Right code:
class MainActHandler(val weakActivity: WeakReference<Activity>): Handler() {
override fun handleMessage(msg: Message?) {
val trueAct = weakActivity.get() ?: return
if (msg?.what == ConversationMgr.MSG_WHAT_NEW_SENTENCE){
val sentence = msg.obj as String?
trueAct.conversation.text = sentence
}
super.handleMessage(msg)
}
}
cannot be resolved code:
class MainActHandler(weakActivity: WeakReference<Activity>): Handler() {
override fun handleMessage(msg: Message?) {
val trueAct = weakActivity.get() ?: return
if (msg?.what == ConversationMgr.MSG_WHAT_NEW_SENTENCE){
val sentence = msg.obj as String?
trueAct.conversation.text = sentence
}
super.handleMessage(msg)
}
}
cannot be resolved code screenshot
The only difference is the "val" has been deleted and cannot be resolve.
Which might be important is that it's a inner class.
BUT
This one class without "val/var" in constructor parameter is working:
class BookInfo(convrMgr: ConversationMgr, id: String, queue: RequestQueue, queueTag:String) {
val TAG = "BookInfo"
var title: String? = ""
init {
val url = "https://api.douban.com/v2/book/$id"
// Request a string response from the provided URL.
val stringRequest = StringRequest(Request.Method.GET, url,
Response.Listener<String> { response ->
Log.d(TAG + " Response", response.substring(0))
// Parse JSON from String value
val parser = Parser()
val jsonObj: JsonObject =
parser.parse(StringBuilder(response.substring(0))) as JsonObject
// Initial book title of book properties.
title = jsonObj.string("title")
Log.d(TAG + " Book title", title)
convrMgr.addNewMsg(title)
},
Response.ErrorListener { error -> Log.e(TAG + " Error", error.toString()) })
// Set the tag on the request.
stringRequest.tag = queueTag
// Add the request to the RequestQueue.
queue.add(stringRequest)
}
}
And if I add var/val before "queue: RequestQueue", I'll get suggestion:
"Constructor parameter is never used as a property less. This inspection reports primary constructor parameters that can have 'val' or 'var' removed. Unnecessary usage of 'val' and 'var' in primary constructor consumes unnecessary memory."
I am just confused about it.
When you write val/var within the constructor, it declares a property inside the class. When you do not write it, it is simply a parameter passed to the primary constructor, where you can access the parameters within the init block or use it to initialize other properties. For example,
class User(val id: Long, email: String) {
val hasEmail = email.isNotBlank() //email can be accessed here
init {
//email can be accessed here
}
fun getEmail(){
//email can't be accessed here
}
}
Constructor parameter is never used as a property
This suggestion is saying that you do not use this property in place apart from the initialization. So, it suggests you to remove this property from the class.
Constructor parameters must use var or val when they are used as a property elsewhere in the class. They do not need to be properties if they are only used for class initialization.
In the example below, the parameter must be a property (var or val) because it is used in a method:
class A(val number: Int) {
fun foo() = number
}
In this other example, the parameter is only used to initialize the class, so it does not need to be a property:
class B(number: Int): A(number) {
init {
System.out.println("number: $number")
}
}
This might be a late answer but the magic lies under the hood:
Based on #BakaWaii's answer:
Putting var/val will make the variable a property of the class and not putting it will make it a parameter of only the constructor function.
So what does it mean, to understand lets look into some code:
class Test(a: Int){}
Now Lets see the decompiled java code:
public final class Test {
public Test(int a) {
}
}
So now if I try to access a using the object of Test() like the below code:
Test t = new Test(10);
t.a //Error
It will give me error. Unresolved reference: a. Why because a is a parameter of the constructor only.
Now if we put var/val in the paramater like below:
class Test(var a: Int){}
The decompliked Java code will become:
public final class Test {
private int a;
public final int getA() {
return this.a;
}
public final void setA(int var1) {
this.a = var1;
}
public Test(int a) {
this.a = a;
}
}
Thus it will not only give you a class property but also give you getter/setters for setting the values.
Now the next question arises if the field a is private how can it be accessed. Simple answer in Java you cannot, i.e. if you are calling the KT class from a Java you will not be able to assign value of a like Test(1).a = 10 but will have to use Test(1).setA(5).
But as kotlin internally handles getters/setters Test(1).a = 5 will be ok.
