29. Abstract Classes

An abstract class cannot be instantiated and may declare abstract members that subclasses must implement. It can also provide concrete members that build on those abstract ones.

abstract class Shape(val name: String) {
    abstract fun area(): Double                   // must be implemented
    fun describe() = "$name has area ${area()}"   // concrete
}

class Circle(val radius: Double) : Shape("Circle") {
    override fun area() = Math.PI * radius * radius
}

fun main() {
    val shape: Shape = Circle(2.0)
    println(shape.describe())
}

describe() is fully defined in Shape, yet it calls the abstract area() — the concrete subclass supplies that piece.

Running it:

$ kotlin run
Circle has area 12.566370614359172

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