I have an enum class SettingsVisibility inside an database entity, which I am converting to a bitmask to store in the database. I am trying to reverse the conversion to bitmask, and get a list of enums as a return value. So if I have an enum with values ONE(1), TWO(2), FOUR(4), then it'll store as Enum(7). I want to take 7 and convert it to {ONE, TWO, FOUR}.
My code is below. I have the SettingsVisibility enum with integer values which are stored in the DB. When I try to retrieve from the database, Objectbox will use the given PropertyConvertor to marshall/unmarshall the data. When I want to convertToEntityProperty, it should return a list of just the saved enums, but at the moment it returns a list of all the enums. I can pass a databaseValue of 12 and it will return all enums instead of just 2 (LOCATION AND PAYMENTS).
I think the issue is the usage of enumClass.enumConstants because it gets all the values, but then the filter doesn't work on this, so I am stuck.
#Entity
data class Settings(
#Id override var id: Long = 0,
#Convert(converter = DocumentVisibilityConverter::class, dbType = Int::class)
val showItems: List<SettingsVisibility>
) : Identifiable<Long> {
lateinit var organisation: ToOne<Organisation>
constructor() : this(
showItems = emptyList(),
)
enum class SettingsVisibility(override val bit: Int) : Flags {
USERS(1),
FINANCE(2),
LOCATION(4),
PAYMENTS(8),
MESSAGES(16),
ERRORS(32),
CANCELLATIONS(64)
}
internal class DocumentVisibilityConverter
: BoxConverters.EnumFlagConverter<SettingsVisibility>(SettingsVisibility::class.java)
}
So for example, if I store the first 3, the database value will be 7 (1+2+4).
The database is ObjectBox and here are the property converters:
abstract class EnumFlagConverter<E>(private val enumClass: Class<E>) : PropertyConverter<List<E>, Int> where E : Enum<E>, E : Flags {
override fun convertToDatabaseValue(entityProperty: List<E>?): Int? {
return entityProperty?.toBitMask()?.value
}
override fun convertToEntityProperty(databaseValue: Int?): List<E>? {
return databaseValue?.let(::BitMask)?.enabledValues(enumClass)
}
}
class BitMask(val value: Int)
interface Flags {
val bit: Int
fun toBitMask() = BitMask(bit)
fun <T> BitMask.enabledValues(enumClass: Class<T>): List<T>? where T : Enum<T>, T : Flags? {
return enumClass.enumConstants?.filter(::hasFlag)
}
infix fun <T : Flags?> BitMask.hasFlag(flag: T): Boolean {
if (value == 0 || (value > 0 && flag?.bit == 0)) {
return false
}
return true
}
Maybe the logic in hasFlag is wrong, because I think that just gets every enum if it isn't 0.
Answer was to replace return true, with:
if (flag?.bit?.toByte() == null) {
return false
}
return (this.value.toByte().and(flag.bit.toByte()) == flag.bit.toByte())
This is basically: bit & mask == bit
Related
I have a situation where I need to create a copy of data class object. I don't know in advance which of the many data classes I have will come in into the function. I do know, however, that only data classes will be used as input to this function.
This is what didn't work:
fun doSomething(obj: Any): Any {
obj.copy(...) // <- there's no 'copy' on Any
...
}
This is what I really like to do:
fun doSomething(obj: KAnyDataClass): KAnyDataClass {
obj.copy(...) // <- works, data classes have a 'copy' method
...
}
I'm not a Kotlin developer, but it looks like the language does not support dynamic dispatch or traits. You might find success with the dynamic type, which just turns off the type-checker so it won't yell at you for using a method that it doesn't know about. However this opens up the possibility of a runtime error if you pass an argument that actually doesn't have that method.
There is no class or interface for data classes, but we know from the documentation of data classes that there are derived functions componentN and copy in each data class.
