Kotlin Data Class Inheritance: Extend a Data Class

1. Use the data Keyword to Define Class in Kotlin
2. Requirements to Declare a Data Class in Kotlin
3. Example of Data Class in Kotlin
4. Extend Data Class in Kotlin
5. Use the Abstract Class to Extend Data Class in Kotlin
6. Use the Interface to Extend Data Class in Kotlin
7. Use the Open Classes to Extend Data Class in Kotlin

The data class in Kotlin is the class that holds an object’s data. This tutorial will show how to extend a data class to leverage the concept of inheritance in Kotlin.

Use the data Keyword to Define Class in Kotlin

Syntax:

data class Tutorials (var name: String, val year: Int)

Declaring a data class in Kotlin automatically generates functions like equals(), toString(), and hashcode().

Requirements to Declare a Data Class in Kotlin

• There should be one or more parameters in the primary constructor.
• The parameters should be initialized as var or val.
• We cannot declare the class as abstract, open, sealed, or inner.

Example of Data Class in Kotlin

Use the main function to declare variable properties in data class.

data class Tutorial(val tutorial_name: String, val year: Int)

fun main(args: Array<String>) {
    val tut = Tutorial("Kotlin", 2022)
    println("Tutorial Name = ${tut.tutorial_name}")
    println("Year = ${tut.year}")
}

Output:

Tutorial Name = Kotlin
Year = 2022

Extend Data Class in Kotlin

Data classes are the replacements of POJOs in Java. Hence, it is natural to think that they would allow for inheritance in Java and Kotlin.

The inheritance of data classes in Kotlin doesn’t execute well. Hence, it is advised not to use inheritance by extending the data class in Kotlin.

But we can use abstract class and interface.

Use the Abstract Class to Extend Data Class in Kotlin

The abstract class can declare all the parameters as abstract in the parent class and then override them in the child class.

fun main(args: Array<String>) {
    abstract class Tutorial {
        abstract var year: Int
        abstract var name: String
    }

    data class Book (
        override var year: Int = 2022,
        override var name: String = "Kotlin Tutorials",
        var isbn: String
    ) : Tutorial()
}

The above code won’t throw any error as it extends the data class. It will create a corresponding class and enable the data class to extend from it.

Use the Interface to Extend Data Class in Kotlin

We can also extend a data class in Kotlin with the help of an interface.

interface Base_Interface {
    val item:String
}

interface Derived_Interface : Base_Interface {
    val derived_item:String
}

fun main(args: Array<String>) {
    
    data class B(override val item:String) : Base_Interface

    data class D(override val derived_item:String,
                 private val b:Base_Interface) :  Derived_Interface, Base_Interface by b
        val b = B("Hello ")
        val d = D("World!", b)

    print(d.item) //from the base class
    print(d.derived_item) //from the derived class
}

Use the Open Classes to Extend Data Class in Kotlin

Another way to extend a data class is by declaring all the parent class properties as open.

We can then use the same override method to override the properties in the sub-class and extend the data class.

fun main(args: Array<String>) {
    open class ParentClass {
        var var1 = false
        var var2: String? = null
    }

    data class ChildClass(
        var var3: Long
    ) : ParentClass()
}