Kotlin vs Java: Detailed Comparison Guide

The selection of the right programming language is an important step in building successful software applications. For Android and modern application development, Java and Kotlin are two of the most widely used options. Java has been trusted for decades in web, mobile, and enterprise application development because of its stability and strong ecosystem. Kotlin, on the other hand, is a modern language officially supported by Google, designed to improve developer productivity with cleaner syntax and advanced features. 

Both languages are powerful, reliable, and supported by large developer communities, but there are considerable differences between them. Understanding these differences can help businesses and developers select the best technology for their project needs. Many businesses also consult a professional Java development company to understand which language aligns with their long-term goals and technical requirements. 

In this blog, we will explore the major differences between Kotlin and Java, including their features, advantages, limitations, and use case scenarios to help you make better decisions.

1. Kotlin

Kotlin is an open-source, object-oriented, and functional programming language developed by the company called JetBrains. It is developed with the vision to improve the way developers write code in Java without compromising security and performance. It runs on the Java Virtual Machine or JVM, which means it can work with existing Java libraries and frameworks while offering a cleaner and more concise syntax. Kotlin reduces unnecessary code and makes programs easier to read and maintain.

1.1 History of Kotlin

These are some of the most significant events in Kotlin’s past:

• In 2016, JetBrains released the first official and stable Kotlin version, Kotlin 1.0.
• Kotlin 1.1, released in 2017, included JavaScript support, coroutines, and improved type inference. 
• In 2019, Google recommended Kotlin as the preferred language for Android app development.
• Kotlin 1.4 in 2020 came with major upgrades, such as a faster front-end compiler, SAM conversions, and language polish.
• Kotlin 1.6 in 2021 brought type inference improvement, stabilized standard builder inference, and included significant concurrency updates. 
• In 2024, Kotlin 2.0 marked the official release of the K2 compiler, providing 94% faster compilation speeds in certain projects.

1.2 Unique Features of Kotlin

The following features of Kotlin make it a developer-friendly programming language: 

• Open-source: Kotlin is an open-source language, so anyone can use and improve it freely. It also offers a simple one-click tool to quickly convert Java code into Kotlin. This makes it easier for developers to switch and start building applications efficiently.
• Java Interoperability: Kotlin works smoothly with Java, allowing both languages to be used together in the same project. Developers can reuse Java libraries and even convert existing code easily, making it simple to switch without rebuilding everything from scratch.
• Data Classes: Kotlin simplifies data classes by removing much of the extra boilerplate code that Java requires. This allows developers to define data structures more quickly and clearly. This reduces errors and makes the code shorter, cleaner, and easier to manage.
• Null Safety: Kotlin prevents null values by default, reducing crashes caused by null pointer errors, which are common in Java applications. Therefore, developers must handle null cases explicitly to develop reliable and safe applications.

1.3 Pros and Cons of Kotlin

Let us discuss some of the advantages and disadvantages of Kotlin:

Benefits of Using Kotlin

• Easy to learn: Kotlin is not developed to replace Java, but to improve the efficiency of Java developers while coding. Experienced Java developers can still use their Java programming skills in Kotlin projects, making it easier for them to adapt and build better applications with improved features and efficiency. 
• Reliable: Kotlin has existed since 2011 and has undergone multiple rounds of Alpha and Beta tests since then, before the release of its final version. It is built on a mature ecosystem, increasing the stability of applications developed using it.
• Easy to maintain: Kotlin allows developers to work smoothly with popular development software like Android Studio and other SDKs. Therefore, they can work in familiar environments, increasing their speed and efficiency without needing to learn completely new tools or workflows.

Disadvantages of Kotlin

The following are the disadvantages of Kotlin:

• Slow compilation: Kotlin may feel slower during compilation, especially when developing a new build. This happens because the compiler handles complex features like type checks and optimizations. It may also use more memory, but incremental builds usually run faster after the initial process.
• Limited learning resources: Kotlin is growing quickly, but its community is still smaller compared to Java. As a result, fewer learning resources and experts are available. It’s difficult, especially for beginners, to find help, guidance, or solutions during development when compared to more established languages.
• Dependency on Tools: Kotlin relies heavily on tools such as Gradle and specific IDEs, which can increase development complexity. It may result in slower Builds, incompatible updates leading to crashes, and complex debugging issues.

