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Java vs. Kotlin – A Guide for Modern Developers

In the world of Android development, a heated debate often arises: Java vs. Kotlin.

While both languages compile to run on the Java Virtual Machine, their strengths differ. Choosing between Java and Kotlin hinges on your project’s needs.

Java, the long-standing champion, boasts an extensive library collection and mature ecosystem. Kotlin, the energetic challenger, brings a modern twist with its concise syntax and focus on developer experience.

This article explores the advantages and disadvantages of different programming languages to help you choose the best fit

History of Kotlin

In 2010, JetBrains, a software development company known for creating IntelliJ IDEA, started developing Kotlin under the name Project Kotlin.

Kotlin aimed to be easier to write and read than Java while integrate seamlessly with existing Java projects.

In February 2012, JetBrains open-sourced Kotlin under the Apache 2 license, allowing for wider community involvement and development.

Early on, Kotlin started gaining traction among developers who appreciated its concise syntax, null safety features, and interoperability with Java.

The first official stable release of Kotlin arrived in February 2016(Kotlin 1.0). This marked a significant milestone and ensured backwards compatibility for future versions.

In 2017, Google announced Kotlin as a first-class language for Android app development alongside Java. This significantly boosted Kotlin’s adoption within the Android development community.

Since then, Kotlin has seen continuous growth in popularity across various development domains beyond Android, including web development, server-side development, and data analysis.

Kotlin’s Syntax

Kotlin’s “static sugar” refers to features in the language that make code more concise, readable, and easier to maintain, without fundamentally changing its functionality. These features don’t alter the underlying behavior of the program but improve the developer experience by reducing boilerplate code and making the logic more explicit.

Type Inference

Type mismatches can lead to runtime errors that can be challenging to identify and fix. Type inference helps catch these errors early in the development process, during compilation. This improves code quality and prevents potential issues in production environments.

While both Java and Kotlin offer some level of type inference, Kotlin’s type inference is generally considered more powerful and flexible.

When variable and expression types are explicit in the code, it becomes easier for developers to understand the purpose of each piece of code and make modifications when needed

Kotlin can often infer the data type of a variable based on its initialization value. This eliminates the need for explicit type declarations in many cases, making code less verbose.

Java’s type inference is primarily limited to local variable declarations where the compiler can determine the type based on the assigned value.

Kotlin uses type inference extensively for various scenarios, including local variables, function return types, complex expressions, collections, and even lambdas.

Let’s take an example of Kotlins when{} block is essentially an advanced form of the switch-case statement known from Java.

val grade = when (score) {

in 90..100 -> "A"

in 80..89 -> "B"

else -> "C"

Type is inferred as String based on the possible outcomes within the when expression (all String literals).

Lambda Expressions with Collections:

val numbers = listOf(1, 2, 3, 4, 5)

val doubledNumbers = { number -> number * 2 }
Type inference considers the lambda parameter (number) as Int and the return value as the result of the multiplication (also Int)

val evenNumbers = numbers.filter { it % 2 == 0 }
Type inference considers the lambda parameter (it) as Int based on the collection content (numbers) and the operation involving Ints

In these examples, the compiler analyzes the entire expression, including nested structures and conditional logic, to infer the most appropriate type for the final result. This allows for concise and expressive code without the need for explicit type declarations in many cases.

Default Parameters

Kotlin allows functions to have default parameter values. This reduces the need for extensive conditional checks within functions to handle cases where specific parameters might be omitted during the call.

For example:

fun greet(name: String, title: String = "Mr./Ms.") {

println("Hello, $title $name!")

Null Safety

Kotlin enforces null safety by design. This means variables must be explicitly declared as nullable if they might hold no value. This eliminates the need for frequent null checks and conditional statements typically used in Java to avoid null pointer exceptions.

Java does offer Optional as a way to handle null values, but it’s not quite the same as Kotlin’s enforced null safety.

While Optional helps manage null values, it requires developers to explicitly handle the empty state using methods like isPresent() and get(). This can add boilerplate code and still requires checking for null within these methods.

