Kotlin File vs Class. Whats the difference?

Question

Just started using Kotlin and created some activities as Kotlin Files. They work, but still, I want to know what the difference between creating a File and a Class.

Stratify

Answer 1

Kotlin Class: Android Studio displays it with no extension if the file contains ONLY ONE class.

Unlike Java, Kotlin allows you to put certain things outside the class like:

• extension functions
• constants

If you, therefore, add any of the above or another class to the file, Android Studio will change it to "Kotlin File":

with extension .kt

Removing the above extras, will again show the file as "Kotlin Class"

When

So, if I put a private const val TAG = "MyActivity" at the top simply for the purpose of Log.d messages, it turns my class into a file. I know it's not really a problem, but is there a better place to put that? – Firehouse

@Firehouse IMO this is the best place to put it if you want a private, static constant. You limit the scope to the file so a class inside this file can use it and this is pretty much the same as private static String from Java. – When

So pretty much ignore it and let AS figure it out! – Box

@Firehouse by using companion object you can achive in class level also – Yore

Terrible design choice (I assume by JetBrains). It's never a good idea to obfuscate a programmer's tools for no reason. Just by showing the ".kt" extension always, this StackOverflow question would be unnecessary and no one would care. – Experimental

Answer 2

The only difference is that creating a file creates a file with no classes, and creating a class creates a file with one class. You can then add more classes to the file, or delete classes, or make any other changes - the end result doesn't depend on how the file was created.

Tetragrammaton

This should be the decent answer – Babble

whats the added advantage with this i mean having more classes in a single file? – Janes

Answer 3

Here is the answer from the official Documentation:

If a Kotlin file contains a single class (potentially with related top-level declarations), its name should be the same as the name of the class, with the .kt extension appended. If a file contains multiple classes, or only top-level declarations, choose a name describing what the file contains, and name the file accordingly. Use camel humps with an uppercase first letter (e.g. ProcessDeclarations.kt).

The name of the file should describe what the code in the file does. Therefore, you should avoid using meaningless words such as "Util" in file names.

so basically a file can -for instance- contains just (helper-)functions without any class declaration.

Leatheroid

Answer 4

As I can see Kotlin file has many classes inside it But a Kotlin class has one class with that name, that's only an observation though.

Syllabub

Answer 5

Yes, if you define more than one class, it will be automatically converted to file, otherwise it will be automatically converted to class. Usually I think if you need to define some variables or methods that can be directly referenced, you can put them in the file.

Kathernkatheryn

Answer 6

Creating a file creates public final class at bytecode level just like creating a class. If You declare a class in a file then two classes will be created at the bytecode level. Therefore, the file exists for convenience - it contains code unrelated to defining the behavior of objects

Kinescope

Answer 7

The difference is:

• Kotlin Class can have only a class.

• Kotlin File can have many classes.

Sherillsherilyn

Answer 8

I found a further difference - when it comes to renaming files, the refactoring mechanism handles Kotlin files and class files differently:

• Class: On renaming the file, the refactoring also renames the contained class itself. This also works vice-versa, so when the class is renamed, also the file is renamed.
• File: When renaming a file, only the file is renamed and the contained class name is untouched.

I assume this is done that way because the class name should be kept consistent to the file name in case of Kotlin class files, for a better overview.

Hwahwan