hi, is there any way to initialize a immutable pro...

# announcements

[nucc]

hi, is there any way to initialize a immutable property in a data class in the constructor? In non-data class I've seen examples for the lazy keyword that might can help for me, but I don't know how I could use it in a data class. I want something like in C++ how we define a const instance variable in the constructor. Here is an example what I would like to do:

data class MyClass(val property: String) {   # <-- mutable
	init {
		this.property = property.capitalize()
	}
}

instance = MyClass("test")
instance.property # Test
instance.property = "test2" # error, immutable variable

but unfortunately the compiler says Val cannot be reassigned when I set this.property. Do you know how I could use immutable instance variable here?

[karelpeeters]

Well val means immutable, I think you're looking for var.

[nucc]

sorry, I made a mistake. immutable.

[nucc]

(edited)

[karelpeeters]

No you can't initialize a parameter val, it is already initalized by the main constructor itself.

[karelpeeters]

If you want to do some preprocessing on data class parameters you can do something like this:

data class MyClass(property: String) {
    val property = property.capitalize()
}

[nucc]

unfortunately it raises the following error if I don't use val or var before the constructor argument: Data class primary constructor must have only property (val / var) parameters

[karelpeeters]

Ah right that's unfortunate.

[Andreas Sinz]

@nucc IMO a data class shouldn't have any code in it, I'd go with a top level factory method

[Eduard Boloș]

why don’t you do the capitalization in a factory method instead?

[karelpeeters]

The problem with that is that the copy function leaks the constructor, so I'd be able to do MyClassFactory("test").copy(property = "test") to get a "broken" instance.

[nucc]

I need to amend the data and if I put the logic to each class where I want to initialize a MyClass object then I need to copy the logic everywhere. Or I should create a factory class, but I don't really want to create a MyClassFactory just to do some modification on a string.

[karelpeeters]

I really which there was a @NoCopy annotation to avoid stuff like this.

[karelpeeters]

You can make the constructor private and name the factory MyClass, Kotlin allows it.

[karelpeeters]

But the issue I mentioned remains.

[nucc]

hmm. okay, thanks

[Eduard Boloș]

what if you add a property to your data class that returns the capitalized string?

[nucc]

yeah, I was thinking on the same.

[karelpeeters]

That works in this case but breaks equality and such when the preprocessing isn't an injective function like Math.abs.

[karelpeeters]

We've had this exact discussion before in this Slack 🙂.

[karelpeeters]

Actually doesn't work for capitalize either, consider MyClass("test") == MyClass("Test").

[nucc]

yes, it's true, fortunately my transformation is injective.

[nucc]

you're right, I used it as an example only