I try to loop through my seal class of
sealed class NavigationItem(val route: String, val icon: Int, val title: String, val color: String) {
object Home : NavigationItem("home", R.drawable.ic_home, "Home", "#FFFF00")
object Music : NavigationItem("music", R.drawable.ic_music, "Music", "#FF00FF")
object Movies : NavigationItem("movies", R.drawable.ic_movie, "Movies", "#00FFFF")
object Books : NavigationItem("books", R.drawable.ic_book, "Books", "#FFAAAA")
object Profile : NavigationItem("profile", R.drawable.ic_profile, "Profile", "#AAAAFF")
}
Using the below
NavigationItem::class.sealedSubclasses.forEach {
it.objectInstance?.apply {
// Do something with each of the object (this)
}
}
However, it complaints
kotlin.jvm.KotlinReflectionNotSupportedError: Kotlin reflection implementation is not found at runtime. Make sure you have kotlin-reflect.jar in the classpath
I want to avoid using Reflection to loop through them, is that possible? Or I have to use Enum for that purpose?
Well, you can try with Enum values() or you must import kotlin-reflection library to your project.
Put this into your dependencies.
impolementation org.jetbrains.kotlin:kotlin-reflect:${Versions.kotlin}
And it will work properly.
One problem of this can be the order of sealedSubclasses. But there are many ways to implement this.
one thing I tried is all the Subclasses have index value and then sort it at the end or you can use mapIndexed pipeline or something.
Related
guys, I am learning kotlin. From https://kotlinlang.org/docs/interfaces.html#properties-in-interfaces it says:
Properties declared in interfaces can't have backing fields, and
therefore accessors declared in interfaces can't reference them.
(I think the pronoun "them" at the end of quoted sentence should refer to "properties" rather than "fields". )
However the following code works. It seems that we can refer to properties. Why is print(prop) highlighted as red then?
interface MyInterface {
val prop: Int // abstract
val propertyWithImplementation: String
get() = "foo"
fun foo() {
print(prop) // this is highlighted red but it works. what's does the author want to say?
}
}
class Child : MyInterface {
override val prop: Int = 29
}
fun main() {
val c = Child()
c.foo()
}
Besides, I noticed that in the above example foo is not accessor. So I tried following example and it works too:
interface User {
val email: String
val nickname: String
get() = email.substringBefore('#') // aren't we referring to a property in accessor? why does this work then?
}
So what does the author want to say in here? what does "them" refer to?
"Them" in this sentence means "fields".
Property is basically a getter (setter) and it could be optionally backed by a field. For technical reasons interfaces can't hold fields, so properties in interfaces have to be "fieldless". Property has to be either abstract or its implementation can only use e.g. other properties/functions, but it can't store/read any data directly. Note that referencing other properties does not break above rule, because, as I said, property is mainly a getter/setter, not a field.
print(prop) is highlighted as red, because... well, this is how automatic highlighter colored it... :-)
So I have a class with a generic type
class GenericClass<T> {
// At some point in the class I have variable item of type T
val name: String = item.name
}
I know for sure that the type T of GenericClass will be used with a class that has the "name" property. But of course at the line I got a "Unresolved reference name". Android Studio generated me this code via "Create extension property T.name"
private val <T> T.name: String
get() {}
I don't really know what to put in the bracket {} after the get. I tried return name but I got a recursive property error.
Any ideas ?
Thanks
If you know that every type T has property name you can declare it implicitly:
// GenericClass.kt
class GenericClass<T : HasName> {
// At some point in the class I have variable item of type T
val name: String = item.name
}
// HasName.kt
// Create new interface with "name" property
interface HasName {
val name: String
}
But also you must implement this new interface for all classes that can be used as T.
I know for sure that the type T of GenericClass will be used with a class that has the "name" property.
Then you need to explicitly declare that. By default, T extends Any?. You need to narrow down possible types of T by declaring some interface, like
interface Named {
val name : String
}
and passing T : Named as a generic paramteter. Also you need to make all classes, you're going to pass as a generic parameter, to implement that interface. By the way, GenericClass<T : Named> class itself could be declared as implementing that interface:
class GenericClass<T : Named> : Named {
override val name: String = item.name
}
I have a Java class that holds generic information on databse entities (i.e. their id).
#Data
public class DbEntity {
protected final String id;
public DbEntity(String id) {
this.id = id;
}
}
We use Lombok #Data to generate getters, toString, equals...
In Java I would simply extend this class and add #Data once again.
