Kotlin create object with params syntax - kotlin

I have an object
class Person {
#JsonProperty("name")
var name: String? = null
#JsonProperty("id")
lateinit var id: String}
There is an only empty constructor and I want to create a person so I wrote:
val person = Person()
person.name = "someName"
person.id = "SomeId"
I'm pretty sure that there is a prettier syntax, something like
val person = Person {name = "someName" , id = "someId"}
but I can't find an example.
am I missing something? should I create a secondary constructor to use this syntax or is there another way?


Please check apply method.
Your code will be like this:
val person = Person().apply {name = "someName", id = "someId"}
Another way - you can change declaration of Person to (e.g. just change brackets, replace var to val and remove lateinit):
class Person (#JsonProperty("name") val name: String? = null,
#JsonProperty("id") val id: String )
Then you will able to do this:
val person = Person(name = "someName", id = "someId")


You can achieve it with the constructor parameter.
class Person(
#JsonProperty("name")
var name: String? = null,
#JsonProperty("id")
var id: String
)
val person = Person(name = "someName", id = "someId")
Another way is make your class and desired variables open to be overridden.
open class Person {
#JsonProperty("name")
open var name: String? = null
#JsonProperty("id")
open var id: String = ""
}
val person = object : Person() {
override var name: String? = "SomeName"
override var id = "SomeId"
}

Related

Retrieve Item from realm sort by id

I've a realm class A in my android project -
open class A(
var id: Int? = null,
var level: String? = null,
var state: String? = null,
var updated_at: String? = null,
var created_at: String? = null,
var person: Person? = null
): RealmObject()
and Person -
open class Person(
var id: Int? = null,
var name: String? = null,
var email: String? = null,
var url: String? = null
): RealmObject()
I can retrieve list of items of class A sorted by Class A's id attribute -
fun getItemsOfA() : List<A> {
val listToBeReturn: MutableList<A> = mutableListOf()
DBManager.getRealm().executeTransection { realm ->
val tempList = realm.where(A::class.java).sort("id").findAll()
for (item in tempList) {
listToBeReturn.add(item)
}
And I'm getting sorted list. But I want to have a sorted list by the Person's id attribute instead of A's id. Any insight on this.
How can I achieve these?
Thanks & Regards

How to define a no-argument constructor in Kotlin file with #Parcelize annotation [duplicate]

I have 10+ variables declared in Kotlin data class, and I would like to create an empty constructor for it like how we typically do in Java.
Data class:
data class Activity(
var updated_on: String,
var tags: List<String>,
var description: String,
var user_id: List<Int>,
var status_id: Int,
var title: String,
var created_at: String,
var data: HashMap<*, *>,
var id: Int,
var counts: LinkedTreeMap<*, *>,
)
Expected usage:
val activity = Activity();
activity.title = "New Computer"
sendToServer(activity)
But the data class requires all arguments to be passed while creating a constructor. How can we simplify this like the Java POJO class constructor?
val activity = Activity(null,null,null,null,null,"New Computer",null,null,null,null)
sendToServer(activity)

You have 2 options here:
Assign a default value to each primary constructor parameter:
data class Activity(
var updated_on: String = "",
var tags: List<String> = emptyList(),
var description: String = "",
var user_id: List<Int> = emptyList(),
var status_id: Int = -1,
var title: String = "",
var created_at: String = "",
var data: HashMap<*, *> = hashMapOf<Any, Any>(),
var id: Int = -1,
var counts: LinkedTreeMap<*, *> = LinkedTreeMap<Any, Any>()
)
Declare a secondary constructor that has no parameters:
data class Activity(
var updated_on: String,
var tags: List<String>,
var description: String,
var user_id: List<Int>,
var status_id: Int,
var title: String,
var created_at: String,
var data: HashMap<*, *>,
var id: Int,
var counts: LinkedTreeMap<*, *>
) {
constructor() : this("", emptyList(),
"", emptyList(), -1,
"", "", hashMapOf<Any, Any>(),
-1, LinkedTreeMap<Any, Any>()
)
}
If you don't rely on copy or equals of the Activity class or don't use the autogenerated data class methods at all you could use regular class like so:
class ActivityDto {
var updated_on: String = "",
var tags: List<String> = emptyList(),
var description: String = "",
var user_id: List<Int> = emptyList(),
var status_id: Int = -1,
var title: String = "",
var created_at: String = "",
var data: HashMap<*, *> = hashMapOf<Any, Any>(),
var id: Int = -1,
var counts: LinkedTreeMap<*, *> = LinkedTreeMap<Any, Any>()
}
Not every DTO needs to be a data class and vice versa. In fact in my experience I find data classes to be particularly useful in areas that involve some complex business logic.