We can use that knowledge to write an abstract copy method that calls the copy method of a given arbitrary data class using reflection:
fun <T : Any> copy(data: T, vararg override: Pair<Int, Any?>): T {
val kClass = data::class
if (!kClass.isData) error("expected a data class")
val copyFun = kClass.functions.first { it.name == "copy" }
checkParameters(override, kClass)
val vals = determineComponentValues(copyFun, kClass, override, data)
#Suppress("UNCHECKED_CAST")
return copyFun.call(data, *vals) as T
}
/** check if override of parameter has the right type and nullability */
private fun <T : Any> checkParameters(
override: Array<out Pair<Int, Any?>>,
kClass: KClass<out T>
) {
override.forEach { (index, value) ->
val expectedType = kClass.functions.first { it.name == "component${index + 1}" }.returnType
if (value == null) {
if (!kClass.functions.first { it.name == "component${index + 1}" }.returnType.isMarkedNullable) {
error("value for parameter $index is null but parameter is not nullable")
}
} else {
if (!expectedType.jvmErasure.isSuperclassOf(value::class))
error("wrong type for parameter $index: expected $expectedType but was ${value::class}")
}
}
}
/** determine for each componentN the value from override or data element */
private fun <T : Any> determineComponentValues(
copyFun: KFunction<*>,
kClass: KClass<out T>,
override: Array<out Pair<Int, Any?>>,
data: T
): Array<Any?> {
val vals = (1 until copyFun.parameters.size)
.map { "component$it" }
.map { name -> kClass.functions.first { it.name == name } }
.mapIndexed { index, component ->
override.find { it.first == index }.let { if (it !== null) it.second else component.call(data) }
}
.toTypedArray()
return vals
}
Since this copy function is generic and not for a specific data class, it is not possible to specify overloads in the usual way, but I tried to support it in another way.
Let's say we have a data class and element
data class Example(
val a: Int,
val b: String,
)
val example: Any = Example(1, "x")
We can create a copy of example with copy(example) that has the same elements as the original.
If we want to override the first element, we cannot write copy(example, a = 2), but we can write copy(example, 0 to 2), saying that we want to override the first component with value 2.
Analogously we can write copy(example, 0 to 3, 1 to "y") to specify that we want to change the first and the second component.
I am not sure if this works for all cases since I just wrote it, but it should be a good start to work with.
I have this enum:
enum class Types(val value: Int) {
FOO(1)
BAR(2)
FOO_BAR(3)
}
How do I create an instance of that enum using an Int?
I tried doing something like this:
val type = Types.valueOf(1)
And I get the error:
Integer literal does not conform to the expected type String
enum class Types(val value: Int) {
FOO(1),
BAR(2),
FOO_BAR(3);
companion object {
fun fromInt(value: Int) = Types.values().first { it.value == value }
}
}
You may want to add a safety check for the range and return null.
Enum#valueOf is based on name. Which means in order to use that, you'd need to use valueof("FOO"). The valueof method consequently takes a String, which explains the error. A String isn't an Int, and types matter. The reason I mentioned what it does too, is so you know this isn't the method you're looking for.
If you want to grab one based on an int value, you need to define your own function to do so. You can get the values in an enum using values(), which returns an Array<Types> in this case. You can use firstOrNull as a safe approach, or first if you prefer an exception over null.
So add a companion object (which are static relative to the enum, so you can call Types.getByValue(1234) (Types.COMPANION.getByValue(1234) from Java) over Types.FOO.getByValue(1234).
companion object {
private val VALUES = values()
fun getByValue(value: Int) = VALUES.firstOrNull { it.value == value }
}
values() returns a new Array every time it's called, which means you should cache it locally to avoid re-creating one every single time you call getByValue. If you call values() when the method is called, you risk re-creating it repeatedly (depending on how many times you actually call it though), which is a waste of memory.
Admittedly, and as discussed in the comments, this may be an insignificant optimization, depending on your use. This means you can also do:
companion object {
fun getByValue(value: Int) = values().firstOrNull { it.value == value }
}
if that's something you'd prefer for readability or some other reason.
The function could also be expanded and check based on multiple parameters, if that's something you want to do. These types of functions aren't limited to one argument.
If you are using integer value only to maintain order, which you need to access correct value, then you don't need any extra code. You can use build in value ordinal. Ordinal represents position of value in enum declaration.
Here is an example:
enum class Types {
FOO, //Types.FOO.ordinal == 0 also position == 0
BAR, //Types.BAR.ordinal == 1 also position == 1
FOO_BAR //Types.FOO_BAR.ordinal == 2 also position == 2
}
You can access ordinal value simply calling:
Types.FOO.ordinal
To get correct value of enum you can simply call:
Types.values()[0] //Returns FOO
Types.values()[1] //Returns BAR
Types.values()[2] //Returns FOO_BAR
Types.values() returns enum values in order accordingly to declaration.
Summary:
Types.values(Types.FOO.ordinal) == Types.FOO //This is true
If integer values don't match order (int_value != enum.ordinal) or you are using different type (string, float...), than you need to iterate and compare your custom values as it was already mentioned in this thread.
It really depends on what you actually want to do.
If you need a specific hardcoded enum value, then you can directly use Types.FOO
If you are receiving the value dynamically from somewhere else in your code, you should try to use the enum type directly in order not to have to perform this kind of conversions
If you are receiving the value from a webservice, there should be something in your deserialization tool to allow this kind of conversion (like Jackson's #JsonValue)
If you want to get the enum value based on one of its properties (like the value property here), then I'm afraid you'll have to implement your own conversion method, as #Zoe pointed out.
