So I made a little number type for a specific soft...

# getting-started

[rednifre]

So I made a little number type for a specific software and noticed that there is a Number super type in Kotlin, which is the super type of numbers like Int Float etc. Should my custom number type (a little Decimal type object that wraps a Long and has some custom rounding rules) also inherit from Number? What are the pros and cons? Would there be any gnarly surprises later?
If I remember correctly, Java does not have a Number super type, so what were the reasons for adding it to Kotlin? What does it do for Kotlin?

[dmitriy.novozhilov]

Java does have Number as a superclass for Integer, Long, Double and other number types
That's the main reason to have it in Kotlin

[rednifre]

Oh, interesting, I did not notice that before. Thanks for pointing it out!
So, Java does not have sealed classes (I think?), so was Java’s Number meant ONLY for the standard number types, or were you also meant to extend it?
Would it even make any practical difference if my number type extended from Number?

[dmitriy.novozhilov]

> Java does not have sealed classes
It does starting from Java 15 IIRC

> or were you also meant to extend it?
Number is an arbitrary abstract class, you can extend it

> Would it even make any practical difference if my number type extended from Number?
Well, it will allow you to use your numbers in some abstract APIs which can work with java.lang.Number. Do you need it or not depends on which libraries/frameworks you use

[Daniel Pitts]

If you look at what the Number class exposes (which I believe is just toInt, toLong, etc...), you can decide whether or not it is useful for your class to extend it. If you find that you're passing instances of your class to a lot of places that take Number, and you end up converting it there anyway, then it might make sense to just extend Number. If you don't, then maybe not.

[Klitos Kyriacou]

Also, consider making your class a @JvmInline value class if you haven't already.

[rednifre]

I considered making it a value class. How well does that work for KMP, i.e. JS and Swift?

[rednifre]

Ah, looks like value classes are not supported by the Objective-C exporter.