Challenge 3: Sealed Class with when — Solution

sealed class Shape
data class Circle(val radius: Double) : Shape()
data class Square(val side: Double) : Shape()
data class Rectangle(val width: Double, val height: Double) : Shape()

fun area(shape: Shape): Double = when (shape) {
    is Circle -> Math.PI * shape.radius * shape.radius
    is Square -> shape.side * shape.side
    is Rectangle -> shape.width * shape.height
}

fun main() {
    println(area(Circle(3.0)))
    println(area(Square(4.0)))
    println(area(Rectangle(5.0, 2.0)))
}

/*
Output:
28.274333882308138
16.0
10.0

Notes:
- Shape is sealed, so Kotlin knows Circle, Square, and Rectangle are the
  only possible subclasses in existence. The "when" expression covers all
  three with "is" branches and needs no "else" — it's exhaustive.
- If a fourth Shape subclass were added later without updating this
  "when", the code would fail to compile until a branch for it was added
  — catching the gap immediately instead of silently falling through.
- Each "is X ->" branch smart-casts shape to that specific subtype, so
  shape.radius / shape.side / shape.width and .height are all accessible
  directly, with no manual casting.
*/