One way to implement this custom conversion is by adding a companion object with the conversion method:
enum class Types(val value: Int) {
FOO(1),
BAR(2),
FOO_BAR(3);
companion object {
private val types = values().associate { it.value to it }
fun findByValue(value: Int): Types? = types[value]
}
}
Companion objects in Kotlin are meant to contain members that belong to the class but that are not tied to any instance (like Java's static members).
Implementing the method there allows you to access your value by calling:
var bar = Types.findByValue(2) ?: error("No Types enum value found for 2")
Note that the returned value is nullable, to account for the possibility that no enum value corresponds to the parameter that was passed in. You can use the elvis operator ?: to handle that case with an error or a default value.
If you hate declaring for each enum type a companion object{ ... } to achieve EMotorcycleType.fromInt(...). Here's a solution for you.
EnumCaster object:
object EnumCaster {
inline fun <reified E : Enum<E>> fromInt(value: Int): E {
return enumValues<E>().first { it.toString().toInt() == value }
}
}
Enum example:
enum class EMotorcycleType(val value: Int){
Unknown(0),
Sport(1),
SportTouring(2),
Touring(3),
Naked(4),
Enduro(5),
SuperMoto(6),
Chopper(7),
CafeRacer(8),
.....
Count(9999);
override fun toString(): String = value.toString()
}
Usage example 1: Kotlin enum to jni and back
fun getType(): EMotorcycleType = EnumCaster.fromInt(nGetType())
private external fun nGetType(): Int
fun setType(type: EMotorcycleType) = nSetType(type.value)
private external fun nSetType(value: Int)
---- or ----
var type : EMotorcycleType
get() = EnumCaster.fromInt(nGetType())
set(value) = nSetType(value.value)
private external fun nGetType(): Int
private external fun nSetType(value: Int)
Usage example 2: Assign to val
val type = EnumCaster.fromInt<EMotorcycleType>(aValidTypeIntValue)
val typeTwo : EMotorcycleType = EnumCaster.fromInt(anotherValidTypeIntValue)
A naive way can be:
enum class Types(val value: Int) {
FOO(1),
BAR(2),
FOO_BAR(3);
companion object {
fun valueOf(value: Int) = Types.values().find { it.value == value }
}
}
Then you can use
var bar = Types.valueOf(2)
Protocol orientated way with type-safety
interface RawRepresentable<T> {
val rawValue: T
}
inline fun <reified E, T> valueOf(value: T): E? where E : Enum<E>, E: RawRepresentable<T> {
return enumValues<E>().firstOrNull { it.rawValue == value }
}
enum class Types(override val rawValue: Int): RawRepresentable<Int> {
FOO(1),
BAR(2),
FOO_BAR(3);
}
Usage
val type = valueOf<Type>(2) // BAR(2)
You can use it on non-integer type, too.
I would build the 'reverse' map ahead of time. Probably not a big improvement, but also not much code.
enum class Test(val value: Int) {
A(1),
B(2);
companion object {
val reverseValues: Map<Int, Test> = values().associate { it.value to it }
fun valueFrom(i: Int): Test = reverseValues[i]!!
}
}
Edit: map...toMap() changed to associate per #hotkey's suggestion.
try this...
companion object{
fun FromInt(v:Int):Type{
return Type::class.java.constructors[0].newInstance(v) as Type
}
}
This is for anyone looking for getting the enum from its ordinal or index integer.
enum class MyEnum { RED, GREEN, BLUE }
MyEnum.values()[1] // GREEN
Another solution and its variations:
inline fun <reified T : Enum<T>> enumFromIndex(i: Int) = enumValues<T>()[i]
enumFromIndex<MyEnum>(1) // GREEN
inline fun <reified T : Enum<T>> enumFromIndex(i: Int) = enumValues<T>().getOrNull(i)
enumFromIndex<MyEnum>(3) ?: MyEnum.RED // RED
inline fun <reified T : Enum<T>> enumFromIndex(i: Int, default: T) =
enumValues<T>().getOrElse(i) { default }
enumFromIndex(2, MyEnum.RED) // BLUE
It is an adapted version of another answer. Also, thanks to Miha_x64 for this answer.
Another option...
enum class Types(val code: Int) {
FOO(1),
BAR(2),
FOO_BAR(3);
companion object {
val map = values().associate { it.code to it }
// Get Type by code with check existing codes and default
fun getByCode(code: Int, typeDefault_param: Types = FOO): Types {
return map[code] ?: typeDefault_param
}
}
}
fun main() {
println("get 3: ${Types.getByCode(3)}")
println("get 10: ${Types.getByCode(10)}")
}
get 3: FOO_BAR
get 10: FOO
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 have several data class with fields, which are used in forms and need them to have a method return true if any of the fields has been filled.
