Kotlin interface implementation behaviour when consuming Java interface

Question

If I have an interface in Kotlin:

interface KotlinInterface {
    val id: String
}

I can implement it like so:

class MyClass : KotlinInterface {
    override val id: String = "id"
}

However, if I were to consume a Java interface like this:

public interface JavaInterface {
    String id = "id";
}

I cannot override the id class variable in a similar way:

class MyClass : JavaInterface {
    override val myId: String = JavaInterface.id //linter says: 'myId' overrides nothing
}

I also cannot make use of id elsewhere despite it having a predefined value:

class MyClass : JavaInterface {
    val myArray: Array<String> = arrayOf(id) // linter would say that id is not defined rather than recognising it as the string “id”
}

It appears I have to use it like so:

class MyClass {
    val id: String = JavaInterface.id
    val myArray: Array<String> = arrayOf(id)
}

Could anyone explain this difference in behaviour and point out anything I may be understanding wrongly?

Jayanth · Accepted Answer · 2020-01-22 07:23:47Z

4

In the java interface, every variable is a static final variable and static variables can't be overridden. That is why you see that lint warning.

Edit 1:

Kotlin interface

  interface Ser {
       var name: String //abstract
    }

is Equivalent to java interface

public interface Ser {
   @NotNull
   String getName();

   void setName(@NotNull String var1);
}

edited Jan 22, 2020 at 7:23

answered Jan 22, 2020 at 6:48

Jayanth

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3 Comments

Cloud Over a year ago

Thanks for the response. I’ve edited my question just as two answers rolled in. If my understanding is correct, Kotlin doesn’t allow interface properties to be initialised with a value at all, while this is possible in Java. However, if I don’t “shadow” or “replace” that final Java property in my Kotlin implementation, I cannot access the property (which already has a value in Java) by name from within the implementation.

Jayanth Over a year ago

Yes when you declare a variable without initializing it, its marked as an abstract variable. and in byte code, kotlin creates getter and setter methods for that variable internally so you can override that variable.

Jayanth Over a year ago

to see how it does just declare the interface with one abstract variable and see its decompiled byte code.

Alexey Romanov · Accepted Answer · 2020-01-22 12:41:58Z

2

I cannot override the id class variable in a similar way:

You can't do it in Java either; the equivalent would be

class MyClass implements JavaInterface {
    @Override String getMyId() {
        return JavaInterface.id;
    }
}

getMyId doesn't override anything.

And if you write in Java

class MyClass implements JavaInterface {
    String id = JavaInterface.id;
}

you aren't overriding anything, because you can't override fields (in addition, MyClass.id is a non-final instance package-private field, while JavaInterface.id is implicitly a final static public field because interfaces don't allow any other kind).

I also cannot make use of id elsewhere despite it having a predefined value:

Again, same as in Java; you need JavaInterface.id in both Java and Kotlin, or import some.package.JavaInterface.id to use just id (import static in Java).

There is a difference if MyClass implements JavaInterface, because then in Java you can refer to JavaInterface.id as MyClass.id or as someInstanceOfMyClass.id; and inside MyClass just as id. But this is often considered a misfeature, so Kotlin's designers avoided it.

answered Jan 22, 2020 at 12:41

Alexey Romanov

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8 Comments

Cloud Over a year ago

Could you elaborate on that last part about referring to the implemented parent interface's initialised variables from within the child class being a "misfeature"? It seems to make sense.

Alexey Romanov Over a year ago

See e.g. softwareengineering.stackexchange.com/questions/49572/… and docs.oracle.com/javase/1.5.0/docs/guide/language/….

Cloud Over a year ago

Thank you, I found the second link particularly helpful. A part of the quote in the first link states, "It is of no consequence to the users of a class that the class implements a constant interface." Yet, wouldn't users of MyClass have access to MyClass().id if it implements JavaInterface?

Cloud Over a year ago

Also, my question arose from the existence of this interface: developer.android.com/reference/android/provider/…. Is it good practice to use it not by "Constant Interface Antipattern" & abusing inheritance, but rather, to simply write something similar to val id: String = JavaInterface.id?

Cloud Over a year ago

Regarding the second part of your main answer, I've just realised that I forgot to implement JavaInterface as part of my 5th code snippet. I've edited it; the original edit was made on mobile and the mistake wasn't apparent at the time. It is as you said, though. It seems I'm unable to access id from within MyClass, such as in arrayOf(id), in Kotlin.

|

Sergey Ushakov · Accepted Answer · 2020-01-22 06:51:23Z

0

You declared constant (public static String id = "id") at Java interface.

Whereas Kotlin interface declared abstract property. See Kotlin reference: https://kotlinlang.org/docs/reference/interfaces.html

answered Jan 22, 2020 at 6:51

Sergey Ushakov

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