#Data
class JavaSubClass extends DbEntity {
public JavaSubClass(String id) {
super(id);
}
}
In a newer service we use Kotlin but would like to reuse standard classes such as DbEntity.
My first approach was to simply declare a data class such as
data class SubClass1(val id: String, val name: String) : DbEntity(id)
Accidental override: The following declarations have the same JVM signature (getId()Ljava/lang/String;):
fun <get-id>(): String defined in com.demo.SubClass1
fun getId(): String! defined in com.demo.SubClass1
After some reading I found several solutions, all of which I'm not super happy with.
Don't use data classes. This works but leaves me with the task of implementing equals etc.
class SubClass4(id: String, val name: String) : DbEntity(id)
Duplicate the field. This works but we end up with two fields that could go out of sync.
data class SubClass3(val subId: String, val name: String) : DbEntity(subId)
Assign a different name to the getter. This fundamentally also duplicates the field, but hides the getter.
data class SubClass2(#get:JvmName("getId_") val id: String, val name: String) : DbEntity(id)
As I said, I'm not happy with any of the solution presented above. Having an abstract super class or an interface instead would certainly be more appropriate. However the Entity class resides in a library that primarily Java projects depend on. I'm hesitant to change it just because of a new Kotlin dependnecy.
Did anyone encounter similar issues and has advice on how to solve them?
As a workaround, until KT-6653 - Kotlin properties do not override Java-style getters and setters is fixed, I would go for a variant of your point 3, i.e.:
data class SubClass(#get:JvmName("bogusId") private val id: String, val name: String) : DbEntity(id)
The benefit of this variant is, that you always access the "original" getId-function. You will not use the bogusId()-function as it is not visible/accessible (accessing it via reflection makes no sense... you are only interested in the actual id-field). This works and looks similar for both sides: from Java as also from Kotlin. Still, under the hood this variant uses 2 fields, but in the best case you can just replace it in future with something like:
data class SubClass(override val id: String, val name : String) : DbEntity(id)
I have two library classes, Item and ItemFood : Item (that is derived from Item), and a library function registerItem(item: Item, name: String). I cannot modify them.
I have two of my own classes (ItemKey : Item and ItemBerry : ItemFood) that are derived from the library classes.
What I want is to store the name: String property in my classes ItemKey and ItemBerry and make them "count" as a NamedItem, so I can write a function like so:
fun registerNamedItem(namedItem: NamedItem) {
registerItem(namedItem, namedItem.name)
}
I cannot just make a class like so: class NamedItem(val name: String) : Item and derive my classes from it, because sometimes I need to derive my classes from ItemFood, not from Item.
I don't want to make a class wrapper like class NamedItem(val item: Item, val name: String), because then every time I want to get the "underlying" Item I will need to manually get the item property: registerItem(namedItem.item, namedItem.name), and this is ugly.
I cannot use an interface INamedItem { val name: String } and implement this interface in ItemKey and ItemBerry, because then I will need to write a function in this way:
fun registerNamedItem(item: Item, namedItem: INamedItem) {
registerItem(item, namedItem.name)
}
, and it is not an improvement at all.
Is there some kind of advanced technique - using an interface, delegation, generic, whatever - so I can implement the registerNamedItem function like I want it - passing to the registerItem(item: Item, name: String) an instance of the NamedItem as the first parameter and the namedItem.name as the second parameter?
Actually, you could use interface just for that:
fun main() {
registerNamedItem(ItemKey("item_key"))
registerNamedItem(ItemBerry("item_berry"))
}
// Cannot change this
open class Item
// Cannot change this
open class ItemFood : Item()
// This is your class
class ItemKey(override val name: String) : NamedItem()
// This is also your class
class ItemBerry(override val name: String) : NamedFoodItem()
// This is the property you would like to enforce
interface INamedItem {
val name: String
}
// Since Item and ItemFood are concrete classes, you don't have much choice there
abstract class NamedItem : Item(), INamedItem
abstract class NamedFoodItem : ItemFood(), INamedItem
// Adapter pattern
fun registerNamedItem(namedItem: NamedFoodItem) {
registerItem(namedItem, namedItem.name)
}
// Adapter pattern
fun registerNamedItem(namedItem: NamedItem) {
registerItem(namedItem, namedItem.name)
}
fun registerItem(namedItem: Item, name: String) {
println("Item $namedItem registered with $name")
}
Delegation won't work in your case, since from your example, Item is a class, and you can only delegate to interfaces.
What is the intended meaning of "companion object"? So far I have been using it just to replace Java's static when I need it.
