I'm defining a Kotlin class with a number of primitive properties:
class Contract (contractEntity : ContractEntity) EntityDao<ContractEntity> {
var id : Long // <- This is a primitive datatype, needs to be initialized
var concept : String //<- This also needs to be initialized or declared abstract
init{
mapFromEntity(contractEntity)
}
override fun mapFromEntity(entity : ContractEntity){
id = entity.id
concept = entity.concept
}
}
Now, I want those properties to be initialized with the function mapFromEntity() but I'm stuck with the init block because those are not initialized. What could be a good way to achieve what I'm trying?
So far, the best solution suggest to remove the function that maps the class and use the constructor parameter.
class Contract (contractEntity : ContractEntity) EntityDao<ContractEntity> {
var id = contractEntity.id
var concept = contractEntity.concept
}
Related
I have a list of clases that implement a specific interface. The ability to construct those clases or not is not static (so it's not possible to use when(className)), and can be configured so I want to be able to create some clases or call some methods based on a hashMap of allowed "constructors". Then if the key identifying a class is in present in the hashmap I can call the corresponding method, otherwise I can safely ignore. Let me illustrate:
Let's say I have an interface like
interface Instanceable {
data class Config(
val bar: Whatever
)
fun getIntance(config: Config): Instanceable
}
Then I have several (let's say 10) classes that implement this interface
class Implementation1() : Instanceable {
companion object {
const val ID = "INSTANCE_1"
}
private lateinit var foo: Whatever
override fun getIntance(config: Config) = Implementation1().also{ this#Implementation1.foo = config.bar }
}
I want to create a hashmap of the methods by the identifiers, so later down the lane I can grab the method from the hashMap by the key ID and just invoke() the value if it's there. Something like:
allowedInstances("INSTANCE_1")?.let{ it.invoke(someConfig) }
In order to do this I tried to create a hashMap of methods like this:
private val allowedInstances = mutableHashMapOf<String, Instanceable.(Instanceable.Config)->Instanceable>()
allowedInstances[Instance1.ID] = Instance1::getIntance
allowedInstances[Instance2.ID] = Instance2::getIntance
allowedInstances[Instance4.ID] = Instance4::getIntance
But it fails with:
Type mismatch.
Required: Instanceable.(Instanceable.Config) → Instanceable
Found: KFunction2<Implementation1, Instanceable.Config, Instanceable>
If I create the hashmap directly and let the compiler infer the types like this:
private val allowedInstances = mutableHashMapOf(
Implementation1.ID to Implementation1::getIntance,
Implementation2.ID to Implementation2::getIntance,
Implementation4.ID to Implementation4::getIntance,
)
Checking the type of the hashmap shows:
HashMap<String, out KFunction2<Nothing, Instanceable.Config, Instanceable>>
In fact I can do:
private val allowedInstances = mutableHashMapOf<String, Nothing.(Instanceable.Config)->Instanceable>()
allowedInstances[Instance1.ID] = Instance1::getIntance
allowedInstances[Instance2.ID] = Instance2::getIntance
allowedInstances[Instance4.ID] = Instance4::getIntance
So the actual question is:
Why the function of the second hashMap parameter has Nothing as the receptor? Why I cannot have the interface Instanceable instead?
Edit: Still not good to have the Nothing there:
allowedInstances["INSTANCE_1"]?.let{ it.invoke(Nothing, someConfig) }
//Fails with: Classifier 'Nothing' does not have a companion object, and thus must be initialized here
Edit 2: All of the errors are in compile time
Your function type
Instanceable.(Instanceable.Config) -> Instanceable
is describing an extension function on an instance of Instanceable. You need to omit the receiver from the function type to be able to match your constructors' signature:
(Instanceable.Config) -> Instanceable
Edit: The other half of the problem is that you define getInstance() as a member function of the class. So you have to create an invalid instance of your class to use to create a valid instance, which doesn't make sense.
I would delete the getInstance() function from your interface, and put the equivalent code in the constructor of your class. Then you can define a function type in your Map that constructs your items.
interface Instanceable {
data class Config(
val bar: Whatever
)
// REMOVE this: fun getIntance(config: Config): Instanceable
}
class Implementation1(config: Config) : Instanceable {
companion object {
const val ID = "INSTANCE_1"
}
private val foo: Whatever = config.bar
}
private val allowedInstances = mutableHashMapOf<String, (Instanceable.Config)->Instanceable>()
allowedInstances[Instance1.ID] = ::Implementation1
// and so on...
