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What is the difference between Java and Kotlin?
Updated 2 min read7 differences
In short
Java is the original JVM language, stable and everywhere; Kotlin is a newer, more concise JVM language with built-in null safety, preferred for Android.
Java
Java is a statically typed, object-oriented programming language that compiles to bytecode for the Java Virtual Machine, so programs run on many platforms.
Read the page on JavaKotlin
Kotlin is a concise, statically typed language that runs on the JVM, works seamlessly with Java, and is widely used for Android and server-side development.
Read the page on KotlinJava and Kotlin compared
| Aspect | Java | Kotlin |
|---|---|---|
| First released | 1995, by Sun Microsystems | 2016, by JetBrains |
| Null safety | Any reference can be null; checks are up to you | Built into the types: String vs String? |
| Verbosity | More explicit; records and var cut some boilerplate | Concise: data classes, type inference, default arguments |
| Asynchronous code | Threads, including lightweight virtual threads | Coroutines with suspend functions |
| Compiles to | JVM bytecode | JVM bytecode, JavaScript, WebAssembly and native code |
| Android | Supported, but no longer the default | Google's preferred language since 2019 |
| Ecosystem | Huge, with decades of libraries and existing code | Every Java library, plus Kotlin-first ones |
The difference, explained
Java, released in 1995, is the language the Java Virtual Machine was built for. Kotlin, created by JetBrains and released as version 1.0 in 2016, also compiles to JVM bytecode and was designed to fix common Java pain points while working seamlessly with it. Kotlin code can call any Java library, Java code can call Kotlin, and both can live in the same project.
The main differences show in everyday code. Kotlin builds null safety into the type system: a String can never be null, and a String? must be checked before use, which prevents most NullPointerException crashes. It also needs less boilerplate, with type inference, data classes, extension functions, default arguments and coroutines for asynchronous code. Java is more explicit and changes more cautiously, with a strong focus on backward compatibility.
Java has been catching up. Records, pattern matching, text blocks, var and virtual threads arrived in recent releases, and a new version ships every six months. Kotlin, in turn, reaches beyond the JVM: it compiles to JavaScript, WebAssembly and native code, and Kotlin Multiplatform shares logic between Android, iOS, desktop and web.
On Android the question is mostly settled: Google declared Kotlin its preferred language in 2019, and modern Android tools such as Jetpack Compose are Kotlin-first. On the server both are common, often with Spring Boot, which supports Kotlin well. A common misconception is that choosing Kotlin means rewriting Java code; most teams mix the two and migrate file by file.
Which one should you use?
Choose Java when…
- You maintain a large Java codebase and want to stay consistent with it.
- Your company standardizes on Java and its tooling.
- You want the widest pool of developers, books and long-term stability.
Choose Kotlin when…
- You build Android apps, where Kotlin and Jetpack Compose are the default.
- You want null safety and concise code on the JVM.
- You want to share business logic across Android, iOS and other platforms.
Readers ask
Is Kotlin replacing Java?
Not overall. Kotlin has largely replaced Java for new Android code, but Java remains one of the most used languages for backends, and the two run side by side on the same JVM.
Can Java and Kotlin be used in the same project?
Yes. Both compile to JVM bytecode, so classes in one language can call the other directly. Many teams write new files in Kotlin and leave existing Java code as it is.
Is Kotlin faster than Java?
At runtime they perform about the same, since both become JVM bytecode. Kotlin builds can take a little longer to compile, but that rarely decides the choice.