I am confused with:
Why is it called "companion"?
Does it mean that to create multiple static properties, I have to group it together inside companion object block?
To instantly create a singleton instance that is scoped to a class, I often write
:
companion object {
val singleton by lazy { ... }
}
which seems like an unidiomatic way of doing it. What's the better way?
What is the intended meaning of "companion object"? Why is it called "companion"?
First, Kotlin doesn't use the Java concept of static members because Kotlin has its own concept of objects for describing properties and functions connected with singleton state, and Java static part of a class can be elegantly expressed in terms of singleton: it's a singleton object that can be called by the class' name. Hence the naming: it's an object that comes with a class.
Its name used to be class object and default object, but then it got renamed to companion object which is more clear and is also consistent with Scala companion objects.
Apart from naming, it is more powerful than Java static members: it can extend classes and interfaces, and you can reference and pass it around just like other objects.
Does it mean that to create multiple static properties, I have to group it together inside companion object block?
Yes, that's the idiomatic way. Or you can even group them in non-companion objects by their meaning:
class MyClass {
object IO {
fun makeSomethingWithIO() { /* ... */ }
}
object Factory {
fun createSomething() { /* ... */ }
}
}
To instantly create a singleton instance that is scoped to a class, I often write /*...*/ which seems like an unidiomatic way of doing it. What's the better way?
It depends on what you need in each particular case. Your code suits well for storing state bound to a class which is initialized upon the first call to it.
If you don't need it to be connected with a class, just use object declaration:
object Foo {
val something by lazy { ... }
}
You can also remove lazy { ... } delegation to make the property initialize on first class' usage, just like Java static initializers
You might also find useful ways of initializing singleton state.
Why is it called "companion"?
This object is a companion of the instances.
IIRC there was lengthy discussion here: upcoming-change-class-objects-rethought
Does it mean that to create multiple static properties, I have to group it together inside companion object block?
Yes. Every "static" property/method needs to be placed inside this companion.
To instantly create a singleton instance that is scoped to a class, I often write
You do not create the singleton instance instantly. It is created when accessing singleton for the first time.
which seems like an unidiomatic way of doing it. What's the better way?
Rather go with object Singleton { } to define a singleton-class. See: Object Declarations
You do not have to create an instance of Singleton, just use it like that Singleton.doWork()
Just keep in mind that Kotlin offers other stuff to organize your code. There are now alternatives to simple static functions e.g. you could use Top-Level-Functions instead.
When the classes/objects with related functionalities belong together, they are like companions of each other. A companion means a partner or an associate, in this case.
Reasons for companionship
Cleaner top-level namespace
When some independent function is intended to be used with some specific class only, instead of defining it as a top-level function, we define it in that particular class. This prevents the pollution of top-level namespace and helps with more relevant auto-completion hints by IDE.
Packaging convenience
It's convenient to keep the classes/objects together when they are closely related to each other in terms of the functionality they offer to each other. We save the effort of keeping them in different files and tracking the association between them.
Code readability
Just by looking at the companionship, you get to know that this object provides helper functionality to the outer class and may not be used in any other contexts. Because if it was to be used with other classes, it would be a separate top level class or object or function.
Primary purpose of companion object
Problem: companion class
Let's have a look at the kinds of problems the companion objects solve. We'll take a simple real world example. Say we have a class User to represent a user in our app:
data class User(val id: String, val name: String)
And an interface for the data access object UserDao to add or remove the User from the database:
interface UserDao {
fun add(user: User)
fun remove(id: String)
}
Now since the functionalities of the User and implementation of the UserDao are logically related to each other, we may decide to group them together:
data class User(val id: String, val name: String) {
class UserAccess : UserDao {
override fun add(user: User) { }
override fun remove(id: String) { }
}
}
Usage:
fun main() {
val john = User("34", "John")
val userAccess = User.UserAccess()
userAccess.add(john)
}
While this is a good setup, there are several problems in it:
We have an extra step of creating the UserAccess object before we can add/remove a User.
Multiple instances of the UserAccess can be created which we don't want. We just want one data access object (singleton) for User in the entire application.
There is a possibility of the UserAccess class to be used with or extended with other classes. So, it doesn't make our intent clear of exactly what we want to do.
The naming userAccess.add() or userAccess.addUser() doesn't seem very elegant. We would prefer something like User.add().
