I have a Kotlin class with named arguments and defaults for non-specified arguments. I am trying to figure out how to create a custom setter for one argument and just can't seem to figure out what I'm doing wrong - although it's probably simple. Here is a simplified version of the class:
class ItemObject(var itemNumber: String = "",
var itemQty: Int = 0)
I can use the properties of this class without issues itemObject.itemQty = itemObject.itemQty + 1 (accessing both the getter and setter).
However, I'd like to make a custom setter to prevent the itemQty from going below zero. So I've tried many variations on the following theme:
class ItemObject(var itemNumber: String = "",
itemQty: Int = 0) {
var itemQty: Int = 0
set(value: Int) =
if (value >= 0) {
itemQty = value // Don't want to go negative
} else {
}
}
This compiles without issue, but seems to keep defaulting itemQty to zero (or something like this - I haven't tracked it down).
Can anyone point me in the correct direction? I'd certainly appreciate it - this is my first Kotlin project and could use a bit of help. :)
This is what vetoable is for:
var quantity: Int by Delegates.vetoable(0) { _, _, new ->
new >= 0
}
Return true to accept the value, return false to reject it.
Well, you initualize the real field to 0 and ignore the passed value ... Instead, initialize the property with the passed constructor parameter:
class Item(_itemQty: Int = 0) {
var itemQty: Int = Math.max(0, _itemQty)
set(v) { field = Math.max(0, v) }
}
(I used two different Identifiers to seperate the parameter and the property, as mentioned in the comments you can also use the same name for the property and the parameter [but be careful, that could add confusion]).
You should also set the backing field, not the setter itself which will end in endless recursion.
If the setter is rather complicated and also needs to be applied to the initial value, then you could initialize the property, and execute the setter afterwards with the parameter:
class Item(_itemQty: Int = 0) {
var itemQty: Int = 0
set(v) { field = Math.max(0, v) }
init { itemQty = _itemQty }
}
As an opinion-based side note, item.itemQty is not really descriptive, item.quantity would be way more readable.
There are different ways of preventing value going below zero.
One is unsigned types. Experimental at the moment. UInt in your case
class ItemObject(var itemNumber: String = "",
var itemQty: UInt = 0u
)
If you don't care about value overflow - it might be an option
Another way is Property Delegation
class ItemObject(var itemNumber: String = "", itemQty: Int = 0) {
var itemQty: Int by PositiveDelegate(itemQty)
}
class PositiveDelegate(private var prop: Int) {
operator fun getValue(thisRef: Any?, property: KProperty<*>): Int = prop
operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
if (value >= 0) prop = value
}
}
Third one with custom setters is described in other answers.
Related
I've got following example class:
fun main(args: Array<String>) {
var car = Car("Ford", 50, 8000)
println(car.vendor)
println("Speed: ${car.speed}")
car.speed = 65 // How does it know which setter to invoke?
println("New speed: ${car.speed}")
}
class Car(vendor: String = "Unknown", speed: Int = 0, price: Int = 0) {
var vendor = vendor
get() = field
set(newVendor: String) {
field = newVendor
}
var speed = speed
get() = field
set(speed: Int) {
field = speed
}
var price = price
get() = field
set(newPrice: Int) {
field = price
}
}
When I change the speed-attribute (please see the commented line): Where does Kotlin know from, which setter-method it has actually to invoke?
There are two setter-methods within my class (speed, price) which both are named "set" and both expect an integer-value.
Is the order, in which the methods are defined, crucial?
The respective getter-/setter-methods have to be written directly after the attribute-definition? Or does it somehow work differently? If so: How?
Are the indentations just a covention? Or are the indentations needed for the compiler?
car.speed = 65 is called Property-access syntax. It is equivalent to car.setSpeed(65).
You don't have two methods named set; you've two mutable properties speed and price both of type Int. They each have corresponding getter and setter methods; in Java Beans convention, the getter for speed would be getSpeed and the setter setSpeed(Int).
