Using @JvmOverloads and @JvmStatic for Kotlin interoperability with Java

Isaac — Fri, 02 Apr 2021 08:51:36 +0000

Kotlin promises interoperability with Java. Today(31/03/2020), I found out that we sometimes need to do a little extra to make interoperability with Java seamless and natural using a couple of annotations. I never really had a use case for Java interoperability, mainly because I have been working in Kotlin-only projects since 2018.

I had the chance to rewrite a simple open source library in Kotlin. During the rewrite, I failed to consider that there could be Android apps written in Java that already depend on the library till the owner of the library drew my attention to that fact. In order to not break existing applications that already depend on this library, I had to find a solution. And that's what led me to @JvmOverloads and @JvmStatic annotations.

The Java implementation of the library exposes a single static method with three(3) overloads. Rewriting this in Kotlin was quite simple.

// Shutdown.java
public static void now(Activity context){
        if (context != null){
            init(context, DEFAULT_MESSAGE, DEFAULT_TIMEOUT);
        }
    }

    public static void now(Activity context, String message){
        if (context != null && !message.isEmpty()){
            init(context, message, DEFAULT_TIMEOUT);
        }
    }

    public static void now(Activity context, long timeout){
        if (context != null && timeout != 0){
            init(context, DEFAULT_MESSAGE, timeout);
        }
    }

    public static void now(Activity context, String message, long timeout){
        if (context != null && !message.isEmpty() && timeout != 0){
            init(context, message, timeout);
        }
    }

// Shutdown.kt

...// inside companion object

fun now(context: Activity?, message: String = DEFAULT_MESSAGE, timeout: Long = DEFAULT_TIME_OUT) {
    context?.let { init(context, message, timeout) }
}

Calling the Kotlin implementation of now(context) from a Kotlin class works without issues. Calling this Kotlin implementation from a Java class will show you the red squiggly line.

// SomeActivityClass.java

Shutdown.now(this); // no issues

//SomeActivityClass.kt

Shutdown.Companion.now(this); // Compile time error. Breaking change?? 😱

The compile-time error message read that three(3) arguments were expected but only one(1) was found. Which was strange because I provided default values to the other two arguments in Kotlin. Upon further investigation, I found that the Kotlin compiler wasn't generating the overloads for now(context) so it was impossible to omit some arguments when calling from Java. In comes the @JvmOverloads annotation. This annotation essentially instructs the Kotlin compiler to generate overloads for now(context).

// Shutdown.kt

@JvmOverloads
fun now(context: Activity?, message: String = DEFAULT_MESSAGE, timeout: Long = DEFAULT_TIMEOUT)
    context?.let { init(it, message, timeout) }        
}

Adding @JvmOverloads resolves the compile-time error. And now we can call now(context) from Java with one(1) or more arguments. But the Java usage still didn't feel natural. It didn't feel like a static method call. And that's what led me to @JvmStatic. This annotation tells the Kotlin compiler to generate an extra static method for now(context).

// Shutdown.kt

@JvmOverloads
@JvmStatic
fun now(context: Activity?, message: String = DEFAULT_MESSAGE, timeout: Long = DEFAULT_TIMEOUT)
    context?.let { init(it, message, timeout) }        
}

// SomeActivity.java

Shutdown.now(this);

Now the Java client doesn't need to access Companion in order to use now(context) from Java. Order has finally been restored in our Java client after the Kotlin rewrite. No more breaking changes! You can find the Pull Request here, if interested.

Thanks to Eyram A. Agbe, Raymond(creator of Playlistor), Waheed Derby and Wilfred for taking the time to review this article.

Tales of a Noob Dev[Part 1]

Isaac — Mon, 03 Jul 2017 12:07:47 +0000

This post was originally posted on medium

One of my dad’s many sayings subconsciously became my personal academic mantra, and it goes(translated from Twi)

Asare, once there is someone standing in front you, teaching you something, anything at all, that thing can be learnt and understood.

With that at the back of my mind, I began taking Construction of Angles and Pie Charts seriously in my Maths classes because they were the cause of my failures in MathsÂ inÂ juniorÂ high.

Fast forward to 2015, a similar situation happened in my first C++ classÂ inÂ yearÂ one, where I was really struggling to understand the concept of variables in programming ( Variables? ugh dude?!?). Variables are supposed to be the easiest things to grasp when learning to program but believe you me when I say I was really really struggling to grasp how it relates to memory and whatnots, and the nerve-wracking thing was almost everybody was nodding their heads in sync to whatever the lecturer was saying, and there I was feeling left out and dumbo(shoutsÂ toÂ Hov). But as time went on I managed to find solace and motivation to do better in the synchronization of nods from my classmates and I did better, with my academic mantra consciously or unconsciously at the back of my extremely big head, and that is how I began my journey on a path of no return, fightingÂ dragons,Â demogorgons, justÂ toÂ mentionÂ aÂ fewÂ mysticÂ beasts.

Moral of the story? It’s all dependent on you, the reader. I’ll be back.

PS: I dislike C++ now

Hi, I'm Isaac.

Isaac — Sat, 27 May 2017 23:14:33 +0000

I have been coding for [2] years.

You can find me on Twitter as @ike__jr

I live in [Ghana].

I work as a [Student]

I mostly program in these languages: Android.

I am currently learning more about [Python].

Nice to meet you.

DEV Community: Isaac

Using @JvmOverloads and @JvmStatic for Kotlin interoperability with Java

Tales of a Noob Dev[Part 1]

Hi, I'm Isaac.