// If there's an implementation that has no config, you can use a lambda:
class NoConfigImplementation : Instanceable {
companion object {
const val ID = "INSTANCE_2"
}
}
allowedInstances[NoConfigImplementation.ID] = { _ -> NoConfigImplementation() }
I have multiple data classes and each class has a corresponding class containing more info. I want to write a function in which I should be able to pass an identifier (table name corresponding to the data class). Based on this identifier, object of the corresponding class should be made, the value changed and this object should be returned as output of the function. I have written a simplified version of it on playground but I am unable to get it to work. Any help is appreciated.
class someClass(
)
class objectForSomeClass(
var value: String
)
class someOtherClass(
)
class objectForSomeOtherClass(
var value: String
)
class doSomething() {
companion object {
val classMap = mapOf(
"someClass" to objectForSomeClass::class,
"someOtherClass" to objectForSomeOtherClass::class,
)
}
// Create a map of class name to a new object based on the class name input
fun dummyFun(className: String, valueInput: String): Map<String, kotlin.Any> {
var returnObject = mutableListOf<Pair<String, kotlin.Any>>()
when(className) {
"SOME_CLASS" -> {
returnObject = mutableListOf<Pair<String, justDoIt.classMap["someClass"]()>>()
}
"SOME_OTHER_CLASS" -> {
returnObject = Map<String, justDoIt.classMap["someOtherClass"]()>
}
}
returnObject[className].value = valueInput
return returnObject
}
}
fun main() {
var obj = doSomething()
var t = obj.dummyFun("SOME_CLASS", "Value to be inserted")
// do something with t
}
Not knowing more about your classes (the ones in your code are not data classes – a data class in Kotlin is a specific type of class) I still think a lot could be simplified down to maybe even this:
fun createObject(className: String, value: String): Any? {
return when (className) {
"SomeClass" -> ObjectForSomeClass(value)
"SomeOtherClass" -> ObjectForSomeOtherClass(value)
// ...
else -> null
}
}
Additionally:
The classMap is not necessary, you can hard-code the cases in the when clause as in my example. There is also no need for reflection, which you would need to create instances from SomeType::class.
With getting rid of classMap you also do not need the companion object holding it anymore, and then you are left with one function for creating instances of your classes, and this function does not have to be in a class. You might put it into a singleton class called object in Kotlin (https://kotlinlang.org/docs/object-declarations.html#object-expressions)
Data classes in Kotlin: https://kotlinlang.org/docs/data-classes.html
You could maybe also replace each class someClass & class objectForSomeClass pair with a class someClass with a companion object.
Suppose I have an abstract superclass A. That class has a property abstract val predicate: (ModelClass) -> Boolean.
Let B be a subclass.
I want to be able to do both of the following:
Use the predicate from an instance aInstance.predicate
Also use that predicate elsewhere, without having to create an instance to get that predicate B.predicate
How can I do this.
I don't think this is possible.
There is no such thing as an abstract static method in Kotlin or Java.
Perhaps this will give more insight.
Does your class need to be abstract? Maybe the code below can be useful:
open class MyClass {
companion object myCompanion {
val myStatic = "MyClass"
}
open val myStatic = myCompanion.myStatic
}
class MySubClass : MyClass() {
companion object myCompanionSubClass {
val myStatic = "MySubClass"
}
override var myStatic = myCompanionSubClass.myStatic
}
fun main() {
println(MyClass.myStatic)
val t = MyClass()
println(t.myStatic)
println(MySubClass.myStatic)
val subClass = MySubClass()
println(subClass.myStatic)
}
In this code you can define a static property and use it from the class or any instance. It is also possible to override the property in a subclass and use it in the same way.
I'm trying to assign a callback implementation of an interface (defined inside a class A) to a variabile defined inside another class B. Let's say that class A has the interface OnSomethingHappens which defines a doSomething method.
Inside class B I've defined my callback variable like this:
private lateinit var callback:A.OnSomethingHappens
I need to create an instance of class A passing callback variabile to the constructor in this way:
myinstanceA = A(callback)
I'm trying to assign an instance of an anonymous class that implements A.OnSomethingHappens using this code:
callback = object : A.OnSomethingHappens {
override fun doSomething(..){
//here I put the implementation of this method
}
}
but the compiler says "expecting member declaration" for my callback variable and "name expected" for object.
