How to create empty constructor for data class in Kotlin Android

Question

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)

score 301 · Accepted Answer · edited May 22 '17 at 18:09

301

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.

edited May 22 '17 at 18:09

Willi Mentzel

21,499
16
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101

answered Jun 17 '16 at 06:33

miensol

32,791
6
103
105

1

Thanks @miensol, Is there any way this can be done using copy fun. eg. https://kotlinlang.org/docs/reference/data-classes.html#copying – Sai Jun 17 '16 at 06:46
@SaiKiran to use `copy` you need a data class instance. To create it you need to invoke a constructor - and here was the issue. – miensol Jun 17 '16 at 06:58
I'm using Kotlin 1.1.2 for Android Studio 2.3 and emptyList is not available :/ – Gonzalo May 29 '17 at 20:58
Nevermind. I didn't add kotlin to my build.gradle config file. – Gonzalo May 30 '17 at 22:02
Using emptyList will allocate memory? . I things it will be increase the memory usage – Muhammad chhota Oct 05 '17 at 05:57
3

@Muhammadchhota `emptyList` will not repeatedly allocate memory. [It returns a singleton](https://github.com/JetBrains/kotlin/blob/1.1.3/libraries/stdlib/src/kotlin/collections/Collections.kt#L77). – miensol Oct 05 '17 at 08:43
How do I create a setter for this model? – Farwa Apr 23 '18 at 11:51
@Farwa what do you mean by setter? The properties above are declared as `var` meaning mutable. – miensol Apr 23 '18 at 12:00
I mean how can I assign values to those programmatically? – Farwa Apr 23 '18 at 12:03

score 85 · Answer 2 · answered Jan 15 '18 at 14:29

85

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()

answered Jan 15 '18 at 14:29

kosiara - Bartosz Kosarzycki

8,153
5
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1

if id is auto-generated then how to use? – Panchal Amit Nov 13 '19 at 12:06
1

Worked for me. For Firebase Chat message : `class FeelComChatMessage (messageText: String = "", messageUser: String = "")` – Jitendra Surve Jun 09 '20 at 16:53
@Panchal Amit Who auto-generates id? Room? You should study it's annotations I think. – The incredible Jan May 28 '21 at 10:57

score 15 · Answer 3 · answered Nov 28 '17 at 11:32

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).

The last commend is wrong. At least for the Gson serializer, in fact, Gson uses the unsafe mechanism to create objects and it will not call your constructor. I have just answered a related question here https://stackoverflow.com/questions/59390294/what-should-i-do-if-i-dont-want-a-devired-class-call-base-classs-constructor-i/59393841#59393841 — Võ Quang Hòa, Dec 18 '19 at 14:37

score 12 · Answer 4 · answered Sep 13 '20 at 17:55

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 .

score 8 · Answer 5 · answered Jun 01 '18 at 01:51

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")

score 6 · Answer 6 · answered Jul 05 '19 at 23:56

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.

score 4 · Answer 7 · answered Apr 13 '19 at 16:25

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)

score 2 · Answer 8 · edited Jan 27 '19 at 18:36

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)

How to create empty constructor for data class in Kotlin Android

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