why can't interfaces inherit from classes ?

# announcements

[LastExceed]

why can't interfaces inherit from classes ?

[ephemient]

that's how Java's object model works. single-inheritance for classes, to avoid diamond MRO. mutiple-inheritance for interfaces, there's no behavior so there's no MRO issues.

[ephemient]

... somewhat violated by the interfaces with default implementations, but that's a later addition (in both Java and Kotlin)

[LastExceed]

i dont quite see how an interface inheriting from a class could cause a diamong problem as long as the interface cannot override the class's behavior

[ephemient]

class A {
    fun foo() = 1
}
class B {
    fun foo() = 2
}
interface C : A
interface D : B
class E : C, D

[LastExceed]

in that example E ends up inheriting from 2 classes, which is illegal

[ephemient]

but multi-inheritance for interfaces is legal

[LastExceed]

so what ?

[ephemient]

so to prevent multi-inheritance of classes, interfaces can't be allowed to extend classes

[LastExceed]

that conclusion doesnt necessarily follow. the compiler could just check if the hierachy contains multiple base classes and throw an error if it does

[ephemient]

Java (and Kotlin) is designed for separate compilation

[ephemient]

you can't check the whole hierarchy, it may not be available at compile time or it may be different at runtime

[LastExceed]

oh you can't ? guess i'll have to take your word on that one cuz im not familiar enough with compilation

[LastExceed]

that's circular reasoning

[ephemient]

it means what it means so that the language works

[LastExceed]

well you could apply the same argument to default implementations

[ephemient]

no, because those are subject to other restrictions

[ephemient]

it's not like there aren't other ways around the issue - Scala's self-type, for example - but they're not safely compatible with Java

[ephemient]

that being said, as far as the JVM is concerned... interfaces cannot extend classes for technical reasons as well. there is no way to invoke the constructor during object initialization

[ilya.gorbunov]

Curiously, when interfaces were called traits in Kotlin before 1.0, they were allowed to define a base class constraint, so that an implementing class must have inherited from that base class.
An example of what it looked like: https://stackoverflow.com/questions/28298152/kotlin-superclass-is-not-accessible-from-trait