Using the default function for an interface in Kotlin

Question

I have a Kotlin interface with a default implementation, for example:

interface Foo {
    fun bar(): String {
        return "baz"
    }
}

This will be fine until I try to implement this interface with Java. When I do this, he says that the class should be marked as abstract or implement the method bar(). Also, when I try to implement a method, I cannot call super.bar().

Answer 1

Please see the related issue .

There is a recommendation in the comments:

Write your interface in Java (with default methods), and the Java and Kotlin classes use these default values ​​correctly

Answer 2

default Java, Kotlin 1.2.40.

@JvmDefault @JvmDefault:

interface Foo {
    @JvmDefault
    fun bar(): String {
        return "baz"
    }
}

- , -Xjvm-default=enable . ( Gradle, ).

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Answer 3

Java8, Kotlin .

Foo Java. Kotlin . .

Java, /. Kotlin , Java-.

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Kotlin binaries were compiled using java-bytecode version 1.8 without standard methods in the interfaces. And they are faced with a decisive problem that solves it.