For example:

Java (using Optional)

Optional name = getCustomerName();

if (name.isPresent()) {

System.out.println("Hello, " + name.get() + "!");

} else {

System.out.println("Customer name is unknown.");

Kotlin Only executes if name is not null

val name: String? = getCustomerName()

name?.let { println("Hello, $it!") }
Data Classes

Data classes in Kotlin are a special type of class designed to hold data and provide a concise way to represent them. They are particularly useful for situations where you need to create classes that primarily focus on storing and manipulating data. Here are some key characteristics of data classes in Kotlin:

When you define a data class, the Kotlin compiler automatically generates several methods for you, eliminating the need to write them yourself

These methods include:

toString(): Returns a human-readable string representation of the object, including its class name and property values.
equals(): Compares two objects for equality based on their property values.
hashCode(): Generates a hash code for the object, which is useful for storing data in collections that use hash-based algorithms.
copy(): Creates a new copy of the object with potentially modified properties.
Data classes are declared using the data keyword followed by the class name and its properties within curly braces.

data class Person(val name: String, val age: Int)
Java records (introduced in Java 16) offer a way to define classes that primarily focus on holding data. But there are differences:

Kotlin Data Classes: Mutable by default, but can be made immutable by declaring properties as val (read-only) or using copy constructors.
Java Records: All fields are final and immutable by design.
Kotlin Data Classes: Enforces null safety by design. Variables must be explicitly declared as nullable (String?) if they might hold no value.
Java Records: No built-in null safety. Variables can be null without explicit declaration.
Kotlin Data Classes: Allow defining default values for properties within the class constructor.
Java Records: Don’t support default parameter values.
Modern Features

Kotlin extension functions are a powerful feature that allows you to add new functionality to existing classes without modifying the original class itself.

You can extend functionality without altering the original class definition, which is beneficial for working with third-party libraries or frameworks.

fun String.reversed(): String {

return this.reversed() // Using the built-in reversed function on the String receiver


val message = "Hello, world!"

val reversedMessage = message.reversed()

println(reversedMessage) // Output: !dlrow ,olleH
Kotlin extension functions are a versatile tool for extending the capabilities of existing classes and promoting clean, maintainable, and expressive code.

Kotlin lambdas, also known as anonymous functions, are a powerful feature that allows you to define concise blocks of code that can be passed around like values.

While both Java and Kotlin support lambdas, Kotlin lambdas offer a more concise, readable, and potentially more powerful approach due to their additional features.

Here are some key characteristics of Kotlin lambdas:

  • Improved Code Readability: Lambdas can make code more concise and readable, especially when dealing with short, well-defined functions.

  • Higher-Order Functions: Kotlin allows functions to take other functions as arguments (higher-order functions). Lambdas are perfect for implementing this concept and expressing logic in a more functional style.

  • Collections and Iterations: Lambdas are heavily used with collections in Kotlin for filtering, mapping, and other operations. They provide a clean way to define the logic for these operations within the context of the collection.

  • Receiver functions: Can directly access the object they’re operating on.

val sum = { x: Int, y: Int -> x + y } // Lambda with two arguments and a return statement

val result = sum(5, 3) // Calling the lambda and passing arguments

println(result) // Output: 8
Java does offer lambdas (anonymous functions), but there are some key differences in their syntax, features, and usage:

A lambda with a receiver function is similar to a regular lambda, but with an additional object specified before the curly braces using an arrow ->. This object becomes the receiver for the lambda body.


objectName.methodName { receiver ->

// Lambda body with access to "receiver"

In contrast, Java lambdas don’t have built-in receiver support.

You might need to pass the object as an argument explicitly within the lambda, making the code less concise and potentially less readable.


Kotlin offers a more concise syntax compared to Java. This means you can write the same functionality with fewer lines of code in Kotlin, making it easier to read and maintain.

Java is notorious for NullPointerException errors, which can cause crashes. Kotlin enforces null safety at compile time, preventing these errors and making your code more robust.

Kotlin code can seamlessly integrate with existing Java code, making it easy to adopt Kotlin gradually within a project.

On the other side, if you’re working on a massive Java codebase, transitioning entirely to Kotlin might be a significant undertaking.

Java’s maturity and vast ecosystem of libraries and frameworks can be advantageous in such cases.Overall, both Java and Kotlin are powerful languages. The choice between them depends on your specific project requirements, team expertise, and desired level of code conciseness and safety features.

Previously published On My Blog where you can find more articles about Java and Spring Framework

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