``` interface Some { fun calculate(str: String? = null) } class SomeImpl : Some { fun calculate(...

# announcements

[rrader]

interface Some {
  fun calculate(str: String? = null)
}
class SomeImpl : Some {
  fun calculate(str: String? = null)
}

An overriding function is not allowed to specify default values for its parameters
Why?

[Luke]

That does not stop you from calling it without parameters, no?

[Luke]

The overloading is “handled” by the interface

[rrader]

but why I can not specify same default parameter in implementation?

[Luke]

I guess it’s to not break the interface’s contract

[streetsofboston]

@rrader That would violate one of the SOLID principles, in this case the L(iskov substitution principal).

[rrader]

How it can break if it is the same default value?

[streetsofboston]

If it is the same, you don’t have to specify it in its implementation.

[rrader]

I open the implementation class and I want to see that it can have a default parameter, if I will not open the interface than I will not know this

[streetsofboston]

But the code that calls the calculate(...) functions, calls it on a variable that is declared to be of type Some, correct, not of type SomeImpl?

val mySome : Some
...  
...
mySome = ... SomeImpl() ...
...
mySome.calculate()

[rrader]

it could be also a type of SomeImpl, why not?

[streetsofboston]

Yup, but usually it won't be for the users of your Some instances (they should not be concerned with the specific implementation of an interface).