android - What is the meaning of "ClassInstance.[Someclass::class.java]" in Kotlin?

Question

I've seen people using ViewModelProvider[Someclass::class.java] instead of ViewModelProvider.get(Someclass::class.java), and it compiles in Android Studio. The problem is that I couldn't find any documentation of such usage online.

Answer 1

With kotlin you can add operator modifiers to your function. So if you have some class with a get function and you might want to access it with [], like an array or map, you could add operator modifier.

Square brackets are translated to calls to get and set with appropriate numbers of arguments.

So this only works for functions with name get or set!

class Provider {
  operator fun get(key: String)
  operator fun set(key: String, value: String) { ... }
}

Then you can call the function like:

Provider().get("key") // IDE hint: should be replaced with indexing operator
Provider()["key"] // calls get()

Provider().set("key", "value") // IDE hint: should be replaced with indexing operator
Provider()["key"] = "value" // calls set()

Reference

• See Kotlin Operator overloading