Solution: companion object
In the User class, we just replace the two words class UserAccess with the two other words companion object and it's done! All the problems mentioned above have been solved suddenly:
data class User(val id: String, val name: String) {
companion object : UserDao {
override fun add(user: User) { }
override fun remove(id: String) { }
}
}
Usage:
fun main() {
val john = User("34", "John")
User.add(john)
}
The ability to extend interfaces and classes is one of the features that sets the companion objects apart from Java's static functionality. Also, companions are objects, we can pass them around to the functions and assign them to variables just like all the other objects in Kotlin. We can pass them to the functions that accept those interfaces and classes and take advantage of the polymorphism.
companion object for compile-time const
When the compile-time constants are closely associated with the class, they can be defined inside the companion object.
data class User(val id: String, val name: String) {
companion object {
const val DEFAULT_NAME = "Guest"
const val MIN_AGE = 16
}
}
This is the kind of grouping you have mentioned in the question. This way we prevent the top-level namespace from polluting with the unrelated constants.
companion object with lazy { }
The lazy { } construct is not necessary to get a singleton. A companion object is by default a singleton, the object is initialized only once and it is thread safe. It is initialized when its corresponding class is loaded. Use lazy { } when you want to defer the initialization of the member of the companion object or when you have multiple members that you want to be initialized only on their first use, one by one:
data class User(val id: Long, val name: String) {
companion object {
val list by lazy {
print("Fetching user list...")
listOf("John", "Jane")
}
val settings by lazy {
print("Fetching settings...")
mapOf("Dark Theme" to "On", "Auto Backup" to "On")
}
}
}
In this code, fetching the list and settings are costly operations. So, we use lazy { } construct to initialize them only when they are actually required and first called, not all at once.
Usage:
fun main() {
println(User.list) // Fetching user list...[John, Jane]
println(User.list) // [John, Jane]
println(User.settings) // Fetching settings...{Dark Theme=On, Auto Backup=On}
println(User.settings) // {Dark Theme=On, Auto Backup=On}
}
The fetching statements will be executed only on the first use.
companion object for factory functions
Companion objects are used for defining factory functions while keeping the constructor private. For example, the newInstance() factory function in the following snippet creates a user by generating the id automatically:
class User private constructor(val id: Long, val name: String) {
companion object {
private var currentId = 0L;
fun newInstance(name: String) = User(currentId++, name)
}
}
Usage:
val john = User.newInstance("John")
Notice how the constructor is kept private but the companion object has access to the constructor. This is useful when you want to provide multiple ways to create an object where the object construction process is complex.
In the code above, consistency of the next id generation is guaranteed because a companion object is a singleton, only one object will keep track of the id, there won't be any duplicate ids.
Also notice that companion objects can have properties (currentId in this case) to represent state.
companion object extension
Companion objects cannot be inherited but we can use extension functions to enhance their functionality:
fun User.Companion.isLoggedIn(id: String): Boolean { }
The default class name of the companion object is Companion, if you don't specify it.
Usage:
if (User.isLoggedIn("34")) { allowContent() }
This is useful for extending the functionality of the companion objects of third party library classes. Another advantage over Java's static members.
When to avoid companion object
Somewhat related members
When the functions/properties are not closely related but only somewhat related to a class, it is recommended that you use top-level functions/properties instead of companion object. And preferably define those functions before the class declaration in the same file as that of class:
fun getAllUsers() { }
fun getProfileFor(userId: String) { }
data class User(val id: String, val name: String)
Maintain single responsibility principle
When the functionality of the object is complicated or when the classes are big, you may want to separate them into individual classes. For example, You may need a separate class to represent a User and another class UserDao for database operations. A separate UserCredentials class for functions related to login. When you have a huge list of constants that are used in different places, you may want to group them in another separate class or file UserConstants. A different class UserSettings to represent settings. Yet another class UserFactory to create different instances of the User and so on.
That's it! Hope that helps make your code more idiomatic to Kotlin.
Why is it called "companion"?
An object declaration inside a class can be marked with the companion keyword:
class MyClass {
companion object Factory {
fun create(): MyClass = MyClass()
}
}
Members of the companion object can be called by using simply the class name as the qualifier:
val instance = MyClass.create()
If you only use 'object' without 'companion', you have to do like this:
val instance = MyClass.Factory.create()
In my understanding, 'companion' means this object is companion with the outter class.
We can say that companion is same as "Static Block" like Java, But in case of Kotlin there is no Static Block concept, than companion comes into the frame.
How to define a companion block:
class Example {
companion object {
fun display(){
//place your code
}
}
}
Calling method of companion block, direct with class name
Example.Companion.display