What Is Kotlin?

Kotlin is a statically typed programming language developed by software company JetBrains. The language combines aspects of Scala and Groovy, two popular high-level programming languages but adds additional usability features by combining object-oriented and functional programming features with a more concise syntax than Java. Since its initial release, Kotlin has been used for front- and back-end development and is the preferred language for Android development.

How Does Kotlin Work?

Similar to Java and Scala, Kotlin is a statically-typed language, where checks on variable types are executed during compilation time. The main advantage of a statically-typed language is protection from runtime errors and faster execution times. This is because the compiler knows the exact data types that are in use and is able to produce optimized machine code. 

Other programming languages can be dynamically typed, where the compiler assigns a type to all the variables at runtime. A good example of a dynamically typed programming language is Python. In Python, programmers can write code faster because they don’t need to specify variable types every time the code runs, which translates into less verbose code. JavaScript is also a dynamically typed language so when we declare a variable we don’t need to specify its type.

As a statically typed language, Kotlin was initially designed to run on a Java virtual machine (JVM). However, it is also possible to compile Kotlin code to JavaScript. What’s more, Kotlin native allows programmers to compile Kotlin code to native binaries, which are executables created to run natively in a specific operating system and that can run without the JVM. Kotlin’s native binaries can run on macOS, iOS, tvOS, watchOS, Linux, Windows and Android NDK. Ultimately, Kotlin produces either Java bytecode, JavaScript-compatible code or native binaries based on the target platform.

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What Is Kotlin Used For?

Kotlin was originally developed with Android development in mind. Therefore, Android developers are increasingly adopting Kotlin and it’s one of Google’s official programming languages. Several large companies, including Netflix, Amazon, Uber, Foursquare and Coursera, have used or are currently using it as well. Kotlin can be useful in the following applications:

• Web development: Kotlin is used for frontend development with the Kotlin/JS library, which compiles Kotlin code with JavaScript. 
• Cross-platform development: Kotlin enables developers to share code with proprietary languages for iOS, desktop and web development.  
• Server-side development: Kotlin runs on the JVM and supports backend tools like Ktor and Coroutines for async processing, and is fully compatible with Java libraries.   
• Data Science: New libraries like KotlinDL and Krangl are making Kotlin useful for data science applications. The language also offers strong access to Java-based libraries for machine learning and analytics. 

Kotlin’s interoperability with the JVM makes it compatible with existing Java code and allows businesses to leverage existing Java ecosystems. This makes Kotlin a great solution for microservices. A microservice architecture is a software architectural style that structures a back-end application as a collection of services that are independently deployable and highly maintainable. DoorDash has a great case study of migration from monolithic architecture to microservices with Kotlin.

Kotlin's Language Features and Syntax

Kotlin offers a more modern and concise syntax than Java, making code easier to read and maintain. This is achieved by how it handles types. At the time of compilation, Kotlin can infer the type of a variable or expression, eliminating the need to explicitly declare it, like in Java. Kotlin also eliminates the need to write out several methods and functions. 

Take the following comparison as an example. Both print “Hello World” to console, but Kotlin is dramatically shorter and easier to read to the human eye.

// Java
public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

 In Kotlin the code reduces to this:

// Kotlin
fun main() {
    println("Hello, World")
}

What Are the Advantages of Kotlin?

In general, Kotlin is easier to understand than Java, which makes code easier to read and comprehend. A good example of this is immutability. An immutable object is one whose state is unchangeable after it’s been declared. In Kotlin, immutable variables are declared as val, whereas mutable variables are declared as var. 

Using val in our code makes it easier to read and incredibly compact. Using the example above, we can see how we don’t need to declare the variable type in the beginning of a HashMap.

// Java
HashMap<String, String> map = new HashMap<String, String>();

// Kotlin
val map = mutableMapOf<String, String>()

Instead, we use val to declare this as an immutable variable. As an immutable variable a HashMap cannot become anything rather than a HashMap, but it can be used as usual to store key value pairs.

Another major advantage of Kotlin is its safety features for null values. Null pointer exceptions are one of the most prevalent problems in Java projects. NullPointerException is an error generated in Java when the code tries to access an object with a null reference. To solve this in Java, programmers use preventive techniques like doing null checks before referencing an object’s methods or properties. Developers can end up doing null checks all over their code without actually understanding the possible states of an object. 

Kotlin is considered null safe. This means it’s not possible to assign a null value to a variable. Kotlin distinguishes between references that can hold a null state and those that cannot. For example, we can declare a regular variable type string as either capable of holding null values or not.

var message_a: String = “Hello World” // This is a non-null variable by default.

message_a = null // This would create a compilation error.

var message_b: String? = “Hello World” // Nullable string which can be set to null.

message_b = null // This would be ok.

Kotlin offers several concise ways to handle nullable variables. One of them is using safe calls. Using the example above, we can safe call our nullable string as follows:

println(message_a.length) // No safe call is required.

println(message_b?.length) // Returns length if not null, otherwise returns null.

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Disadvantages of Kotlin

Kotlin is a higher-level language than Java, with more built-in features and abstraction. But this comes at a cost during clean builds, when a project is compiled from scratch. Kotlin tends to be slower than Java in this aspect. However, in terms of incremental builds, Kotlin is definitely faster than Java. Also called a regular build, incremental builds compile only the source files that need to be compiled on that particular run based on the latest changes. This is distinct from a clean build, which deletes everything and recompiles all the source files again from scratch. 

There are two general principles behind Kotlin’s speed for incremental builds: compile avoidance and incremental compilation. The key idea behind these principles is to only recompile affected modules (compile avoidance) and affected files (incremental compilation).

A Brief History of Kotlin and Future Releases

Kotlin was developed by JetBrains and led by Dmitry Jemerov, who began work on the programming language in 2010. He wanted a modern language that offered features Java didn’t have, and found those features in Scala and Groovy, which are other programming languages that could run on the JVM (C# and JavaScript also influenced Jemerov). In Scala, basic types like numbers and booleans are not represented as primitive types; instead they are objects. Also, Scala provides more concise code. Meanwhile, Groovy provides a concise and efficient way of handling variable nullability. That said, Scala and Groovy both take a long time to compile so Jemerov wanted a new language that had all the benefits of Scala and Groovy but compiled as fast as Java. 

Kotlin was officially released in February 2016. Adoption accelerated quickly thereafter, with Google officially supporting the language for Android development in May 2017. 

JetBrains continues to develop Kotlin. As of 2025, the current stable version is Kotlin 2x, which introduces features like context receivers, improved multiplatform support and a refactored compiler for better performance and error handling. Looking ahead, future Kotlin releases will focus on finalizing K2 compiler adoption and continued improvements for better cross-platform development. 

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