Kotlin, set var/val once to make it final, is that possible

Question

In Kotlin, we have val that is final and can't be change. e.g.

val something = "Something"

If a value that is is initialized later, we use lateinit var.

lateinit var something: String

But this is var instead of val. I wanted to set something once (not in constructor), and have it as final. How could I achieve this?

Answer 1

Reading into the conventions of Kotlin, a late-initialized variable which is final is impossible.

Consider its use case:

Normally, properties declared as having a non-null type must be initialized in the constructor. However, fairly often this is not convenient. For example, properties can be initialized through dependency injection, or in the setup method of a unit test. In this case, you cannot supply a non-null initializer in the constructor, but you still want to avoid null checks when referencing the property inside the body of a class.

lateinit var is providing relative sanity when dealing with a variable that may not have yet been initialized, such as the case with injected fields (like Spring and @Autowired). Then, speaking strictly in the context of dependency injection, if you don't have a way to concretely instantiate the variable at compile time, then you cannot leave it as a final field.

From a Java to Kotlin world, having a late initialized variable come in as final would look as paradoxical as this from Spring:

@Autowired
private final Interface something;

Answer 2

What do you think the behavior should be when you attempt to set it again? Do you expect this to be enforced at compile time? Should it cause a crash at runtime or just do nothing?

If you expect it to happen at compile time, I'm pretty sure it's not possible for a compiler to catch something like that.

If you want some other behavior, you can make it a private variable with a public set method that does whatever you want if it's been already set.

Or you could encapsulate it in an instance of a custom class that does whatever behavior you want.