For #Parcelize to work you need to open up the super's properties and override them in the child:
abstract class Goal(open var number: Int, open var name: String) : Parcelable
#Parcelize
class OperationalGoal(override var number: Int, override var name: String, var description: String) : Goal(number, name)```
In very simple terms, use var or val in class constructor parameters when you want to use that variable, say, inside a method within that class. Thus you're effectively turning them into properties and not just mere constructor or method parameters.
class User(var name: String, age: Int) {
var str = "John"
var num = 18
fun setName(){
name = str // due to using var on our class constructor parameter, we can access the constructor variable *name* inside this setter method. *name* is a property parameter thanks to the var keyword.
}
fun setAge(){
age = num // this will result in a compiler error, because *age* is just a parameter, notice that var wasn't used in the *age* parameter within the class constructor, which means we can't access it like we did with *name*
}
}
Run this Kotlin Playground code to get a clearer idea of what's going on.
Related
I want to reference an object within this class I have below:
class HerbData {
object Dill {
const val herbName: String = "This is Dill!"
const val scientificName: String = "Anethum Graveolens"
val dullThumbnail: Int = R.drawable.dill_thumbnail_attr
}
object Peppermint {
val herbName: String = "This is Peppermint!"
}
}
Is there anyway that I can reference the object by using a string in Kotlin? Here is somewhat what I mean:
HerbData."Dill".herbname
I can't find anything on this topic for Kotlin.
Another way you could do this is with an enum class. The advantage over a map is that you have a data structure you can reference directly in code, so you could use HerbData.Dill as well as HerbData["Dill"]. And that will enable you to take advantage of compile-time checking and lint warnings, refactoring, exhaustive pattern matching, code completion etc, because the data is defined in your code
enum class HerbData(
val herbName: String,
val scientificName: String? = null,
val dullThumbnail: Int? = null
) {
Dill("This is Dill!", "Anethum Graveolens", R.drawable.dill_thumbnail_attr),
Peppermint("This is Peppermint!");
companion object {
operator fun get(name: String): HerbData? =
try { valueOf(name) } catch(e: IllegalArgumentException) { null }
}
}
fun main() {
// no guarantee these lookups exist, need to null-check them
HerbData["Peppermint"]?.herbName.run(::println)
// case-sensitive so this fails
HerbData["peppermint"]?.herbName.run(::println)
// this name is defined in the type system though! No checking required
HerbData.Peppermint.herbName.run(::println)
}
>> This is Peppermint!
null
This is Peppermint!
Enum classes have that valueOf(String) method that lets you look up a constant by name, but it throws an exception if nothing matches. I added it as a get operator function on the class, so you can use the typical getter access like a map (e.g. HerbData["Dill"]). As an alternative, you could do something a bit neater:
companion object {
// storing all the enum constants for lookups
private val values = values()
operator fun get(name: String): HerbData? =
values.find() { it.name.equals(name, ignoreCase = true) }
}
You could tweak the efficiency on this (I'm just storing the result of values() since that call creates a new array each time) but it's pretty simple - you're just storing all the enum entries and creating a lookup based on the name. That lets you be a little smarter if you need to, like making the lookup case-insensitive (which may or may not be a good thing, depending on why you're doing this)
The advantage here is that you're generating the lookup automatically - if you ever refactor the name of an enum constant, the string label will always match it (which you can get from the enum constant itself using its name property). Any "Dill" strings in your code will stay as "Dill" of course - that's the limitation of using hardcoded string lookups
The question really is, why do you want to do this? If it's pure data where no items need to be explicitly referenced in code, and it's all looked up at runtime, you should probably use a data class and a map, or something along those lines. If you do need to reference them as objects within the code at compile time (and trying to use HerbData."Dill".herbName implies you do) then an enum is a fairly easy way to let you do both
Declare a Data Class
data class HerbData (
val scientificName: String,
val dullThumbnail: Int
)
Initialize a muteable map and put data in it
val herbData = mutableMapOf<String, HerbData>()
herbData.put("Dill", HerbData("Anethum Graveolens", R.drawable.dill_thumbnail_attr))
herbData.put("Peppermint", HerbData("Mentha piperita", R.drawable.peppermint_thumbnail_attr))
You can now just
herbData["Dill"]?.scientificName
class HerbData {
interface Herb {
val herbName: String
val scientificName: String
}
object Dill : Herb {
override val herbName: String = "This is Dill!"
override val scientificName: String = "Anethum Graveolens"
}
object Peppermint: Herb {
override val herbName: String = "This is Peppermint!"
override val scientificName: String = "Mentha piperita"
}
companion object {
operator fun get(name: String): Herb? {
return HerbData::class
.nestedClasses
.find { it.simpleName == name }
?.objectInstance as? Herb
}
}
}
println(HerbData["Dill"]?.herbName) // Prints: This is Dill!
println(HerbData["Peppermint"]?.scientificName) // Prints: Mentha piperita
println(HerbData["Pepper"]?.herbName) // Prints: null
I have a function filter here
fun filter(category: String) {
...
