Choosing a programming language for creating mobile applications for the Android operating system has always been a stumbling block for beginners and experienced developers. For many years, the ecosystem was built on the foundation laid by Java, and millions of lines of code around the world were written in this language. However, since 2017, when Google officially announced support Kotlin, the development landscape began to change rapidly, offering an alternative that promised greater conciseness and security.
Today the situation is such that a new project started on pure Javamay look like an anachronism, although it will remain completely workable. Kotlin positioned as a modern, cross-platform language that solves many of the problems inherent in its predecessor, such as redundant code and NullPointerExceptions. But does this mean that knowledge of Java has become useless? Not at all, since legacy code is not going anywhere, and understanding the principles of a virtual machine JVM remains critical for both languages.
In this article we will conduct an in-depth comparative analysis to understand what is better to choose for starting a career or for a new commercial project. We'll look not only at syntactic differences, but also at the impact of language on coding speed, readability, community support, and employer demands in the current job market.
Historical context and official support from Google
The history of Android development is inextricably linked with the Java language, which has been used since the launch of the platform in 2008. For a long time, it was the only official tool, and all documentation, tutorials and libraries were oriented towards it. Android Studio, the official development environment, was initially designed taking into account the specifics of Java, which created a strong, but sometimes clumsy ecosystem.
The turning point came at the Google I/O 2017 conference, when the search giant announced Kotlin first class of supported languages for Android. This was a revolutionary announcement, as Google rarely makes such moves without making sure the technology is mature. Developers no longer had to use third-party plugins or take risks when introducing a new tool into production.
⚠️ Attention: Although Google is actively promoting Kotlin, this does not mean that support for Java has ended. You can still write applications in Java, compile them and publish them on Google Play without any restrictions from the store.
By 2019, Google's approach became even more radical: development style Android KTX and new Jetpack libraries began to be created with an eye primarily on Kotlin. This created a situation where using new functionality in Java became increasingly inconvenient and required writing a lot of boilerplate wrapper code.
Syntactic differences and code readability
One of the main arguments in favor of Kotlin is its brevity. Code written in this language often requires 20-40% fewer lines compared to a similar solution in Java. This is achieved through the absence of mandatory semicolons, the ability to infer variable types, and smarter constructs for working with data.
Consider the classic situation of creating a data model class (POJO in Java terminology). In Java, you would have to write a constructor, getters, setters, methods equals, hashCode and toString. In Kotlin, all this routine is replaced by one line with an annotation data class, which significantly speeds up development and reduces the risk of copying errors.
// Example in Kotlin
data class User(val name: String, val age: Int)
// An analogue in Java would require ~50 lines of code
In addition, Kotlin introduces the concept extension functionsthat allows you to add new methods to existing classes without inheritance or the use of wrapper patterns. This makes the code more expressive and allows you to organically extend the functionality of third-party libraries that you cannot change directly.
Use the code conversion function in Android Studio (Code -> Convert Java File to Kotlin File) to quickly see the difference in code size using real-life examples.
Code security: combating NullPointerException
Problem NullPointerException (NPE) is so famous in the Java world that it is often called the "billion dollar bug." In Java, any variable of an object type can contain a value by default, and the compiler does not force the developer to check this before use, which leads to applications crashing at runtime. null, and the compiler does not force the developer to check this before use, which leads to applications crashing in runtime.
Kotlin solves this problem at the type system level. In this language, default types cannot contain null. If you want to allow a variable to be empty, you must explicitly indicate this by adding a question mark, for example String?. An attempt to assign null to a variable of type String will cause a compilation error before the application even starts.
- 🛡️ Secure calls: The
?.operator allows you to safely access the properties of an object that may be null, returning null instead of an exception. - 🚀 Elvis operator: Design
?:allows you to set a default value if the expression on the left is null, eliminating cumbersome if-else checks. - 🔍 Smart Casts: The Kotlin compiler automatically casts types after checking for null, which eliminates the need for explicit casting.
This approach forces the developer to think about possible missing values at the architecture design stage, and not while debugging crashes in production. This significantly increases the stability of the final product.
The Kotlin type system eliminates an entire class of runtime errors by shifting the responsibility for checking null references to the compilation stage.
