Abstract Classes
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Declaring an Abstract Class
abstract class marks a class that cannot be instantiated on its own -- it exists only to be subclassed, often to define a shared shape for related types.
Example: Declaring an Abstract Class
abstract class Shape {
abstract fun area(): Double
}
class Circle(val radius: Double) : Shape() {
override fun area(): Double = Math.PI * radius * radius
}
fun main() {
val circle = Circle(3.0)
println("Area: ${circle.area()}")
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Abstract Members Have No Body
An abstract fun or abstract val declares only a signature, with no implementation -- every concrete subclass is required to provide one using override.
Example: Abstract Members Have No Body
abstract class Employee(val name: String) {
abstract fun monthlySalary(): Double
}
class Manager(name: String, val baseSalary: Double) : Employee(name) {
override fun monthlySalary(): Double = baseSalary * 1.2
}
fun main() {
val manager = Manager("Ana", 5000.0)
println("${manager.name}'s salary: ${manager.monthlySalary()}")
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Mixing Abstract and Concrete Members
An abstract class can define fully implemented members alongside abstract ones, sharing common behavior while still leaving specific details to subclasses.
Example: Mixing Abstract and Concrete Members
abstract class Shape {
abstract fun area(): Double
fun describe(): String = "This shape has area ${area()}"
}
class Square(val side: Double) : Shape() {
override fun area(): Double = side * side
}
fun main() {
val square = Square(5.0)
println(square.describe())
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Abstract Classes vs Regular Open Classes
Use an abstract class when there is no sensible default implementation and instantiating the base type directly would not make sense; use a regular open class when a default, usable implementation exists.
Example: Abstract Classes vs Regular Open Classes
abstract class PaymentMethod {
abstract fun pay(amount: Double): String
}
class CreditCard : PaymentMethod() {
override fun pay(amount: Double): String = "Paid $$amount with credit card"
}
fun main() {
val payment: PaymentMethod = CreditCard()
println(payment.pay(49.99))
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- Trying to instantiate an abstract class directly with
AbstractClass(), which the compiler always rejects. - Forgetting that abstract members are implicitly open and don't need an extra
openkeyword. - Not implementing every abstract member in a concrete subclass, leaving the subclass abstract too without realizing it.
abstract classcannot be instantiated directly; only its concrete (non-abstract) subclasses can be.abstract fun/abstract valdeclare members with no implementation that subclasses must provide.- Abstract members are implicitly open, so a subclass overriding them just needs
override, notopentoo. - An abstract class can still have concrete (fully implemented) members alongside its abstract ones.
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