JvmField Annotation in Kotlin

I am Amit Shekhar, Co-Founder @ Outcome School, I have taught and mentored many developers, and their efforts landed them high-paying tech jobs, helped many tech companies in solving their unique problems, and created many open-source libraries being used by top companies. I am passionate about sharing knowledge through open-source, blogs, and videos.

In this blog, we are going to learn about JvmField annotation in Kotlin.

This article was originally published at Outcome School.

The best thing about Kotlin is that it is designed with Java interoperability in mind. It means that the existing Java code can be called from Kotlin, and also the Kotlin code can be called from Java. Both ways are supported.

Today, we will focus on calling the Kotlin code from Java as we want to learn about the JvmField annotation.

The best way to learn this is by taking an example.

Assume that we have a data class Session in Kotlin as below:

data class Session(val name: String, val date: Date = Date())

We can create the object and get the name in Kotlin as below:

val session = Session("Session", Date())
val name = session.name

It works as expected.

But, when we create the object and get the name from Java as below:

Session session = new Session("Session", new Date());
String name = session.name; // compilation error

It will not compile. From Java, we will have to use the getter method as below:

Session session = new Session("Session", new Date());
String name = session.getName();

Now, it will compile and work as expected.

So, the question is: Can we use it without the getter method as we use in Kotlin?

The answer is yes. By using the JvmField annotation. So, if we want a field to be used as a normal field and not as a getter or setter then we will have to tell the compiler not to generate any getter and setter for the same and expose it as a field by using the @JvmField annotation.

Let’s update our data class Session as below:

data class Session(@JvmField val name: String, val date: Date = Date())

Notice that we have used @JvmField over the field name to instruct the Kotlin compiler not to generate any getter and setter for the same and expose it as a field.

Now, if we create the object and get the name from Java as below:

Session session = new Session("Session", new Date());
String name = session.name;

Now, it works perfectly. It will compile as expected as the Kotlin compiler will not generate any getter and setter for the same and expose it as a field.

This is how we can use the JvmField annotation in Kotlin.

Master Kotlin Coroutines from here: Mastering Kotlin Coroutines

That’s it for now.

Thanks

Amit Shekhar

Co-Founder @ Outcome School

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