2. Java

Java was created by James Gosling of Sun Microsystems Inc. in 1995. In 2009, Oracle Corporation acquired the rights to the language. Java is considered a high-level language since it simplifies and streamlines the process of writing, compiling, and debugging code.

Java is an object-oriented, statically typed programming language that uses classes and follows the “write once, run anywhere” approach. Applications written in Java can be executed on any computer with a Java Virtual Machine (JVM) installed. Java code is very popular and easy to grasp since it is so close to C and C++. According to a recent Stack Overflow research report, Java is among the most used programming languages by developers.

2.1 History of Java

Java was developed by James Gosling and the “Green Team” at Sun Microsystems in 1991. It was initially named “Oak”.

• In 1994, a web development language named “Java” was introduced as a shift from a language for embedded systems to a language for the internet.
• Java 1.0 was officially released in 1995.
• In 2006, OpenJDK was released, and Java became an open-source language.
• In 2014, support for functional programming was added through Lambdas, Stream API, and Optional. 
• 2017 saw the introduction of Module Syster and the var keyword.
• In 2021, Sealed Classes, Pattern Matching for instance of features were added
• In September 2023, modern features like string Templates, Sequenced Collections, Generational ZGC, Record Patterns, Pattern Matching for switch, etc., were added.

2.2 Unique Features of Java

The following are the characteristic features of the Java programming language: 

1. Object-Oriented: Java is a complete object-oriented programming language where real-world entities are denoted as objects and objects with similar behaviour are grouped together in a class. Hence, Java supports all four fundamental OOP principles, i.e., encapsulation, abstraction, polymorphism, and inheritance. 
2. Platform Independent: The Java compiler converts the source code into platform-independent bytecode that can run on any JVM installed operating system. Therefore, there’s no need to write the same code multiple times for different machines. Thus, Java follows the “Write Once, Run Anywhere” (WORA) principle. 
3. Focuses on Security: Java offers strong security through multiple built-in protections. Java programs run inside a controlled virtual environment (JVM), limiting harmful actions. Features like bytecode verification by the JVM, class loading by the class loader, and access control help prevent unauthorized actions, increasing the security and reliability of Java applications.
4. Multithreaded: Java has built-in support for multithreading, such as the Thread class, runnable interface, and java.util.concurrent package. This allows developers to execute several tasks in parallel, optimizing CPU utilization and improving system performance.

2.3 Pros and Cons of Java

Below is a list of the advantages and disadvantages of the Java programming language.

Benefits of Java

• Community Support: Java has been in use for more than thirty years, resulting in a well-established ecosystem contributing in the form of libraries, frameworks, tutorials, tools, documentation, and online support. Hence, it prevents developers from getting stuck in the midst of development. 
• Automatic Memory Management: Java manages memory automatically using a garbage collection process, which removes unused objects from the system. This reduces the chances of memory leaks and errors. Developers do not need to handle memory manually, making applications more stable and easier to maintain over time.
• Dynamic: Java loads classes and libraries only when they are needed, instead of loading everything at the same time. This makes programs more efficient and flexible. It also supports runtime linking and updates, allowing applications to change or expand without restarting completely.

Disadvantages of Java

• Verbose Syntax: Java often uses lengthy, detailed syntax, which can be difficult to remember and slow down coding. The huge amount of boilerplate code increases the development time, which can be a drawback for projects requiring quick release.
• Poor GUI: Creating graphical interfaces in Java can be difficult because its GUI Builder and Java GUI frameworks are not very advanced and often produce inconsistent designs. These limitations make it harder to build complex, modern desktop applications with a native look and smooth user experience.
• High memory consumption: Java often consumes more memory than languages like C or C++. Its automatic garbage collection and runtime system can slow performance and cause sudden delays, especially for applications with limited memory that require fast and efficient execution.

