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 lt String gt       var description  String      var user id  List lt Int gt       var status id  Int      var title  String      var created at  String      var data  HashMap lt      gt       var id  Int      var counts  LinkedTreeMap lt      gt      Expected usage  val activity    Activity    activity title    quot New Computer quot  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  quot New Computer quot  null null null null  sendToServer activity

User · Answer

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 .

User · Answer

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 lt String gt    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

User · Answer

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 lt String gt    emptyList        var description  String           var user id  List lt Int gt    emptyList        var status id  Int   -1      var title  String           var created at  String           var data  HashMap lt      gt    hashMapOf lt Any  Any gt         var id  Int   -1      var counts  LinkedTreeMap lt      gt    LinkedTreeMap lt Any  Any gt        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 lt String gt     null      var description  String    null      var user id  List lt Int gt     null      var status id  Int    null      var title  String    null      var created at  String    null      var data  HashMap lt      gt     null      var id  Int    null      var counts  LinkedTreeMap lt      gt     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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

You have 2 options here    Assign a default value to each primary constructor parameter   data class Activity      var updated on  String           var tags  List lt String gt    emptyList        var description  String           var user id  List lt Int gt    emptyList        var status id  Int   -1      var title  String           var created at  String           var data  HashMap lt      gt    hashMapOf lt Any  Any gt         var id  Int   -1      var counts  LinkedTreeMap lt      gt    LinkedTreeMap lt Any  Any gt        Declare a secondary constructor that has no parameters   data class Activity      var updated on  String      var tags  List lt String gt       var description  String      var user id  List lt Int gt       var status id  Int      var title  String      var created at  String      var data  HashMap lt      gt       var id  Int      var counts  LinkedTreeMap lt      gt          constructor     this     emptyList                                  emptyList    -1                                    hashMapOf lt Any  Any gt                               -1  LinkedTreeMap lt Any  Any gt                                    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 lt String gt    emptyList        var description  String           var user id  List lt Int gt    emptyList        var status id  Int   -1      var title  String           var created at  String           var data  HashMap lt      gt    hashMapOf lt Any  Any gt         var id  Int   -1      var counts  LinkedTreeMap lt      gt    LinkedTreeMap lt Any  Any gt        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

User · Answer

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 lt String gt    emptyList        var description  String           var user id  List lt Int gt    emptyList        var status id  Int   -1      var title  String           var created at  String           var data  HashMap lt      gt    hashMapOf lt Any  Any gt         var id  Int   -1      var counts  LinkedTreeMap lt      gt    LinkedTreeMap lt Any  Any gt            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

[android] How to create empty constructor for data class in Kotlin Android

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