Kotlin vs Java: Which One is Right for Your Project?

Java has been the foundation of Android development, enterprise software, and large-scale backend systems for years. However, with the development of technology, developers are seeking cleaner, more efficient code, so Kotlin has appeared as a modern alternative that solves many of Java’s long-standing issues.

According to the latest programming language rankings, Java holds a significant 30.3% usage rate, making it one of the most widely used languages. Kotlin, though still growing, stands at 9.4%, reflecting its increasing adoption.

So, Kotlin vs Java: which is better? The debate between Kotlin and Java continues as developers and businesses question whether to switch to Kotlin or if Java will remain a key player in software development. This article will show the key differences, and highlight their strengths, weaknesses, as well as the best use cases, helping you decide which language is right for your project.

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What is Kotlin?

Kotlin is a modern language operating on the Java Virtual Machine (JVM). It is popular for its concise syntax, improved safety features, and seamless Java compatibility. Google officially announced Kotlin for Android development in 2017, recognizing it as the top choice for mobile app developers. For years, Kotlin has also expanded to backend development, web development, and cross-platform projects. 

Key Characteristics of Kotlin

• Make the development process faster with cleaner, less repetitive code.
• Built-in null safety to reduce crashes from NullPointerException
• Work with Java smoothly.
• Help designers work on both multi-threaded applications and background workload management.
• Include higher-order functions together with lambdas and extension functions.

What is Java?

Java is a general-purpose, object-oriented programming language created in 1995 by Sun Microsystems (now owned by Oracle). It is widely used in enterprise applications, Android development, and backend systems. This language has its biggest strength “write once, run anywhere” (WORA) which allows developers to create applications that can run on any platform with a Java Virtual Machine (JVM). 

Key Features of Java

• Java Virtual Machines, the only requirement for running Java code on any system.
• Supports encapsulation, inheritance, and polymorphism for structured coding.
• Uses garbage collection to handle system memory efficiently.
• Supports concurrent execution for better performance.
• Has a vast library collection, frameworks like Spring, and extensive community support.

>> Read more: Top 12 Java Project Ideas for Beginners to Professionals

Key Differences Between Kotlin and Java For Developers

Null Safety

Kotlin has the built-in null safety feature which assists developers in preventing null-related crashes from happening. Variables will only become null if it is explicitly marked, forcing developers to handle null values properly. This reduces NullPointerException (NPE) errors that frequently occur in Java applications. 

In contrast, Java does not have built-in null safety, so developers must manually check for null values to prevent crashes. This means developers have to add extra conditional statements, which results in more complex and harder-to-manage code. 

Data Classes

Kotlin makes working with data easier by providing automatic data classes. Using the data keyword, Kotlin automatically generates important methods like hashCode(), toString(), and equals(), so developers don’t have to write them manually. This saves time and keeps the code clean and easy to manage.

In Java, since it does not have built-in data classes, developers must manually create all methods, including getters, setters, and object comparison functions. This leads to longer and more repetitive code. Some Java developers can use Lombok to reduce boilerplate, but it still requires extra setup.

Type Inference

Kotlin automatically detects variable types, so developers don’t need to specify them manually. The smart casts will automatically detect them while retaining the option to declare types explicitly when needed for clarity or constraints. For variable declaration, Kotlin uses val (immutable) and var (mutable) keywords allowing the compiler to determine the type automatically. This reduces redundancy, making the code cleaner and easier to read.

Java, on the other hand, requires developers to name variable types clearly at the declaration stage, causing the code longer and more repetitive. This is because of the lack of automatic type inference features by default like smart casts in Kotlin. Java implemented the var keyword to allow type inference, but it only works for local variables and is not as flexible as Kotlin.

You can explore the Kotlin vs Java code examples below to see how Kotlin and Java differ in declaring variable types.

Kotlin code automatically detects variable types, so developers don’t need to explicitly specify them. However, they still have the option to do so when needed.

fun main() {
    val name = "Kotlin"  // Compiler infers String
    val age = 25         // Compiler infers Int
    val isActive = true  // Compiler infers Boolean

    val list = listOf(1, 2, 3)  // Compiler infers List<Int>

    println("Name: $name, Age: $age, Active: $isActive")
}

In Java, developers must explicitly declare types when initializing variables, making the code more wordy.

public class Main {
    public static void main(String[] args) {
        String name = "Java";
        int age = 25;
        boolean isActive = true;

        List<Integer> list = Arrays.asList(1, 2, 3);

        System.out.println("Name: " + name + ", Age: " + age + ", Active: " + isActive);
    }
}

Implicit Conversions

Kotlin does not support implicit type conversions. This means that developers must manually convert values when assigning different numeric types. 

