How do you ensure interface compatibility when working with Kotlin?

2 Use @JvmStatic and @JvmField annotations

Another difference between Kotlin and Java is that Kotlin does not have static members, but instead uses companion objects and object declarations to define singleton instances. However, this can also cause interface compatibility issues when you want to access these members from Java code, because Java expects them to be static. To solve this, you can use the @JvmStatic annotation to mark a function or a property in a companion object or an object declaration as static for Java. For example, if you have a companion object with a property called PI, you can use @JvmStatic to make it accessible from Java as MyClass.PI. Similarly, you can use the @JvmField annotation to mark a property in a companion object or an object declaration as a field for Java, instead of a getter and a setter. For example, if you have an object declaration with a property called INSTANCE, you can use @JvmField to make it accessible from Java as MyObject.INSTANCE.

3 Use @JvmOverloads annotation

Another feature of Kotlin that differs from Java is that it supports default arguments and named arguments for functions and constructors, which can simplify your code and reduce the need for overloading. However, this can also create interface compatibility issues when you want to call these functions or constructors from Java code, because Java does not support these features. To solve this, you can use the @JvmOverloads annotation to generate multiple overloaded methods for Java, each with a different number of parameters. For example, if you have a function called greet with two parameters, name and message, and the second one has a default value, you can use @JvmOverloads to make it callable from Java as greet(name) or greet(name, message).

4 Use @JvmDefault annotation

Another feature of Kotlin that differs from Java is that it supports default implementations for interface methods, which can provide common behavior and avoid boilerplate code. However, this can also create interface compatibility issues when you want to implement these interfaces from Java code, because Java does not support default methods. To solve this, you can use the @JvmDefault annotation to mark an interface method as default for Java, which will generate a default method in the bytecode. For example, if you have an interface called Drawable with a method called draw with a default implementation, you can use @JvmDefault to make it implementable from Java without overriding the method.

5 Use @JvmSuppressWildcards annotation

Another feature of Kotlin that differs from Java is that it uses declaration-site variance for generic types, which means that you can specify whether a type parameter is covariant, contravariant, or invariant at the point of declaration. However, this can also create interface compatibility issues when you want to use these generic types from Java code, because Java uses use-site variance, which means that you can specify the variance at the point of use. To solve this, you can use the @JvmSuppressWildcards annotation to suppress the generation of wildcard types for Java, which can cause compilation errors or runtime exceptions. For example, if you have a function that returns a List<out Any>, you can use @JvmSuppressWildcards to make it return a List<Any> for Java, instead of a List<? extends Object>.

6 Use @JvmWildcard and @JvmName annotations

Finally, there are some cases where you may want to use wildcard types or change the name of a generic type for Java, even if it is not required for interface compatibility. For example, you may want to use a wildcard type to make your code more flexible or expressive, or you may want to change the name of a generic type to avoid conflicts or confusion. To do this, you can use the @JvmWildcard and @JvmName annotations to modify the generated bytecode for Java. For example, if you have a function that takes a List<Any> as a parameter, you can use @JvmWildcard to make it take a List<?> for Java, or you can use @JvmName("foo") to make it take a List<foo> for Java.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?