What I'm doing wrong?
Instead, I'm able to define and at the same time assign the callback variable in this way:
private var callback = object : A.OnSomethingHappens {
override fun doSomething(..){
//here I put the implementation of this method
}
}
Why? Which are the differences and a possible solution?
I'm trying to assign an instance of an anonymous class that implements A.OnSomethingHappens using this code: ...
This should work, but only inside a method:
class B {
private lateinit var callback:A.OnSomethingHappens
fun someMethod() {
callback = object : A.OnSomethingHappens { ... }
}
...
}
Given the error message and that private var compiles (which doesn't inside a method), you are trying to set it directly in the body of the class instead:
class B {
private lateinit var callback:A.OnSomethingHappens
callback = object : A.OnSomethingHappens { ... }
...
}
This is illegal: the only code you can write there is member definitions and init blocks.
Also, if you can initialize callback directly where it's defined or inside init, there's no point to lateinit in the first place.
It's not obvious from the code snippets cut down to such small pieces, but your issue is that you're writing down the assignment inside the body of a class, but not inside a function.
Here's an example of a valid declaration and immediate assignment:
class A {
var x: X? = X()
}
Here's an example of an invalid assignment, which places an arbitrary expression in the body of a class:
class A {
lateinit var x: X
x = X() // expression placed inside the class body, invalid syntax
someFunction() // extra example, calling functions here is invalid in the same way
}
Instead, you could put this initialization inside a function:
class A {
lateinit var x: X
fun initializeX() {
x = X()
}
}
Or inside an initializer block (in this case, you don't even need lateinit):
class A {
var x: X
init {
x = X()
}
}
While I couldn't explain how to solve your exact problem, because I can't quite understand what code is in which class, I hope these examples and explanation helped.
Hmm, let me propose a variant. It's more simple for me:
import android.util.Log
class SomeClass {
fun mainMethod() {
ClassWithCallback(
{ myBackValue: String ->
logMyString(myBackValue)
}
)
//simplify
ClassWithCallback({ logMyString(it) })
}
private fun logMyString(myBackValue: String) {
Log.d("SomeClass", myBackValue)
}
}
class ClassWithCallback(private val myCallBack: (myBackValue: String) -> Unit) {
init {
// we do something here and back it by callback
val myString = "Hello! Pass me back!"
myCallBack.invoke(myString.toUpperCase())
}
}
Using Kotlin lambdas. Hope this will help you.
I have been reading about properties in Kotlin, including custom getters and setters.
However, I was wondering if it is possible to create a custom getter with extra parameters.
For example, consider the following method in Java:
public String getDisplayedValue(Context context) {
if (PrefUtils.useImperialUnits(context)) {
// return stuff
} else {
// return other stuff
}
}
Note that the static method in PrefUtils has to have Context as a parameter, so removing this is not an option.
I would like to write it like this in Kotlin:
val displayedValue: String
get(context: Context) {
return if (PrefUtils.useImperialUnits(context)) {
// stuff
} else {
// other stuff
}
}
But my IDE highlights all of this in red.
I am aware I can create a function in my class to get the displayed value, but this would mean I would have to use .getDisplayedValue(Context) in Kotlin as well instead of being able to refer to the property by name as in .displayedValue.
Is there a way to create a custom getter like this?
EDIT: If not, would it be best to write a function for this, or to pass Context into the parameters of the class constructor?
As far as I know, property getter cannot have parameter. Write a function instead.
You can do this by having a property that returns an intermediate object that has a get and/or set operator with the parameters that you want, rather than returning the value directly.
Having that intermediate object be an inner class instance may be useful for providing easy access to the parent object. However, in an interface you can't use inner classes so in that case you might need to provide an explicit constructor parameter referencing the parent object when constructing your intermediate object.
For instance:
class MyClass {
inner class Foo {
operator fun get(context: Context): String {
return if (PrefUtils.useImperialUnits(context)) {
// return stuff
} else {
// return other stuff
}
}
}
val displayedValue = Foo()
}
...
val context : Context = whatever
val mc : MyClass = whatever
val y: String = mc.displayedValue[context]
You can do for example:
val displayedValue: String by lazy {
val newString = context.getString(R.string.someString)
newString
}