Java allows implicit type conversions between compatible numeric types. Developers can assign a smaller type to a larger type without extra code. This can make coding faster but increases the risk of data loss when converting larger types into smaller ones. While Kotlin might seem restrictive, it prevents this problem effectively.

Checked Exceptions

Kotlin does not require developers to handle exceptions with try-catch blocks. This makes error handling more flexible. However, developers need to be careful and handle exceptions properly to avoid crashes. 

Meanwhile, Java enforces checked exceptions. Developers must either catch errors using try-catch or declare them with throws-in method signatures. Although this makes the code longer and more repetitive, it helps prevent unexpected runtime failures by enforcing strict error handling at compile time.

Extension Functions

Kotlin allows developers to build new functions to existing classes without changing the original code. Developers do not need to create sub-classes or modify original classes to add functionality. 

Java requires developers to create extra classes and write more code to add functions. It does not support extension functions, so developers must create additional classes and write more code to achieve the same functionality. This leads to more maintenance work and longer, less readable code.

Public Fields

Kotlin does not have public fields like Java. Instead, it uses properties with built-in getters and setters, ensuring better data safety. This design keeps the code clean and structured while still allowing control over how values are accessed and modified.

Java, on the other hand, allows public fields, meaning variables can be accessed directly from outside the class. This can be useful when callers need to modify an object’s representation without changing the public API. However, using public fields reduces encapsulation and can make the code harder to maintain in the long run.

Asynchronous Programming

Coroutines in Kotlin

Kotlin allows developers to run tasks without blocking the main thread through coroutines, making background task management easier. The lightweight and efficient feature of coroutines is ideal for apps that need smooth UI performance, memory management optimization, and fast database operations. Suspend and async/await functions also simplify concurrency, reducing the complexity of handling multiple tasks for developers. 

Thread Management in Java

Meanwhile, Java uses threads to handle asynchronous tasks. Developers have to set up and manage them manually through tools like Runnable, Thread, and ExecutorService. Managing synchronization, error handling, memory management, and thread lifecycles are also more complex. Since each thread has its own memory allocation, Java’s approach consumes more resources, making it harder to manage many tasks at once.

Compilation and Execution Speeds

The compilation speed of Kotlin is slightly slower because it has to execute extra steps such as type inference, coroutine handling, and null safety checks. Although it takes more time and complexity during compilation, it helps reduce runtime errors and improve code quality when executing.

In contrast, Java proceeds faster due to its straightforward process which directly converts code to bytecode without extra processing. This makes Java a good choice for enterprise applications and CI/CD pipelines where fast build times are important. 

Performance Impact

Kotlin’s null safety feature prevents crashes and its coroutines also improve background task performance, ensuring stable and efficient applications. By reducing errors and optimizing memory management, Kotlin supports improving app stability and background task performance, making it ideal for mobile applications where smooth user experience is critical.

The mature JVM optimizations and strong platform support allow Java to be compatible with large-scale enterprise systems. However, it is less efficient for real-time applications and high-concurrency tasks because it lacks built-in asynchronous features and requires manual thread management.

Tooling & Ecosystem

Kotlin is fully supported in Android Studio and JetBrains tools like IntelliJ IDEA. It includes debugging tools, code completion, and refactoring features, making it the top choice for Android development. These benefits make it the top choice for Android developers. 

However, Java has been developed for nearly three decades with a vast ecosystem of libraries, frameworks, and third-party tools. Its long-standing presence means strong community support, detailed documentation, and a stable platform for large-scale applications. With this large collection of libraries, a strong market presence, and community support, Java still offers detailed documentation and a stable platform for long-term development.

Common Applications

Kotlin is widely used in mobile app development, backend systems, cross-platform applications, and data science. Here are some key areas where Kotlin is commonly applied:

• Android App Development: Approved by Google for Android development, it is used in apps like Pinterest, Uber, and Trello because of its cleaner syntax, null safety, and coroutine support.
• Backend Development: Server-side apps use Kotlin a lot through Ktor, Spring Boot, and Micronaut to offer lightweight options that still work with Java. 
• Cross-Platform Development: Kotlin Multiplatform allows code sharing across Android, iOS, web, and desktop, reducing development time. 
• Data Science & Machine Learning: Works with Apache Spark for big data processing and supports AI models through KotlinDL (Deep Learning for Kotlin). 
• Game Development: Used with libGDX and Java-based game engines to build mobile games, benefiting from concise syntax and Java interoperability.