If you give default values to all the fields - empty constructor is generated automatically by Kotlin.
data class User(var id: Long = -1,
var uniqueIdentifier: String? = null)
and you can simply call:
val user = User()

the modern answer for this should be using Kotlin's no-arg compiler plugin which creates a non argument construct code for classic apies more about here
simply you have to add the plugin class path in build.gradle project level
dependencies {
....
classpath "org.jetbrains.kotlin:kotlin-noarg:1.4.10"
....
}
then configure your annotation to generate the no-arg constructor
apply plugin: "kotlin-noarg"
noArg {
annotation("your.path.to.annotaion.NoArg")
invokeInitializers = true
}
then define your annotation file NoArg.kt
#Target(AnnotationTarget.CLASS)
#Retention(AnnotationRetention.SOURCE)
annotation class NoArg
finally in any data class you can simply use your own annotation
#NoArg
data class SomeClass( val datafield:Type , ... )
I used to create my own no-arg constructor as the accepted answer , which i got by search but then this plugin released or something and I found it way cleaner .

Along with #miensol answer, let me add some details:
If you want a Java-visible empty constructor using data classes, you need to define it explicitely.
Using default values + constructor specifier is quite easy:
data class Activity(
var updated_on: String = "",
var tags: List<String> = emptyList(),
var description: String = "",
var user_id: List<Int> = emptyList(),
var status_id: Int = -1,
var title: String = "",
var created_at: String = "",
var data: HashMap<*, *> = hashMapOf<Any, Any>(),
var id: Int = -1,
var counts: LinkedTreeMap<*, *> = LinkedTreeMap<Any, Any>()
) {
constructor() : this(title = "") // this constructor is an explicit
// "empty" constructor, as seen by Java.
}
This means that with this trick you can now serialize/deserialize this object with the standard Java serializers (Jackson, Gson etc).

If you give a default value to each primary constructor parameter:
data class Item(var id: String = "",
var title: String = "",
var condition: String = "",
var price: String = "",
var categoryId: String = "",
var make: String = "",
var model: String = "",
var year: String = "",
var bodyStyle: String = "",
var detail: String = "",
var latitude: Double = 0.0,
var longitude: Double = 0.0,
var listImages: List<String> = emptyList(),
var idSeller: String = "")
and from the class where the instances you can call it without arguments or with the arguments that you have that moment
var newItem = Item()
var newItem2 = Item(title = "exampleTitle",
condition = "exampleCondition",
price = "examplePrice",
categoryId = "exampleCategoryId")

Non-empty secondary constructor for data class in Kotlin:
data class ChemicalElement(var name: String,
var symbol: String,
var atomicNumber: Int,
var atomicWeight: Double,
var nobleMetal: Boolean?) {
constructor(): this("Silver", "Ag", 47, 107.8682, true)
}
fun main() {
var chemicalElement = ChemicalElement()
println("RESULT: ${chemicalElement.symbol} means ${chemicalElement.name}")
println(chemicalElement)
}
// RESULT: Ag means Silver
// ChemicalElement(name=Silver, symbol=Ag, atomicNumber=47,
// atomicWeight=107.8682, nobleMetal=true)
Empty secondary constructor for data class in Kotlin:
data class ChemicalElement(var name: String,
var symbol: String,
var atomicNumber: Int,
var atomicWeight: Double,
var nobleMetal: Boolean?) {
constructor(): this("", "", -1, 0.0, null)
}
fun main() {
var chemicalElement = ChemicalElement()
println(chemicalElement)
}
// ChemicalElement(name=, symbol=, atomicNumber=-1,
// atomicWeight=0.0, nobleMetal=null)

From the documentation
NOTE: On the JVM, if all of the parameters of the primary constructor
have default values, the compiler will generate an additional
parameterless constructor which will use the default values. This
makes it easier to use Kotlin with libraries such as Jackson or JPA
that create class instances through parameterless constructors.

I'd suggest to modify the primary constructor and add a default value to each parameter:
data class Activity(
var updated_on: String = "",
var tags: List<String> = emptyList(),
var description: String = "",
var user_id: List<Int> = emptyList(),
var status_id: Int = -1,
var title: String = "",
var created_at: String = "",
var data: HashMap<*, *> = hashMapOf<Any, Any>(),
var id: Int = -1,
var counts: LinkedTreeMap<*, *> = LinkedTreeMap<Any, Any>()
)
You can also make values nullable by adding ? and then you can assing null:
data class Activity(
var updated_on: String? = null,
var tags: List<String>? = null,
var description: String? = null,
var user_id: List<Int>? = null,
var status_id: Int? = null,
var title: String? = null,
var created_at: String? = null,
var data: HashMap<*, *>? = null,
var id: Int? = null,
var counts: LinkedTreeMap<*, *>? = null
)
In general, it is a good practice to avoid nullable objects - write the code in the way that we don't need to use them. Non-nullable objects are one of the advantages of Kotlin compared to Java. Therefore, the first option above is preferable.
Both options will give you the desired result:
val activity = Activity()
activity.title = "New Computer"
sendToServer(activity)

How do I pass data from a variable into a constructor using the body of the variable?