3. Difference Between Kotlin and Java

We’ll now understand the differences between Java and Kotlin by comparing them against the following twelve parameters:

3.1 Null Safety

• Java: NullPointerException is one of the most common runtime exceptions during the execution of Java programs, as variables can hold null values, which can lead to runtime exceptions if not handled properly. Although tools and practices exist to reduce such issues, developers still require manual effort and careful attention during coding.
• Kotlin: Kotlin does not allow null values to be assigned to variables or objects. Attempting to do so results in compile-time errors. Thus, Kotlin does not throw any NullPointerExceptions. A null value cannot be assigned to a variable unless the programmer marks it as nullable. One must insert a punctuation mark to do this:

  val number: Int? = null

  val number: Int? = null

3.2 Extension Functions

• Java: There is no extension function in Java that coders can directly use to extend the functionality of any class into another class. They need to create a new class and inherit the capabilities of the parent class to extend the functionality of an existing class.

  class derived-class extends base-class { //methods and fields }   class derived-class extends base-class { //methods and fields }

  class derived-class extends base-class { //methods and fields } class derived-class extends base-class { //methods and fields }

• Kotlin: Kotlin doesn’t need developers to use inheritance to extend class functionalities. The programmer must prefix the name of the class being extended with the name of the new function using the “.” notation to define an extension function.

  open class baseClass (x:Int ) { .......... } class derivedClass(x:Int) : baseClass(x) { ........... }   open class baseClass (x:Int ) { .......... } class derivedClass(x:Int) : baseClass(x) { ........... }

  open class baseClass (x:Int ) { .......... } class derivedClass(x:Int) : baseClass(x) { ........... } open class baseClass (x:Int ) { .......... } class derivedClass(x:Int) : baseClass(x) { ........... }

See how to convert an auxiliary function in Java to an extension function in Kotlin

So you've created an auxiliary function, Java style, and then remembered that in Kotlin you can define it as an extension. No problem! Convert a parameter to a receiver of an extension function automatically. The reverse action also works: pic.twitter.com/x6JzuHpBfL

— Kotlin by JetBrains (@kotlin) September 15, 2020

3.3 Syntax

• Java: Writing Java code often feels lengthy because developers must clearly declare data types and structure everything inside classes. This leads to extra boilerplate and repeated patterns. They need to write multiple lines even for performing simple tasks, which can reduce readability and slow down development, especially for beginners or when building smaller features.
• Kotlin: Kotlin offers a concise and cleaner way to write code, reducing unnecessary repetition. It can automatically detect data types, so developers need to write less to achieve the same results. Features like data classes and optional semicolons make programs easier to read, helping developers focus more on logic rather than on writing syntax properly.

3.4 Coroutines Support

• Java: Long-running tasks in Java are handled using threads, callbacks, and other concurrency mechanisms, which can become complex to manage. Developers often need to carefully control execution through background threads to avoid blocking the main thread. There has been an improvement in Java concurrency with newer updates like virtual threads, but it’s still quite hard to manage in practice.
• Kotlin: Kotlin offers a better and more accessible alternative to create multiple threads to manage asynchronous tasks, which is known as coroutines. Coroutines don’t need a stack, so programmers can pause the running program and resume it later. This enables asynchronous code that appears synchronous while avoiding blocking. For this reason, coroutines prevent the need for an excessive number of threads, each of which the developer must eventually manage. They also provide a more precise and streamlined solution than Java.

3.5 Data Classes

• Java: Creating a class in Java requires defining multiple things, such as variables, constructors, and methods like getters, setters, equals(), and toString() to store data. This process can be repetitive and time-consuming, resulting in longer and harder-to-maintain code, especially when working with simple data-focused classes.
• Kotlin: Kotlin provides a much easier and shorter way to create classes and store data through data classes. You need to use a simple keyword, “data,” and many common methods are automatically generated by the compiler. This reduces repetitive coding and keeps the program clean, allowing developers to focus more on logic rather than writing standard supporting code.

3.6 Checked Exceptions

• Java: Checked exceptions in Java must be handled during development, either by catching them with try-catch blocks or declaring them with throws in method signatures. This rule increases code length and complexity but ensures that potential errors are considered early, reducing the chance of unexpected failures during program execution.
• Kotlin: In Kotlin, there are no checked exceptions, so developers are not forced to handle errors explicitly. This keeps the code shorter and easier to read. However, it also means programmers must be more cautious, as missed exception handling can lead to runtime issues if not properly managed.