Besides, Java is widely used in enterprise software, backend development, web applications, and mobile apps. Here are some key areas where Java excels:

• Enterprise Software: Many large organizations, such as banking, finance, and government, rely on Java for their core business applications.
• Backend Development: Powers server-side applications and is commonly used in microservices architectures for handling large-scale distributed systems.
• Embedded Systems & IoT Application: Java is used in smart devices, automotive software, and healthcare systems due to its platform independence and ability to run on embedded devices.

When Do Developers Should Switch From Java to Kotlin?

Developers should switch from Java to Kotlin when:

• You’re building a new Android app. Kotlin can ensure better Jetpack and API support, faster development, and long-term compatibility.
• Your project has too much boilerplate code. It will remove repetitive syntax, making the code shorter, cleaner, and easier to maintain.
• Your app suffers from NullPointerException (NPE) crashes. Kotlin’s built-in null safety can make the app more stable.
• Your apps may freeze, slow down, or become unresponsive. In this case, you need a better asynchronous programming app. Kotlin can make background tasks lighter and more efficient.
• You want faster development with fewer bugs product. Kotlin’s concise syntax, type inference, and extension functions help speed up coding and reduce errors.
• You’re maintaining a Java project but want improvements with extensive features. Kotlin can be gradually introduced into an existing Java codebase, allowing for step-by-step migration without rewriting everything.

When to Stick With Java?

Besides, several cases require your team to be consistent with Java in development, such as when:

• Your project depends much on Java-based frameworks like Spring, Hibernate, or Jakarta EE. In this case, troubleshooting and debugging are easier in Java because of the wider resources available.
• Your team is experienced in Java, and your project does not have enough time for training developers, switching to Kotlin would require extra training and slow down development.
• You are maintaining a large legacy Java codebase, where migrating to Kotlin is not practical or necessary.
• Your application is backend-focused, using Java for server-side processing, microservices, or cloud computing will be a better choice due to its strong framework support, proven scalability, and concurrency handling.
• You need cross-platform support beyond Android, such as enterprise software, web applications, embedded systems, or IoT solutions, where Java has stronger tools and libraries.
• You prioritize long-term stability for your businesses’ apps, as Java’s large ecosystem, strong community support, and scalability make it a reliable choice for enterprise applications.

FAQs

1. Will Kotlin replace Java in the future?

No, Kotlin cannot replace Java in the future. Although Kotlin’s popularity has grown rapidly, Java still has its niche. Experts predict that both languages will coexist, with Kotlin leading in mobile development and Java maintaining its role in enterprise and backend solutions.

2. Which language should you choose for your project?

Choosing between Kotlin and Java depends on your project needs. If you’re working on Android development or a modern software project, Kotlin is the better choice. However, if your project is enterprise-focused, Java remains a solid option. If flexibility allows, using both Kotlin and Java together can be great to take advantage of both languages. 

3. Which language uses less energy?

Kotlin generally uses less energy than Java in mobile apps. However, Java’s Just-In-Time (JIT) compiler and mature optimizations often result in faster execution and lower CPU usage for long-term backend applications. Overall, energy usage depends on the specific use case and optimization techniques used.

4. Which language is easy to learn between Kotlin and Java?

Java is easier to learn for beginners since it has a structured learning path, documentation, and a larger community with more tutorials and resources. Meanwhile, Kotlin is easier once you understand Java's basic programming concepts.

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Conclusion

The debate between Kotlin vs Java: which is better? doesn’t have a one-size-fits-all answer. When comparing Kotlin vs Java syntax, Kotlin offers cleaner, more readable code with fewer lines, thanks to null safety, type inference, and extension functions. Meanwhile, Java follows a more traditional structure, making it easier for developers already experienced in object-oriented programming. 

The best choice depends on your project. If you're building a new Android app or scalable backend service, Kotlin is likely the better fit. But if you’re working on enterprise systems, high-performance applications, or legacy projects, Java still provides unmatched reliability. At the end of the day, whether you go with Kotlin, Java, or a mix of both, the key to success lies in understanding your project’s requirements and leveraging the strengths of each language to build powerful, scalable, and future-proof applications.

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