I don't want to rewrite this for all the classes, so I'm doing it like this at the moment:
data class Order(var consumer: String, var pdfs: List<URI>): Form {
override val isEmpty(): Boolean
get() = checkEmpty(consumer, pdfs)
}
data class SomethingElse(var str: String, var set: Set<String>): Form {
override val isEmpty(): Boolean
get() = checkEmpty(str, set)
}
interface Form {
val isEmpty: Boolean
fun <T> checkEmpty(vararg fields: T): Boolean {
for (f in fields) {
when (f) {
is Collection<*> -> if (!f.isEmpty()) return false
is CharSequence -> if (!f.isBlank()) return false
}
}
return true;
}
}
This is obviously not very pretty nor type-safe.
What's a more idiomatic way of doing this, without abstracting every property into some kind of Field-type?
Clarification: What I'm looking for is a way to get exhaustive when, for example by providing all the allowed types (String, Int, List, Set) and a function for each to tell if they're empty. Like an "extension-interface" with a method isEmptyFormField.
It's kinda hacky but should work.
Every data class creates set of method per each constructor parameters. They're called componentN() (where N is number starting from 1 indicating constructor parameter).
You can put such methods in your interface and make data class implicitly implement them. See example below:
data class Order(var consumer: String, var pdfs: List) : Form
data class SomethingElse(var str: String, var set: Set) : Form
interface Form {
val isEmpty: Boolean
get() = checkEmpty(component1(), component2())
fun checkEmpty(vararg fields: T): Boolean {
for (f in fields) {
when (f) {
is Collection -> if (!f.isEmpty()) return false
is CharSequence -> if (!f.isBlank()) return false
}
}
return true;
}
fun component1(): Any? = null
fun component2(): Any? = null
}
You can also add fun component3(): Any? = null etc... to handle cases with more that 2 fields in data class (e.g. NullObject pattern or handling nulls directly in your checkEmpty() method.
As I said, it's kinda hacky but maybe will work for you.
If all you are doing is checking for isEmpty/isBlank/isZero/etc. then you probably don't need a generic checkEmpty function, etc.:
data class Order(var consumer: String, var pdfs: List<URI>) : Form {
override val isEmpty: Boolean
get() = consumer.isEmpty() && pdfs.isEmpty()
}
data class SomethingElse(var str: String, var set: Set<String>) : Form {
override val isEmpty: Boolean
get() = str.isEmpty() && set.isEmpty()
}
interface Form {
val isEmpty: Boolean
}
However, if you are actually do something a bit more complex then based on your added clarification I believe that "abstracting every property into some kind of Field-type" is exactly what you want just don't make the Field instances part of each data class but instead create a list of them when needed:
data class Order(var consumer: String, var pdfs: List<URI>) : Form {
override val fields: List<Field<*>>
get() = listOf(consumer.toField(), pdfs.toField())
}
data class SomethingElse(var str: String, var set: Set<String>) : Form {
override val fields: List<Field<*>>
get() = listOf(str.toField(), set.toField())
}
interface Form {
val isEmpty: Boolean
get() = fields.all(Field<*>::isEmpty)
val fields: List<Field<*>>
}
fun String.toField(): Field<String> = StringField(this)
fun <C : Collection<*>> C.toField(): Field<C> = CollectionField(this)
interface Field<out T> {
val value: T
val isEmpty: Boolean
}
data class StringField(override val value: String) : Field<String> {
override val isEmpty: Boolean
get() = value.isEmpty()
}
data class CollectionField<out C : Collection<*>>(override val value: C) : Field<C> {
override val isEmpty: Boolean
get() = value.isEmpty()
}
This gives you type-safety without changing your data class components, etc. and allows you to "get exhaustive when".
You can use null to mean "unspecified":
data class Order(var consumer: String?, var pdfs: List<URI>?) : Form {
override val isEmpty: Boolean
get() = checkEmpty(consumer, pdfs)
}
data class SomethingElse(var str: String?, var set: Set<String>?) : Form {
override val isEmpty: Boolean
get() = checkEmpty(str, set)
}
interface Form {
val isEmpty: Boolean
fun <T> checkEmpty(vararg fields: T): Boolean = fields.all { field -> field == null }
}
The idea here is the same as that of an Optional<T> in Java but without the extra object, etc.
You now have to worry about null safety but if your fields are meant to have a concept of absent/empty then this seems appropriate (UsingAndAvoidingNullExplained ยท google/guava Wiki).
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)
}
}