The general pattern to create constants in Kotlin seems to be using companion objects. However, I can also define a constant at the file level. Why is that not so popular? Am I missing something?
With companion object:
class Example {
companion object {
const val CONSTANT = "something"
}
On top level:
const val CONSTANT = "something"
class Example {
}
In Java you're forced to put all static field and method declarations in a class and often you even have to create a class just for that purpose. Coming to Kotlin, many users look for the equivalent facility out of habit and end up overusing companion objects.
Kotlin completely decouples the notions of a file and a class. You can declare any number of public classes is the same file. You can also declare private top-level functions and variables and they'll be accessible only to the classes within the same file. This is a great way to organize closely associated code and data.
Compared to top-level declarations, the syntax of companion objects is quite unwieldy. You should use them only when you specifically want to associate some public static code or data with a class and want your users to qualify access to it with the class's name. The use cases for this are quite rare and in most cases the top-level declarations are more natural.
Whenever you have some private static code/data that you want to couple to a class, you'll be better served with private top-level declarations.
Finally, sometimes the concern of the generated bytecode matters. If, for whatever reason, you have to produce a Java class with Kotlin code such that the class has a static member, you must resort to a companion object and a special annotation.
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