Ktolin or Java for android app development?

I'd choose on the quality of the libraries, not the language itself.

I'm unfamiliar with Kotlin myself.

Look at the development environment / IDE as well - particularly for android/iphone development, integration with the final gui of the program in the IDE makes a big difference in your quality of life.

@Buffy I have a simple vocab question for you.

Say you have a Rectangle class that uses Points.

Rectangle would clearly utilize Point based on its interface.

What do we call the members of this interaction?

I would want to use the words Provider (Point) and Client (Rectangle), but not only can't I find support for those words elsewhere, I can find no vocabulary to describe the members of the interaction.

I might still go ahead and use those words, but I wonder if there are more accepted words that I am just, somehow, still unaware of after all of these years.

I'm assuming you mean some use of composition here. A rectangle "has" points. If so, then "part", "whole" might work. And note that the "composition" might be abstract only, rather than direct. A single point, A vector, and an integer length are enough to specify a rectangle (in 2 space). But the other points can be computed.

Hmmm. Make that a "real" length, of course. Not integer.

Not composition per se -- the user of the interface of an object

It it the object's client, their user, or... some other word?

If A invokes methods of B then A is B's client, yes.

Good, that's the word I've gravitated towards. And B is A's....

Provider?

Sorcerer?

Drug Dealer?

Service provider?

Resource?

Ecumenical Ministerial Council Representative?