90% of single-implementation interfaces (in Kotlin on Android projects I've seen) are internal (package/module private, more or less.) So no, they are not meant to be extended or substituted, and tests are their only raison d'etre (irony: I've almost never seen any actual tests...) This is insane because there are other tools you can use for testing, like an all-open compiler plugin or testing frameworks that can mock regular classes without issues.
An interface with a single implementation sometimes makes sense, but in the code I've seen, such things are cludges/workarounds for technical limitations that haven't been there for more than a decade already. At least, it looks that way from the perspective of a polyglot programmer who has worked with multiple interface-less OOP languages, from Smalltalk to Python to C++.
An interface with a single implementation sometimes makes sense, but in the code I've seen, such things are cludges/workarounds for technical limitations that haven't been there for more than a decade already. At least, it looks that way from the perspective of a polyglot programmer who has worked with multiple interface-less OOP languages, from Smalltalk to Python to C++.