Performance and compilation speed
The issue of performance is often controversial, since both languages are compiled into bytecode for the virtual machine. ART (Android Runtime). In most use cases, the difference in code execution speed between Java and Kotlin is negligible and unnoticeable to the end user.
However, there are nuances. Kotlin adds some overhead through the use of higher order functions and lambda expressions, which can create additional objects in memory. But modern compilers and ART optimizations successfully cope with this, inlining many calls.
High
Same
Low
| Parameter | Java | Kotlin |
|---|---|---|
| APK size | Standard | A little more (due to runtime) |
| Compilation speed (Clean Build) | Average (may be 10-15 slower%) | |
| Execution speed (Runtime) | The same (with minor nuances) | |
| Memory consumption | A little higher due to wrapper objects |
It is worth noting that the speed of clean compilation (Clean Build) in Kotlin may be lower, especially in large projects, due to the complexity of type analysis. However, incremental compilation in modern versions of Android Studio works quite effectively, eliminating this drawback in everyday work.
⚠️ Attention: When adding Kotlin to an existing Java project, the APK size may increase by several tens of kilobytes due to the inclusion of the standard Kotlin library. In new projects on Pure Kotlin, this impact is minimal thanks to the R8 mechanism.
Compatibility and work in mixed projects
A huge advantage of the Android ecosystem is the full compatibility of Java and Kotlin. You can call Java code from Kotlin and vice versa without any complex setup. This allows companies to gradually migrate older projects by rewriting modules one at a time rather than doing a complete (rewrite) from scratch.
When working in a mixed project, it is important to consider annotations. Kotlin uses special annotations such as @JvmStatic, @JvmOverloads and @Nullableto make the code clear and easy to call from a Java class. Without them, a Java developer may encounter unexpected method signatures.
Challenges of mixed development
When calling Kotlin code from Java, difficulties may arise with named arguments and properties that look like fields in Kotlin, but are actually getters/setters.
There is also the problem of interoperability with collections. Collections in Kotlin are immutable by default in terms of interface, while in Java they are mutable. When transferring collections between languages, you need to be careful to avoid errors in changing data where it was not intended.
☑️ Migrating a project from Java to Kotlin
Labor market and development prospects
If you look at the vacancies for Android developers over the past year, you will notice a clear trend. The requirement to know Kotlin has become a de facto standard for Middle and Senior positions. Employers are looking for specialists who can support a modern technology stack, including Coroutines, Flow and Jetpack Composethat are natively supported in Kotlin.
However, Java is not dying. A huge number of banking applications, corporate software and old startups are written in Java. Supporting these systems requires qualified personnel, so the demand for Java developers in the Android sector remains stable, although it is shifting towards support for legacy code.
For a beginner, the entry threshold into Kotlin may seem a little higher due to the abundance of magical constructs and syntactic sugar, but in the long run it is an investment in a career. Knowledge of Java remains fundamental, since understanding how code is executed at the bytecode level is universal for both languages.
To start a career in 2026-2026, Kotlin is the priority language, but reading code in Java cannot be ignored due to the volume of the existing knowledge base.
Frequently asked questions (FAQ)
Is it possible to completely rewrite an application from Java to Kotlin?
Yes, this is possible and is often practiced. However, completely refactoring a large project may not be cost-effective. Typically, a gradual migration strategy is used: new features are written in Kotlin, and old modules are rewritten as needed or when bugs are found.
Do you need to learn Java before learning Kotlin for Android?
No, this is not necessary. Kotlin is self-sufficient, and many modern courses teach development directly on it. However, a basic understanding of object-oriented programming (OOP), which is often explained using Java as an example, will be very useful.
Does the choice of language affect how you work with Jetpack Compose?
Yes, it does critically. Jetpack Compose is a modern toolkit for creating interfaces, which is developed using the advantages of Kotlin (lambdas, extension functions). Using Compose with Java is extremely inconvenient and has virtually no community support.
Will the application become heavier if I use Kotlin?
In modern versions of Android and when using R8 (shrinker) technology, the difference in APK size between Java and Kotlin applications is minimal and is often less than 100 KB, which is invisible to the user at high speed Internet.
Which language is faster in execution?
In 99% of cases there is no difference. Both languages are compiled into bytecode and executed by the ART virtual machine. Micro-optimizations are possible in both directions, but they rarely impact overall application performance.