3.7 Functional Programming: Higher-Order Functions and Lambdas

• Java: Java is mainly an object-oriented programming language, which can feel restrictive at times. However, it has gradually added features from functional programming. Since Java 8, developers can use Java lambda expressions to write shorter, more flexible code. These lambdas can be treated as objects, returned from methods, and used to create a more functional programming style.
• Kotlin: Kotlin combines object-oriented and functional programming paradigms, giving developers the flexibility to write code. It treats functions as important elements, allowing them to be passed, returned, and reused easily. The language also supports lambda expressions and anonymous functions, enabling concise and expressive coding, especially when working with calculations using mathematical functions.

Example of lambda expression: 

In Kotlin:

val sum: (Int, Int) -> Int = { a: Int, b: Int -> a + b }
 
val sum: (Int, Int) -> Int = { a: Int, b: Int -> a + b }

In Java:

int sum = (a,b) -> (a+b);
int sum = (a,b) -> (a+b);

3.8 Primitive Types

• Java: The eight primitive data types in Java, such as int, char, float, boolean, double, etc., are not objects. They are stored directly in memory and are more efficient than objects, but cannot use the methods of a class. When needed, developers must manually use wrapper classes to treat them as objects, which increases the programming overhead.
• Kotlin: In Kotlin, there are no separate primitive types, as everything is treated as an object, creating a more uniform and simple type system. Developers do not need to manage wrappers manually, which makes coding easier and more consistent.

3.9 Public Fields

• Java: Java allows fields to be declared as public, which means they can be accessed and changed directly from outside the class. However, this creates tight coupling between code and internal structure, making future changes risky. As a result, Java developers usually prefer private fields with getter and setter methods to protect data and improve maintainability.
• Kotlin: Kotlin does not have a public fields concept. Instead, it makes use of properties that have built-in getter and setter methods to protect internal data while allowing controlled access. As a result, Kotlin code is more structured and safer when changes are made.

3.10 Implicit Conversions

• Java: In Java, implicit conversion is allowed between compatible primitive data types. A small numeric type value automatically converts into a larger value, such as int to long and float to double. However, implicit conversion is not possible between incompatible types, such as from string to int, without an inheritance relationship or interface implementation. This makes programming easier by reducing manual casting.
• Kotlin: In Kotlin, implicit conversion between different numeric types is not supported. Even when converting from smaller to larger types, developers must explicitly use functions like toInt() or toDouble(). This strict rule improves type safety and makes type changes clear in code. It helps avoid hidden errors that may occur due to automatic or unexpected conversions.

Example: 

Java:

public class myClass{ 
   public static void main(String args[]) { 
      int a = 81; 
      System.out.println("Integer value : "+a);   // Implicit Type Casting 
      long b = a;                                // Int val to long data type                                            
   }
}

But in Kotlin, we can not directly assign an integer value to the long data type.

var x = 100
var x = 100
var y : Long = x       
// Compiler error
// Type mismatch: inferred type is Int but Long was expected
 
 
var x = 100
var y: Long = x.toLong()     
// compiles successfully

4. Tabular Comparison: Kotlin vs Java

5. Kotlin Vs. Java: Which is Better?

The choice between Java and Kotlin depends on the project requirements, as both languages have their associated benefits and limitations. Let’s discuss some of the scenarios where you can decide which language is the most appropriate to choose:

5.1 When to Choose Java?

The following are the scenarios where Java is the most suitable choice:

• You have a team of experienced Java developers, and the project deadline is near, so you can’t spend time on training them to switch to Kotlin.
• You already have legacy software using Java code, which makes migration difficult and time-consuming.
• The application demands wider cross-platform support, which is possible in Java due to its huge libraries and tooling support. 

5.2 When to Choose Kotlin?

The following are the scenarios where Kotlin is the most suitable choice:

• If the application is complex and there’s a need to use coroutines with less boilerplate code. 
• If the project is going to be a long-term project, then it must be easier to maintain the codebase.
• Your existing project is using Java, but you want to introduce changes as per the future development trends. Kotlin can be introduced here gradually.

6. Final Thoughts

Java and Kotlin are modern-day programming languages with a bright future. Java has been around for decades and hence has evolved over a period of time. It has formed a well-established community that provides strong support in the development process. Kotlin is quite new, and it was introduced to address the shortcomings of Java programming without much difficulty in migration. Project goals and requirements play a major role in deciding the programming language, which ultimately determines the success of your project. Therefore, thoroughly evaluate them with your team and move forward practically.