Kotlin Abstract Class

Like Java, abstract keyword is used to declare abstract classes in Kotlin. An abstract class cannot be instantiated (you cannot create objects of an abstract class). However, you can inherit subclasses from can them.

The members (properties and methods) of an abstract class are non-abstract unless you explictly use abstract keyword to make them abstract. Let's take an example:

abstract class Person {
    
    var age: Int = 40

    fun displaySSN(ssn: Int) {
        println("My SSN is $ssn.")
    }

    abstract fun displayJob(description: String)
}

Here,

• an abstract class Person is created. You cannot create objects of the class.
• the class has a non-abstract property age and a non-abstract method displaySSN(). If you need to override these members in the subclass, they should be marked with open keyword.
• The class has an abstract method displayJob(). It doesn't have any implementation and must be overridden in its subclasses.

Note: Abstract classes are always open. You do not need to explicitly use open keyword to inherit subclasses from them.

Example: Kotlin Abstract Class and Method

abstract class Person(name: String) {

    init {
        println("My name is $name.")
    }

    fun displaySSN(ssn: Int) {
        println("My SSN is $ssn.")
    }

    abstract fun displayJob(description: String)
}

class Teacher(name: String): Person(name) {

    override fun displayJob(description: String) {
        println(description)
    }
}

fun main(args: Array<String>) {
    val jack = Teacher("Jack Smith")
    jack.displayJob("I'm a mathematics teacher.")
    jack.displaySSN(23123)
}

When you run the program, the output will be:

My name is Jack Smith.
I'm a mathematics teacher.
My SSN is 23123.

Here, a class Teacher is derived from an abstract class Person.

An object jack of Teacher class is instantiated. We have passed "Jack Smith" as a parameter to the primary constructor while creating it. This executes the initializer block of the Person class.

Then, displayJob() method is called using jack object. Note, that the displayJob() method is declared abstract in the base class, and overridden in the derived class.

Finally, displaySSN() method is called using jack object. The method is non-abstract and declared in Person class (and not declared in Teacher class).

Recommended Reading: Kotlin Interfaces

Kotlin interfaces are similar to abstract classes. However, interfaces cannot store state whereas abstract classes can.

Meaning, interface may have property but it needs to be abstract or has to provide accessor implementations. Whereas, it's not mandatory for property of an abstract class to be abstract.