Why is this forbidden?

# announcements

[Grigorii Yurkov]

Why is this forbidden?

[Marc Knaup]

The compiler cannot create copy() or component1() here.

[Marc Knaup]

Just make it a regular class. No point in having a data class if there is no data 🤔

[Casey Brooks]

If you do actually want DataClass to be a data class, it might make more sense to have MyClass be an interface instead

[Grigorii Yurkov]

@Marc Knaup It's just an example, I can have 15 other properties in data class, but it doesn't let me to add 1 more non val or var property

[Grigorii Yurkov]

@Casey Brooks It can't be an interface, because I want to incapsulate field in it

[Marc Knaup]

Can’t you make the field open and use override val?

[Marc Knaup]

It’s not computed then though.

[Marc Knaup]

Otherwise you’ll have to drop the data and implement the needed functions yourself.

[Grigorii Yurkov]

I can, but I want to incapsulate backing field in superclass

[Marc Knaup]

No way to do that then.
The compiler won’t know what property of a supertype to use instead of the constructor property.

[Tyler Hodgkins]

This is simply part of the language definition of data class. All data classes must only have property-based constructor parameters.

As Mark mentioned - without this requirement the compiler wouldn’t be able to actually provide the guarantees that a data class implies (component decomposition, copy/equals/toString implementations, etc).

[Grigorii Yurkov]

Just don't use this property to generate copy/equals etc

[Grigorii Yurkov]

Like with properties that are located not in constructor

[Marc Knaup]

It cannot implement copy without knowing what value to pass for non-val constructor parameter property.

[Grigorii Yurkov]

Yeah, I got it, thank you

[Javier]

data classes doesnt allow not use val/var I think