Java vs Kotlin: Key Differences and Which to Choose

The short answer

Kotlin is a modern language from JetBrains that runs on the same JVM as Java and is fully interoperable with it. Google recommends Kotlin as the primary language for Android, and new apps there are almost always written in it. On the backend, Java remains extremely common, while Kotlin is a convenient alternative you can adopt gradually.

Null safety

In Java any reference can be null, and the mistake only shows up at runtime as a NullPointerException. In Kotlin nullability is part of the type, and the compiler forces you to handle it.

// Java
String name = user.getName();
int len = name != null ? name.length() : 0;

// Kotlin
val name: String? = user.name
val len = name?.length ?: 0

A Kotlin String cannot be null; a String? can. This removes a whole class of bugs before the code even runs.

Verbosity

Kotlin is noticeably shorter. The classic example is a data model class.

// Java (record, modern syntax)
public record User(String name, int age) {}

// Kotlin
data class User(val name: String, val age: Int)

Modern Java has narrowed the gap with record, var and an improved switch. Kotlin still offers more conveniences: default parameter values, named arguments, extension functions, when and string templates.

Coroutines vs threads

• Java has traditionally relied on OS threads, ExecutorService and CompletableFuture. Newer versions add virtual threads, which make blocking code cheap and scalable.
• Kotlin offers coroutines: asynchronous code is written sequentially using suspend functions.

suspend fun loadProfile(id: Long): Profile {
    val user = api.getUser(id)        // suspends without blocking a thread
    val orders = api.getOrders(id)
    return Profile(user, orders)
}

On Android, coroutines are the standard way to work with the network and database without blocking the UI.

Interoperability

Kotlin and Java can live in the same project: you call Java classes from Kotlin and vice versa. That means:

• you can migrate a project to Kotlin file by file;
• every Java library is available: Spring, Hibernate, Jackson and more;
• Gradle or Maven builds work for both languages.

There are caveats: Java types arrive in Kotlin as "platform types", and the compiler does not know whether they can be null. @Nullable/@NonNull annotations in Java code help.

Side-by-side comparison

How to choose

• A new Android app: Kotlin, together with Jetpack Compose.
• An existing Android project in Java: write new modules in Kotlin and convert old code as you touch it.
• A new backend: both are good options. Kotlin brings conciseness and coroutines; Java brings a larger hiring pool and predictability.
• A large Java codebase and a Java team: do not switch for fashion; weigh the benefits against the cost of retraining.

Common mistakes

• Writing Kotlin "like Java", without data class, null safety or extension functions.
• Overusing the !! operator, which disables the null check.
• Mixing coroutines with blocking calls on the main thread.