Sealed Classes Example

📚 Official Books Kotlin Apprentice
1

I’m not understanding the example code found on page 238

Specifically, the code block under “And functions defined on Shape can distinguish between the different subtypes using a when expression:”

fun size(shape: Shape): Int {
  return when (shape) {
    is Shape.Circle -> shape.radius
    is Shape.Square -> shape.sideLength
  }
}
circle1.size // radius of 4
square2.size // sideLength of 2

When I try to call that function as exemplified circle1.size I get Unresolved Reference errors. When I move that function inside the sealed Shape class as a method, I still get errors as the IDE is expecting the arguments for shape in the function. I ended up adjusting the code as follows and placed it inside the Shape sealed class as a method:

fun size(): Int = when (this) {
            is Shape.Circle -> this.radius
            is Shape.Square -> this.sideLength
        }

At least now I can call is like so
circle1.size()

However, am I missing something? This is apparently the reason why someone would want to use a sealed class?

2

Thanks for the question @chuck_taylor!

You mentioned “When I try to call that function as exemplified circle1.size I get Unresolved Reference errors.” The reason for that, with the code as written in the book, is that size() is not a method on the Shape enum class, it’s just a stand-alone function. So you would call it like this:

size(circle1)

I think that is what you were missing. Thanks again for the question!