Inheritance from an interface with ‘@jvmdefault’ members is only allowed with -xjvm-default option

Inheritance from an Interface with ‘@jvmdefault’ Members

The error message “inheritance from an interface with ‘@jvmdefault’ members is only allowed with -xjvm-default option” indicates that you are trying to extend an interface that includes default method implementations defined with the ‘@jvmdefault’ annotation. This is only supported when using the ‘-xjvm-default’ compiler option in Kotlin.

Default methods in interfaces provide a way to define a default implementation for a method. These default implementations can be inherited by classes or interfaces that extend or implement the interface. However, since default methods in Kotlin are compiled to Java 8’s default methods, which Kotlin doesn’t fully support, the compiler requires the ‘-xjvm-default’ option to enable this feature and avoid potential compatibility issues.

Here’s an example to illustrate this concept:

    interface MyInterface {
        @JvmDefault
        fun doSomething() {
            println("Doing something")
        }
    }

    class MyClass : MyInterface // Error: inheritance from an interface with '@jvmdefault' members is only allowed with -xjvm-default option
  

In the above example, the ‘MyInterface’ interface includes a default method ‘doSomething()’ with the ‘@jvmdefault’ annotation. When trying to inherit this interface in the ‘MyClass’ class, you will encounter the mentioned error unless you use the ‘-xjvm-default’ compiler option.

To fix the error, you need to provide the ‘-xjvm-default’ option to the Kotlin compiler while building your project. This option enables the support for default methods in interfaces compiled to Java 8’s default methods, allowing the inheritance of interfaces with ‘@jvmdefault’ members.

The exact steps to enable the ‘-xjvm-default’ option may vary depending on your build system and IDE. For example, if you’re using Gradle, you can include the following configuration in your ‘build.gradle’ file:

    kotlinOptions {
        jvmTarget = '1.8' // Ensure Java 8 compatibility
        freeCompilerArgs = ["-Xjvm-default=enable"] // Enable '-xjvm-default' option
    }
  

By providing the ‘-xjvm-default’ option, you will no longer encounter the error and the inheritance from an interface with ‘@jvmdefault’ members will work as expected.