I am trying to assign data to my class using the variable that I created. I want to use the variable as an instance of the membership class. Every time I pass any values I get an error.
error: no value passed for parameter '_number'
var member1 = membership()
error: unresolved reference: _number
member1._number = 1
I am getting this for all properties I try to pass anything to.
I tried to create the class without any constructors but I would still get errors. My goal is to create a list of the variables.
class membership(_number:Int, _name:String, _address:String, _zip:String, _phone:String, _memberSince:String, _memberType:Char)
{
var number: Int = _number;
var name: String = _name;
var address: String = _address;
var zip: String = _zip;
var phone: String = _phone;
var memberSince: String = _memberSince;
var memberType: Char = _memberType;
}
fun main(args: Array<String>) {
var member1 = membership()
member1._number = 1
member1._name = "George Jetson";
member1._address ="123 Main St.";
member1._zip = "99207";
member1._memberSince = "12/01/1997";
member1._memberType = 'L';
}

You have declared the class membership with the primary constructor that expects 7 parameters. Hence, you need to provide values for these parameters when you're instantiating this class:
var member1 = membership(
_number = 1,
_name = "George Jetson",
_address ="123 Main St.",
_zip = "99207",
_memberSince = "12/01/1997",
_memberType = 'L'
)
If you want to create an instance of the class first and then initialize its properties one-by-one, you need it to have a parameterless constructor:
class membership() {
However then you'll have to make all its properties nullable or lateinit because now you cannot provide their initial values upon construction:
class membership() {
var number: Int? = null
var name: String? = null
var address: String? = null
// etc
}
This way you'll be able to initialize them as you want in your question.

Kotlin data class + Gson: optional field

I have the following data class in Kotlin:
import com.google.gson.annotations.SerializedName
data class RouteGroup(
#SerializedName("name") var name: String,
#SerializedName("id") var id: Int
)
Sometimes I need to create an object with both fields, sometimes with only one of them.
How can I do this?
EDIT
This is not the duplicate of this question: Can Kotlin data class have more than one constructor?
That question shows how to set a default value for a field. But in my case, I don't need to serialize the field with the default value. I want a field to be serialized only when I explicitly assign a value to it.

it is easy you have to use the nullable operator
import com.google.gson.annotations.SerializedName
data class RouteGroup #JvmOverloads constructor(
#SerializedName("name") var name: String? = null,
#SerializedName("id") var id: Int? = null
)

You may need something like this:
sealed class RouteGroup
data class RouteGroupWithName(
#SerializedName("name") var name: String
) : RouteGroup()
data class RouteGroupWithId(
#SerializedName("id") var id: Int
) : RouteGroup()
data class RouteGroupWithNameAndId(
#SerializedName("name") var name: String,
#SerializedName("id") var id: Int
) : RouteGroup()
EDIT 1:
Or you can use nullable fields and named parameters like this:
data class RouteGroup(
#SerializedName("name") var name: String? = null,
#SerializedName("id") var id: Int? = null
)
val routeGroupWithName = RouteGroup(name = "example")
val routeGroupWithId = RouteGroup(id = 2)
val routeGroupWithNameAndId = RouteGroup(id = 2, name = "example")

type-safe builder example kotlin

I want to have following person object in Kotlin :
var p = person {
age = 22
gender = "male"
name {
first = "Ali"
last = "Rezaei"
}
}
I have following code to build it :
data class Person(var age: Int? = null, var gender: String? = null
, var name : Name? = null) {
}
fun name(init: Name.() -> Unit): Name {
val n = Name()
n.init()
return n
}
data class Name(var first: String? = null, var last : String? = null)
fun person(init: Person.() -> Unit): Person {
val p = Person()
p.init()
return p
}
But when I print it, the result is following :
Person(age=22, gender="male", name=null)
What is wrong with my code?

You could make name an extension function on Person that assigns the Name to the Person instead of returning it:
fun Person.name(init: Name.() -> Unit) {
val n = Name()
n.init()
this.name = n
}
You could even consider a more concise syntax for the same, like this:
fun Person.name(init: Name.() -> Unit) {
this.name = Name().apply(init)
}
Shameless plug for my repository discussing DSL design and containing examples.

You need to assign to name. This ended up working for me...
var p = person {
age = 22
gender = "male"
name = name {
first = "Ali"
last = "Rezaei"
}
}