}
and a Class with many constant string
object Constants {
val CAT_SPORT = "CAT_SPORT"
val CAT_CAR = "CAT_CAR"
...
}
How to ensure the parameter category is a constant string from Constants (or throw warning)?
I am looking for something like #StringRes.
I know Enum may do the trick but prefer not to code refactor at this moment.
Using androidx.annotation you can do something like this:
object Constants {
#Retention(AnnotationRetention.SOURCE)
#StringDef(CAT_SPORT, CAT_CAR)
annotation class Category
const val CAT_SPORT = "CAT_SPORT"
const val CAT_CAR = "CAT_CAR"
}
fun filter(#Constants.Category category: String) {
...
}
I need to create a list of events with a string as name and a list of pairs as properties, some events properties are the static value, some need dynamically changed, so I create specific functions inside the enum entry to update it but complied with error unresolved reference:
Actually, what I want to implement is a list of enums with associated values,
something like these articles mentioned:
KT-4075 Allow setters overloading for properties, or
Kotlin: single property with multiple setters of different types, or
Using Kotlin’s sealed class to approximate Swift’s enum with associated data
Because I have more than 100 events, 95% of them are static, only several of them need to be updated during runtime, so sealed class might not suit my situation:
enum class Event(val eventName: String, vararg eventProperties: Pair<String, String?>) {
LOGIN_CLICKED("Login", ("View" to "button clicked")),
LOGIN_SUCCEED("Login", ("Type" to "succeed")),
LOGIN_ERROR("Login") {
fun errorMessage(errorMessage: String) {
eventProperties = listOf("ErrorType" to errorMessage)
}
},
// ... some other events
LIST_ITEM_CLICKED("LIST") {
fun listItemName(itemName: String) {
eventProperties = listOf("View" to itemName)
}
};
var eventProperties: List<Pair<String, String?>>? = listOf(*eventProperties)
// Although this approach can fix my problem, but I don't prefer it,
// because these functions are only meaningful to specific enum item,
// I don't want them be opened to all enum items.
//
// fun errorMessage(errorMessage: String) {
// eventProperties = listOf("ErrorType" to errorMessage)
// }
// fun listItemName(itemName: String) {
// eventProperties = listOf("View" to itemName)
// }
}
fun main(args: Array<String>) {
// unresolved reference
println(Event.LOGIN_ERROR.eventProperties)
Event.LOGIN_ERROR.errorMessage("error password")
println(Event.LOGIN_ERROR.eventProperties)
}
Because I have more than 100 events, 95% of them are static, only several of them need to be updated during runtime, so sealed class might not suit my situation
Why wouldn't it? If you are bothered with slightly longer declarations:
object LoginClicked : Event("Login", mapOf("View" to "button clicked"))
\\ vs
LOGIN_CLICKED("Login", mapOf("View" to "button clicked"))
you can create a helper enum class for them:
sealed class Event(val eventName: String, val eventProperties: Map<String, String?>) {
enum class Basic(val eventName: String, val eventProperties: Map<String, String?>) {
LOGIN_CLICKED("Login", mapOf("View" to "button clicked")),
LOGIN_SUCCEED("Login", mapOf("Type" to "succeed")),
...
}
class BasicEvent(b: Basic) : Event(b.eventName, b.eventProperties)
class LoginError(errorMessage: String) : Event("Login", mapOf("ErrorType" to errorMessage))
...
}
I want to implement a class with two constrcutors. And empty constructor and another with a param user:FirebaseUser
But i'm getting the message error:
"There is a cycle in the delegation change"
class UserModel(user: FirebaseUser) {
var uid: String?
val email: String?
val phoneNumber: String?
val photoUrl: String
val displayName: String?
