interface CrudRepo {
fun save()
fun saveAll()
}
interface CustomRepo : CrudRepo {
fun validate()
override fun save() {
validate()
saveAll() // can call saveAll, I need to call save on CrudRepo here
}
}
CrudRepo
has a function save which doesn't do validation. I decide to write my own interface which extends CrudRepo
and overrides the save method. In my own definition of save
I want to validate and then call the save
method on the CrudRepo
.
What should I try ?
If you try using super
, you'll get an error that says "abstract member cannot be accessed directly". This applies to interfaces and classes; you can't call bodyless abstract methods on super
. Which means this doesn't compile:
interface CrudRepo {
fun save()
fun saveAll()
}
interface CustomRepo : CrudRepo {
fun validate()
override fun save() {
validate()
super.saveAll()
}
}
But change the CrudRepo interface to this:
interface CrudRepo {
fun save()
fun saveAll(){
}
}
And it does.
If you want an abstract example, here you go:
abstract class CrudRepo {
abstract fun save()
abstract fun saveAll()
}
class CustomRepo : CrudRepo() {
override fun saveAll() { }
override fun save() {
super.saveAll()
}
}
You'll get the same error message. This is because you cannot call unimplemented versions of a method. Also, let's take a look at inheritance.
interface CustomRepo : CrudRepo
class MyClass : CustomRepo
means MyClass
is a child of both CrudRepo and CustomRepo. Both MyClass() as CustomRepo
and MyClass() as CrudRepo
would compile.
Now, what does this mean for your problem?
There is no save
method in CrudRepo
from the CustomRepo
's point of view. It isn't implemented, and as such, you can't call it directly. Calling super.saveAll()
as I showed above means call the saveAll method on my parent
. super
is a very strict keyword. However, saveAll()
isn't overridden. You can call save
, but you cannot call it on the super interface, because the super interface doesn't have a save method with a body. The last three words of that sentence are incredibly important here; since the method defined in the CrudBody interface doesn't have a body, you can't call it.
And, in addition, since you've overridden the method, any calls to it would result in recursion. If you do this:
override fun save(){
save()
}
it will call itself over and over, until it crashes. See What is a StackOverflowError?.
Now, if you have an actual class:
class CustomRepoImpl : CustomRepo{
override fun saveAll() {
}
override fun save() {
super.save()
}
override fun validate() {
}
}
notice how it calls super
; it will now call the super method in CustomRepo
. It's not required to, however.
And when you override save
in CustomRepo
, remember one thing: you're overriding a method of CrudRepo. Any child classes are no longer required to override it at all. The example implementation I had would compile without it. The reason saveAll
works (without the super
keyword) is because it is abstract, and doesn't reference the method in which it is called from. See the StackOverflowError link. If you call an overridden method, the one in the child class is called, unless the method is invoked on super
.
And you can't call super
on bodyless methods in a superclass or interface, because the language has no idea what you're pointing to, because there's nothing there.
TL;DR: you can't call the super method of an abstract/bodyless method. saveAll
refers to the first implementation of the method, or the superclass if defined in a child. Calling save
would result in a StackOverflowError.
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