How to create empty constructor for data class in Kotlin Android

You have 2 options here:

1. 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>()) 

2. 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()

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).