//error message: There is a cycle in the delegation change
constructor():this() {}
init {
this.displayName = user.displayName
this.email = user.email
this.phoneNumber = user.phoneNumber
this.photoUrl = user.photoUrl!!.toString()
this.uid = user.uid
}
companion object {
#Exclude
val CURRENT_LOCATION = "location"
}
}
I've tried several approaches without success. Any help?
All the secondary constructors have to call the primary constructor, either directly or indirectly. Which means:
class X(var x: Int){
constructor() : this(0.0);
constructor(x: Double) : this(x.toInt());
}
However, you can't do this:
class X(var x: Int){
constructor() : this();
constructor(x: Double) : this();
}
Because it would result in a Stack Overflow Exception.
The above example is horrible code, but it's just as a demo.
So with this line:
constructor():this() {}
You make the secondary constructor call itself.
Instead, call the primary constructor. This means you need to pass a FirebaseUser as an argument. I'm not familiar with Firebase, so I'll leave that to you.
But as an example, you basically need to do this instead:
constructor() : this(FirebaseUser());
Either initialize directly, or get it from a method. If you can't get one, you could of course just make it nullable.
But if you're dealing with nullables, assuming the majority of your code is in Kotlin, you can just make it nullable with a default value and remove the secondary constructor:
class UserModel(user: FirebaseUser? = null){
init{
// Now that `user` is nullable, you need to change the assignments to include null-safe or non-null assertion (?. or !!. respectively)
}
}
You have to call into the primary constructor from every secondary constructor you have, since its parameters may be used in property initializers and initializer blocks, like you've used user in the init block in your code.
With this code, the secondary constructor just recursively calls itself:
constructor() : this() {}
What you should do instead is call into the primary constructor so that the class' properties can be initialized:
constructor() : this(FirebaseUser()) {} // get FirebaseUser from somewhere
Alternatively, if what you meant to do is leave everything null when the secondary no-param constructor is called, you could opt for something like this:
class UserModel(user: FirebaseUser?) {
var uid: String? = user?.uid
val email: String? = user?.email
val phoneNumber: String? = user?.phoneNumber
val photoUrl: String? = user?.photoUrl?.toString()
val displayName: String? = user?.displayName
constructor() : this(null)
}
As i have one User class having 2 parameters : first_name, last_name. So my kotlin class with be :
data class User(val first_name:String, val last_name:String)
Now i want a constructor which will accept only first_name, or you can say just one parameter. How can i define it with Kotlin?
I know we can pass default value and in that way we can ignore second parameter, but how can we write multiple constructor?
You can define extra constructors in the class body
data class User(val firstName: String, val lastName: String) {
constructor(firstName: String) : this(firstName, "")
}
These 'secondary constructors' have to call through to the primary constructor or a different secondary constructor. See the Official documentation on constructors.
So, in effect this is the same as just a primary constructor with default argument, which would be the idiomatic way to go.
data class User(val firstName: String, val lastName: String = "")
I hope this will help you
class Person(val name: String, val age: Int = 0) {
override fun toString(): String {
return name + " is " + age + " years Old"
}
}
fun main(args: Array<String>) {
var person = Person(name = "vignesh")
var personNew = Person("vignesh", 23)
println(person.toString())
println(personNew.toString())
}
Output
vignesh is 0 years Old
vignesh is 23 years Old
If you are using data class, then you won't require another constructor. Just pass default value to your last_name parameter.
If you are using a normal class then you can have secondary constructor
Lets say you have class A
class A(val param:String,val param2:String){
constructor(val param:String):this(param,"")
}
If you wish manipulate these values you can use init{} block where you can play around your constructor values.
I hope this will help.
A class in Kotlin can have a primary constructor and one or more secondary constructors. The primary constructor is part of the class header: it goes after the class name (and optional type parameters).
class Person constructor(firstName: String) {
}
If the primary constructor does not have any annotations or visibility modifiers, the constructor keyword can be omitted:
class Person(firstName: String) {
}
Note that parameters of the primary constructor can be used in the initializer blocks. They can also be used in property initializers declared in the class body:
class Customer(name: String) {
val customerKey = name.toUpperCase()
}
You can also follow this link as per your need : Kotlin
This sample of code works fine for me, you can customize them to your need.
data class Booking(
var user: String,
var bike: String
){
constructor(
user: String,
bike: String,
taken_at: String,
returned_at: String
) : this (user, bike)
}