See https://kotlinlang.org/docs/reference/properties.html for more details.
I am trying to find a solution for a nice kotlin data class solution. I have already this:
data class Object(
var classMember: Boolean,
var otherClassMember: Boolean,
var example: Int = 0) {
fun set(block: Object.() -> kotlin.Unit): Object {
val copiedObject = this.copy()
copiedObject.apply {
block()
}
return copiedObject
}
fun touch(block: Object.() -> kotlin.Unit): Object {
return this.set {
classMember = true
otherClassMember = false
block() }
}
}
val test = Object(true,true,1)
val changedTest = test.touch { example = 2 }
the result of this method is that the changedTest object has classMember = true, otherClassMember = false and example = 2
The problem with this solution is, the class properties are not immutable with var declaration. Does somebody have an idea how to optimize my methods to change var to val?
val says that a variable can't change it's value after initialization at the definition point. Kotlin's generated copy method does not modify an existing copy after construction: this method actually uses retrieved values from an object, replaces these values with ones that provided in copy method (if any), and after that just constructs a new object using these values.
So, it is not possible to perform such an optimization if you are going to change object's state after construction.
If I understood what you want correctly, you can do
data class Object(
val classMember: Boolean,
val otherClassMember: Boolean,
val example: Int = 0) {
fun touch(example: Int = this.example): Object {
return copy(
classMember = true,
otherClassMember = false,
example = example)
}
}
val test = Object(true,true,1)
val changedTest = test.touch(example = 2)
Though you need to repeat parameters other than classMember and otherClassMember but without reflection you can't do better.
num should be nullable when set, but what it returns should always be non-nullable (have a default value).
class Test {
var num: Int? = null
get() = field ?: 5 // default value if null
}
The following does not compile even though the returned value is always non-null which makes sense to me, because the type is not inferred but taken from the backing field:
val a: Int = Test().num
Type mismatch: inferred type is Int? but Int was expected
The question is how can I change the return type of that getter to be non-nullable? If I do so, the compiler says:
Getter return type must be equal to the type of the property, i.e.
'Int?'
I know that I could solve it with another property numNotNullable (without a backing field).
class Test {
var num: Int? = null
get() = field ?: 5 // default value if null
val numNotNullable: Int
get() = num ?: 5
}
val c: Int = Test().numNotNullable
But this is not what I want. Is there another way?
var num: Int? = null
This is your property signature. It doesn't matter, if you internally ensure that no null value is returned. The signature says, that the value is nullable.
This implicates:
You are allowed to set null to this field
All classes using this field, must handle the fact that the property can return null
Your Solution with a second property is good.
You of course can replace the property with plain old java bean, but I wouldn't advise that, because than you have to access the prop with getNumb and setNum.
class Test {
private var num: Int = 5
fun setNum(num: Int?) {
this.num = num ?: 5
}
fun getNum() = num
}
I don't believe this is possible in Kotlin. You can't override the type of the the property for get/set. So if your property is an Int? you're going to have to return an Int? and check if it is null when you use it.
There's technically a feature request for what you're looking for, but it's been years since it was made.
You can achive this using delegated properties
import kotlin.properties.ReadWriteProperty
import kotlin.reflect.KProperty
class LazyVar<T : Any>(private var initializer: () -> T) : ReadWriteProperty<Any?, T> {
private var value: T? = null
override fun getValue(thisRef: Any?, property: KProperty<*>): T {
if (value == null) {
value = initializer()
print(value)
}
return value as T
}
override fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
this.value = value
}
}
class Test {
var num: Int by LazyVar { 5 }
}
val a: Int = Test().num
Note, that this code is not thread-safe. Also with this code sample you can't set null value for you field (so no way back to default value).
