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Which one should you choose for your next project Kotlin vs Java?

Which one should you choose for your next project Kotlin vs Java?
Android APP 0 Comments March 07, 2024

If you are a developer looking for a modern, concise and interoperable programming language, you might have heard of Kotlin. Kotlin is a statically typed language that runs on the Java Virtual Machine (JVM) and can also be compiled to JavaScript or native code. Kotlin was developed by JetBrains, the company behind popular IDEs like IntelliJ IDEA, and was officially endorsed by Google as a preferred language for Android development in 2019.

But how does Kotlin compare to Java, the most widely used and mature programming language in the world? Java has been around since 1995 and has a huge community of developers and libraries. Java is also known for its reliability, performance and compatibility across different platforms.

In this blog post, we will explore some of the main differences and similarities between Kotlin and Java, and help you decide which one is better suited for your next project.


One of the most noticeable differences between Kotlin and Java is the syntax. Kotlin has a more concise and expressive syntax than Java, which means you can write less code with fewer errors. For example, Kotlin supports type inference, which allows you to omit the type declaration when the compiler can infer it from the context. Kotlin also has null safety built into the language, which helps you avoid the infamous NullPointerException that plagues many Java applications.

Here is a simple example of how to define a variable in Kotlin and Java:

// Kotlin

val name = “Alice” // immutable variable

var age = 25 // mutable variable

// Java

final String name = “Alice”; // immutable variable

int age = 25; // mutable variable

As you can see, Kotlin does not require semicolons at the end of statements, nor does it require specifying the type of the variable if it can be inferred. The val keyword in Kotlin corresponds to the final keyword in Java, which means the variable cannot be reassigned once initialized. The var keyword in Kotlin allows you to declare a mutable variable that can be changed later.

Another example of how Kotlin simplifies the syntax is the way it handles classes and functions. In Kotlin, you can define a class with a primary constructor and some properties in a single line:

// Kotlin

class Person(val name: String, var age: Int)

// Java

public class Person {

    private final String name;

    private int age;

    public Person(String name, int age) {

        this.name = name;

        this.age = age;


    public String getName() {

        return name;


    public int getAge() {

        return age;


    public void setAge(int age) {

        this.age = age;



In Java, you have to write a lot more boilerplate code to achieve the same functionality. You have to define private fields, a public constructor, and getter and setter methods for each property. In Kotlin, all of this is generated automatically by the compiler from the primary constructor parameters. You can also define secondary constructors, init blocks, and custom getter and setter logic if needed.

Kotlin also has some features that Java does not have, such as extension functions, data classes, sealed classes, object declarations, coroutines, and more. These features can help you write more concise and idiomatic code that is easier to read and maintain.


Another important aspect to consider when comparing Kotlin and Java is the interoperability between them. As mentioned earlier, Kotlin runs on the JVM and can interoperate with Java code seamlessly. This means you can use any existing Java library or framework in your Kotlin project, and vice versa. You can also mix Kotlin and Java files in the same project, and the code will work together without any problems.

This interoperability is possible because Kotlin uses the same class file format as Java, and matches the Java types to the corresponding Kotlin types. For example, the primitive types in Java, such as int, long, boolean, etc., are mapped to the equivalent built-in types in Kotlin with the same name. The reference types in Java, such as String, List, Map, etc., are mapped to nullable types in Kotlin with a question mark at the end, such as String?, List?, Map?, etc. This indicates that these types can hold a null value, unlike in Java where nullability is not part of the type system.

To facilitate the interoperability between Kotlin and Java, there are some annotations that you can use in your code, such as @JvmName,




etc. These annotations instruct the compiler how to generate the bytecode for your Kotlin code so that it can be easily called from Java code. For example,

allows you to specify a different name for a function or property than its actual name in Kotlin. This can be useful if you want to avoid conflicts with existing Java methods or follow different naming conventions.


Another factor that might influence your choice between Kotlin and Java is the performance of the code. In general, Kotlin and Java have comparable performance, as they both run on the same JVM. However, there are some cases where Kotlin can offer some benefits or drawbacks in terms of performance.

One of the benefits of Kotlin is that it supports inline functions, which means the function body is copied to the call site instead of being invoked as a normal function. This can help reduce the overhead of function calls, especially for higher-order functions that take other functions as parameters, such as lambdas. Inline functions can also enable some optimizations, such as eliminating boxing and unboxing of primitive types or enabling reified type parameters.

One of the drawbacks of Kotlin is that it relies on some additional runtime libraries that are not present in Java. These libraries add some functionality and compatibility support for Kotlin, such as the kotlin-stdlib, kotlin-reflect, and kotlin-coroutines libraries. Depending on how you use Kotlin, these libraries can increase the size of your application and the memory usage at runtime. However, you can use tools such as ProGuard or R8 to remove unused code and reduce the size of the libraries.

Another potential drawback of Kotlin is that some features, such as null safety and extension functions, require more bytecode instructions than the equivalent Java code. This can affect the method count limit of your application, which is 65536 by default. However, this limit can be increased by using multidex or by targeting newer versions of Android.

What are some Kotlin libraries?

If you are interested in using Kotlin for your next project, you might want to check out some of the popular and useful libraries that are available for Kotlin. Here are some examples:

  • Ktor: A framework for building asynchronous servers and clients in connected systems using Kotlin.
  • Anko: A library that provides a set of DSLs for creating Android UIs and SQLite databases in Kotlin.
  • Arrow: A library that provides functional programming features such as typeclasses, optics, effects, and more in Kotlin.
  • kotlinx. coroutines: A library that provides a way to write asynchronous and non-blocking code using coroutines in Kotlin.
  • Exposed: A library that provides a DSL for working with SQL databases in Kotlin.
  • Fuel: A library that provides a simple and easy-to-use HTTP client for Kotlin.
  • Klaxon: A library that provides a way to parse JSON data in Kotlin.
  • Koin: A lightweight dependency injection framework for Kotlin.
  • MockK: A mocking library for Kotlin that supports mocking regular classes, objects, interfaces, and more.
  • Spek: A specification framework for Kotlin that allows you to write test cases in a descriptive and readable way.

These are just some of the many libraries that you can use with Kotlin. You can find more libraries on awesome-kotlin, a curated list of awesome Kotlin resources.


In conclusion, Kotlin and Java are both powerful and mature programming languages that can be used for a variety of projects. Kotlin offers some advantages over Java in terms of syntax, features, and interoperability, while Java has a larger and more established community and ecosystem. Ultimately, the choice between Kotlin and Java depends on your personal preference, project requirements, and team skills. You can also use both languages in the same project and enjoy the best of both worlds.

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