why can a `val` from an interface be overwritten as `var` in

Question

why can a `val` from an interface be overwritten as `var` in the implementation

Burt Beckwith · Answer

gt You can also override a val property with a var property but not vice versa This is allowed because a val property essentially declares a getter method and overriding it as a var additionally declares a setter method in the derived class

Burt Beckwith · Answer

Marko Mitic · Answer

From point of anyone using the interface that s a `val`

thana · Answer

but OTOH one could argue that the `set` of the `val` is private and declaring it `var` in the implementation grants access that shouldn t be allowed

thana · Answer

yeah indeed it looks like a `val` and then it changes

Alowaniak · Answer

you can change private to public in a child class

thana · Answer

you cannot even access it from a child class

Marko Mitic · Answer

`val` properties are read only not immutable

thana · Answer

yeah that s a good point indeed

Marko Mitic · Answer

``` val counter Int get = counter++ ```

Marko Mitic · Answer

or ``` val isEmpty Boolean get = this size == 0 ```

thana · Answer

yeah point taken thank you slightly smiling face

Nir · Answer

It all depends how you look at it but it is a bit odd It basically implies the underlying data is protected conceptually

Nir · Answer

In Java iirc more typical is that the data is private and then you implement a public getter in this case a derived class cannot add a setter

gildor · Answer

gt derived class cannot add a setter same in Kotlin you just override gettere and setters underlying backing field is just implementation detail

Nir · Answer

If the base class has a val you can override with var This is the same as adding a setter

Nir · Answer

That s how the discussion started slightly smiling face

Nir · Answer

The point is just that if you are trying to understand properties by analogy of private data + public getter and maybe setter which is how Kotlin itself tries to describe it

Nir · Answer

then this particular model breaks down when you consider inheritance

gildor · Answer

Interface cannot have any state and data it s just a contract so I it s not different from Java

Nir · Answer

Overriding properties is not limited to interfaces

Nir · Answer

It works with classes too

Nir · Answer

``` open class Foo open val x Int get class Bar1 Foo override val x Int = ```

Nir · Answer

Example from kotlin docs

Nir · Answer

No matter how you slice it you can t change the fact that Kotlin is granting access to the underlying data to derived classes but not outside the class

Nir · Answer

Basically it means that the closest java analogy to Kotlin properties is protected data with public or whatever getter and setter functions

gildor · Answer

Sure it works with classes same way in Java It doesn t give access to underlying data outside of available API