Consider this Kotlin code:
var parent: T? = null
get() = if (isParent) this as T else field
set(value) { field = if (value == null) null else value.parent }
val isParent: Boolean
get() = parent == null
var description = ""
get() = if (isParent) field else parent!!.description
set(value) { if (isParent) field = value else parent!!.description = value }
Assume that isParent returns true if this instance is a parent instance. If not getParent() will return the parent instance. In Java you are allowed to access directly field of a different instance of same class like this:
String getDescription() { return getParent().description; }
void setDescription(String value) { getParent().description = value; }
(I am not saying that is a best thing to do, I simplified it for demostration). Comparing to Java, it would be nice to be able to do following:
var description = ""
get() = parent.field
set(value) { parent.field = value }
However this does not work and unfortunately it makes the code less readable. Especially if you have a lot of such variables, which are bound to this parent.
A backing field of a property can only be accessed from a getter or setter of that property, and only for the instance on which the getter or setter has been invoked. If you need to provide multiple ways to access an attribute of a class, you need to define two distinct properties, one of which has a backing field to store the data and another has a getter and setter referring to the first property.
class Foo {
var parent: Foo? = null
val parentOrSelf: Foo get() = parent ?: this
private var _description: String? = null
var description = ""
get() = parentOrSelf._description
set(value) { parentOrSelf._description = value }
}
Given the following Kotlin class:
data class Test(val value: Int)
How would I override the Int getter so that it returns 0 if the value negative?
If this isn't possible, what are some techniques to achieve a suitable result?
After spending almost a full year of writing Kotlin daily I've found that attempting to override data classes like this is a bad practice. There are 3 valid approaches to this, and after I present them, I'll explain why the approach other answers have suggested is bad.
Have your business logic that creates the data class alter the value to be 0 or greater before calling the constructor with the bad value. This is probably the best approach for most cases.
Don't use a data class. Use a regular class and have your IDE generate the equals and hashCode methods for you (or don't, if you don't need them). Yes, you'll have to re-generate it if any of the properties are changed on the object, but you are left with total control of the object.
class Test(value: Int) {
val value: Int = value
get() = if (field < 0) 0 else field
override fun equals(other: Any?): Boolean {
if (this === other) return true
if (other !is Test) return false
return true
}
override fun hashCode(): Int {
return javaClass.hashCode()
}
}
Create an additional safe property on the object that does what you want instead of having a private value that's effectively overriden.
data class Test(val value: Int) {
val safeValue: Int
get() = if (value < 0) 0 else value
}
A bad approach that other answers are suggesting:
data class Test(private val _value: Int) {
val value: Int
get() = if (_value < 0) 0 else _value
}
The problem with this approach is that data classes aren't really meant for altering data like this. They are really just for holding data. Overriding the getter for a data class like this would mean that Test(0) and Test(-1) wouldn't equal one another and would have different hashCodes, but when you called .value, they would have the same result. This is inconsistent, and while it may work for you, other people on your team who see this is a data class, may accidentally misuse it without realizing how you've altered it / made it not work as expected (i.e. this approach wouldn't work correctly in a Map or a Set).
You could try something like this:
data class Test(private val _value: Int) {
val value = _value
get(): Int {
return if (field < 0) 0 else field
}
}
assert(1 == Test(1).value)
assert(0 == Test(0).value)
assert(0 == Test(-1).value)
assert(1 == Test(1)._value) // Fail because _value is private
assert(0 == Test(0)._value) // Fail because _value is private
assert(0 == Test(-1)._value) // Fail because _value is private
In a data class you must to mark the primary constructor's parameters with either val or var.
I'm assigning the value of _value to value in order to use the desired name for the property.
I defined a custom accessor for the property with the logic you described.
The answer depends on what capabilities you actually use that data provides. #EPadron mentioned a nifty trick (improved version):
data class Test(private val _value: Int) {
val value: Int
get() = if (_value < 0) 0 else _value
}
That will works as expected, e.i it has one field, one getter, right equals, hashcode and component1. The catch is that toString and copy are weird:
println(Test(1)) // prints: Test(_value=1)
Test(1).copy(_value = 5) // <- weird naming
To fix the problem with toString you may redefine it by hands. I know of no way to fix the parameter naming but not to use data at all.
