()).value
inline fun hashCode(vararg objects: Any?): Int = hashCode {
var hash = this
objects.forEach {
hash = hash.combine(it)
}
hash
}
inline fun hashCode(vararg hashes: Int): Int = hashCode {
var hash = this
hashes.forEach {
hash = hash.combineHash(it)
}
hash
}
inline fun <T : Any> equals(
lhs: T,
rhs: Any?,
allowSubclasses: Boolean = false,
builder: Equals.(T, T) -> Equals
): Boolean {
if (rhs == null) return false
if (lhs === rhs) return true
if (allowSubclasses) {
if (!lhs::class.isInstance(rhs)) return false
} else {
if (lhs::class != rhs::class) return false
}
#Suppress("unchecked_cast")
return builder(Equals(), lhs, rhs as T).value
}
}
This allows you to write a equals/hashcode implementation as follows:
data class Foo(val title: String, val bytes: ByteArray, val ignore: Long) {
override fun equals(other: Any?): Boolean {
return Objects.equals(this, other) { lhs, rhs ->
combine(lhs.title, rhs.title)
.combineEquals { lhs.bytes contentEquals rhs.bytes }
}
}
override fun hashCode(): Int {
return Objects.hashCode(title, bytes.contentHashCode())
}
}
I'm new to Kotlin, so I have this interface.
interface User {
var nickName : String
}
Now I want to create a class PrivateUser that implements this interface. I have also to implement the abstract member nickName.
Via constructor it's very simple
class PrivateUser(override var nickName: String) : User
However when I try to implement member inside the class Idea generates me this code
class Button: User {
override var nickName: String
get() = TODO("not implemented")
set(value) {}
}
It's confusing to me how to implement it further.
Properties must be initialized in Kotlin. When you declare the property in the constructor, it gets initialized with whatever you pass in. If you declare it in the body, you need to define it yourself, either with a default value, or parsed from other properties.
Some examples:
class Button : User {
override var nickname = "Fred"
}
class Button(val firstName: String, val lastName: String) : User {
override var nickname = "${firstname[0]}$lastname"
}
The code generated by IDEA is useful if you want a non-default getter and/or setter, or if you want a property without a backing field (it's getter and setter calculate on the fly when accessed).
More examples:
class Button : User {
override var nickname = "Fred"
get() = if (field.isEmpty()) "N/A" else field
set(value) {
// No Tommy
field = if (value == "Tommy") "" else value
}
}
class Button(val number: Int) : User {
var id = "$number"
private set
override var nickname: String
get() {
val parts = id.split('-')
return if (parts.size > 1) parts[0] else ""
}
set(value) {
field = if (value.isEmpty()) "$number" else "$value-$number"
}
}
I have a class that writes a user to SharedPreferences every time it is set:
class UserManager #Inject constructor(
val prefs: SharedPreferences,
val jsonAdapter: JsonAdapter<User>
) {
companion object {
val USER = "user"
}
var user: User = User()
set(value) {
field = value
prefs.edit().putString(USER, jsonAdapter.toJson(user)).apply()
}
init {
val userString = prefs.getString(USER, null)
if (userString != null) {
user = jsonAdapter.fromJson(userString)
}
}
}
Problem: If the user is set in the init block, it calls the setter and writes the user that we just got from the shared prefs... to the shared prefs.
Question 1: How can I directly set the property from the init block?
Question 2: Why do I have to initialize the User when I define a custom setter, but can omit the initialization when the default setter is used?
You need to directily initiliaze the property with the correct value. You can do this using the run function from the stdlib:
class UserManager #Inject constructor(
val prefs: SharedPreferences,
val jsonAdapter: JsonAdapter<User>
) {
companion object {
val USER = "user"
}
var user: User = run {
val userString = prefs.getString(USER, null)
if (userString != null) {
jsonAdapter.fromJson(userString)
} else {
User()
}
}
set(value) {
field = value
prefs.edit().putString(USER, jsonAdapter.toJson(user)).apply()
}
}
Shorter syntax proposed by Ilya Ryzhenkov on the Kotlin Slack:
var user: User = prefs.getString(USER, null)?.let { jsonAdapter.fromJson(it) } ?: User()
set(value) {
field = value
prefs.edit().putString(USER, jsonAdapter.toJson(user)).apply()
}
I believe the best solution is to use the 'backing property' concept described here: https://kotlinlang.org/docs/reference/properties.html#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
return _table ?: throw AssertionError("Set to null by another thread")
}
Then initialize the backing property in the constructor and do <backingproperty> = value instead of field = value as well as point the getter to the backing property.
Take a look at by map delegate, seems like this is the pattern you want:
class User(val map: MutableMap<String, Any?>) {
var name: String by map
var age: Int by map
}
https://kotlinlang.org/docs/reference/delegated-properties.html#storing-properties-in-a-map