What makes the String class in Kotlin to be able to work with square brackets?

Question

In Kotlin you can do something like this:

val s: String = "Hey"
println(s[1])

or simply:

println("Hey"[1])

and you will print e.

But what if you want to extend this behavior to your own classes? What interface do you need to implement to accomplish this syntax?

I looked at String's supertypes and only found Comparable<String> and CharSequence, neither of which had any other supertypes themselves, so my quest ended early.

For example in Python, by defining a method called __getitem__ in a class, you can bestow the objects of that class the ability to be used with square brackets syntax like this a[i]; I was wondering how this was possible in Kotlin.

Thanks in advance.

Answer 1

You can use Kotlin's operator overloading, specifically the overloading for indexed access operators.

The syntax "Hey"[1] is just an alias for "Hey".get(1) (if not on the left-hand side of an assignment, where it would be an alias for "Hey".set(1, ...)).