I have seen your answer, I agree that data classes are meant for holding data only, but sometimes we need to make somethings out of them.
Here is what i'm doing with my data class, I changed some properties from val to var, and overid them in the constructor.
like so:
data class Recording(
val id: Int = 0,
val createdAt: Date = Date(),
val path: String,
val deleted: Boolean = false,
var fileName: String = "",
val duration: Int = 0,
var format: String = " "
) {
init {
if (fileName.isEmpty())
fileName = path.substring(path.lastIndexOf('\\'))
if (format.isEmpty())
format = path.substring(path.lastIndexOf('.'))
}
fun asEntity(): rc {
return rc(id, createdAt, path, deleted, fileName, duration, format)
}
}
I know this is an old question but it seems nobody mentioned the possibility to make value private and writing custom getter like this:
data class Test(private val value: Int) {
fun getValue(): Int = if (value < 0) 0 else value
}
This should be perfectly valid as Kotlin will not generate default getter for private field.
But otherwise I definitely agree with spierce7 that data classes are for holding data and you should avoid hardcoding "business" logic there.
I found the following to be the best approach to achieve what you need without breaking equals and hashCode:
data class TestData(private var _value: Int) {
init {
_value = if (_value < 0) 0 else _value
}
val value: Int
get() = _value
}
// Test value
assert(1 == TestData(1).value)
assert(0 == TestData(-1).value)
assert(0 == TestData(0).value)
// Test copy()
assert(0 == TestData(-1).copy().value)
assert(0 == TestData(1).copy(-1).value)
assert(1 == TestData(-1).copy(1).value)
// Test toString()
assert("TestData(_value=1)" == TestData(1).toString())
assert("TestData(_value=0)" == TestData(-1).toString())
assert("TestData(_value=0)" == TestData(0).toString())
assert(TestData(0).toString() == TestData(-1).toString())
// Test equals
assert(TestData(0) == TestData(-1))
assert(TestData(0) == TestData(-1).copy())
assert(TestData(0) == TestData(1).copy(-1))
assert(TestData(1) == TestData(-1).copy(1))
// Test hashCode()
assert(TestData(0).hashCode() == TestData(-1).hashCode())
assert(TestData(1).hashCode() != TestData(-1).hashCode())
However,
First, note that _value is var, not val, but on the other hand, since it's private and data classes cannot be inherited from, it's fairly easy to make sure that it is not modified within the class.
Second, toString() produces a slightly different result than it would if _value was named value, but it's consistent and TestData(0).toString() == TestData(-1).toString().
Seems to be an old but interesting question.
Just want to contribute an option:
data class Test(#JvmField val value: Int){
fun getValue() = if(value<0) 0 else value
}
Now you can override getValue, and still have component1() working.
This seems to be one (among other) annoying drawbacks of Kotlin.
It seems that the only reasonable solution, which completely keeps backward compatibility of the class is to convert it into a regular class (not a "data" class), and implement by hand (with the aid of the IDE) the methods: hashCode(), equals(), toString(), copy() and componentN()
class Data3(i: Int)
{
var i: Int = i
override fun equals(other: Any?): Boolean
{
if (this === other) return true
if (other?.javaClass != javaClass) return false
other as Data3
if (i != other.i) return false
return true
}
override fun hashCode(): Int
{
return i
}
override fun toString(): String
{
return "Data3(i=$i)"
}
fun component1():Int = i
fun copy(i: Int = this.i): Data3
{
return Data3(i)
}
}
You can follow the Builder Pattern for this I think it'd be much better.
Here is an example:
data class Test(
// Fields:
val email: String,
val password: String
) {
// Builder(User):
class Builder(private val email: String) {
// Fields:
private lateinit var password: String
// Methods:
fun setPassword(password: String): Builder {
// Some operation like encrypting
this.password = password
// Returning:
return this
}
fun build(): Test = Test(email, password)
}
}