Java vs Kotlin: Android Development Comparison

Java and Kotlin comparison for Android development

Google announced Kotlin as a first-class language for Android development in 2017. Two years later, they went further: Kotlin became the preferred language for Android apps. This shift left many Java developers wondering whether they needed to switch.

The short answer is that both languages work fine for Android. They compile to the same bytecode, run on the same virtual machine, and can coexist in the same project. But Kotlin offers enough quality-of-life improvements that most new Android projects start with it by default.

Here’s how the two languages compare.

A Brief History

Java has been around since 1995. Android adopted it as the primary development language when the platform launched in 2008. For nearly a decade, if you wanted to build Android apps, you wrote Java.

JetBrains created Kotlin in 2011 and released version 1.0 in 2016. The language was designed to fix Java’s pain points while maintaining full interoperability. JetBrains builds IntelliJ IDEA and Android Studio, so they understood exactly what frustrated developers.

Today, over 60% of professional Android developers use Kotlin as their primary language, according to the 2024 Stack Overflow Developer Survey. But Java isn’t going anywhere. Millions of lines of Android code exist in Java, and plenty of companies maintain Java-based apps.

Null Safety

The biggest difference between the two languages is how they handle null values.

In Java, any object reference can be null. Call a method on a null reference, and your app crashes with a NullPointerException. These crashes account for a significant portion of Android app errors.

// Java - null danger lurks everywhere
String name = getUserName();  // Could return null
int length = name.length();   // Crashes if name is null

Kotlin distinguishes between nullable and non-nullable types at the compiler level. A regular String can never be null. If you want to allow null, you declare it as String? with a question mark.

// Kotlin - null safety built in
val name: String = getUserName()    // Compiler ensures this isn't null
val length = name.length            // Safe

val maybeName: String? = findUser() // Explicitly nullable
val len = maybeName?.length         // Safe call, returns null if maybeName is null

The compiler catches potential null pointer errors before your code ever runs. This alone prevents entire categories of crashes.

Conciseness

Kotlin requires less boilerplate than Java. The same functionality often takes half the lines of code.

Consider a simple data class that holds user information:

// Java - 30+ lines for a basic data class
public class User {
    private String name;
    private int age;
    private String email;

    public User(String name, int age, String email) {
        this.name = name;
        this.age = age;
        this.email = email;
    }

    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    
    public int getAge() { return age; }
    public void setAge(int age) { this.age = age; }
    
    public String getEmail() { return email; }
    public void setEmail(String email) { this.email = email; }

    @Override
    public boolean equals(Object o) {
        // Multiple lines of comparison logic
    }

    @Override
    public int hashCode() {
        // Hash computation
    }

    @Override
    public String toString() {
        return "User{name='" + name + "', age=" + age + ", email='" + email + "'}";
    }
}

// Kotlin - one line does it all
data class User(var name: String, var age: Int, var email: String)

The Kotlin data class automatically generates equals(), hashCode(), toString(), and copy() methods. You get the same functionality with dramatically less typing.

Extension Functions

Kotlin lets you add methods to existing classes without modifying their source code or using inheritance.

// Add a method to String class
fun String.addExcitement(): String {
    return this + "!!!"
}

val boring = "Hello"
val exciting = boring.addExcitement()  // "Hello!!!"

This feature proves especially useful with Android’s built-in classes. You can add convenience methods to View, Context, or any other class you don’t control.

// Make any View easy to hide or show
fun View.show() {
    visibility = View.VISIBLE
}

fun View.hide() {
    visibility = View.GONE
}

// Usage
myButton.show()
myTextView.hide()

Java has no equivalent. You’d need utility classes with static methods, which feels clunkier to use.

Coroutines vs Threads

Asynchronous programming in Android traditionally meant callbacks, AsyncTask (now deprecated), or RxJava. All of these approaches add complexity.

Kotlin coroutines provide a cleaner model. Asynchronous code looks almost like synchronous code:

// Kotlin coroutine - clean async code
suspend fun fetchUserData(): User {
    val user = apiService.getUser()      // Suspends, doesn't block
    val posts = apiService.getPosts(user.id)
    return user.copy(posts = posts)
}

// Call it from a coroutine scope
lifecycleScope.launch {
    val user = fetchUserData()
    updateUI(user)
}

The equivalent Java code would involve callbacks or CompletableFuture chains that are harder to read and debug.

// Java callback approach - nested and harder to follow
apiService.getUser(new Callback<User>() {
    @Override
    public void onSuccess(User user) {
        apiService.getPosts(user.getId(), new Callback<List<Post>>() {
            @Override
            public void onSuccess(List<Post> posts) {
                user.setPosts(posts);
                updateUI(user);
            }
            @Override
            public void onError(Exception e) { /* handle error */ }
        });
    }
    @Override
    public void onError(Exception e) { /* handle error */ }
});

Coroutines have become the recommended approach for asynchronous work in Android. Google’s Jetpack libraries are built around them.

Smart Casts

When you check an object’s type in Java, you still need to cast it explicitly:

// Java - explicit cast required
if (animal instanceof Dog) {
    Dog dog = (Dog) animal;  // Must cast even after checking
    dog.bark();
}

Kotlin remembers type checks and casts automatically:

// Kotlin - smart cast
if (animal is Dog) {
    animal.bark()  // Already knows it's a Dog
}

Small convenience, but it adds up across a codebase.

Interoperability

Kotlin and Java work together seamlessly. You can call Java code from Kotlin and Kotlin code from Java. A single Android project can mix both languages freely.

This matters for practical reasons. Most Android libraries are written in Java. You don’t need Kotlin versions. Just use them directly:

// Kotlin calling Java library code
val retrofit = Retrofit.Builder()
    .baseUrl("https://api.example.com/")
    .addConverterFactory(GsonConverterFactory.create())
    .build()

Teams can also migrate gradually. Convert one file at a time. Android Studio even has a built-in converter that translates Java files to Kotlin automatically (though it sometimes produces awkward code that benefits from manual cleanup).

When Java Still Makes Sense

Despite Kotlin’s advantages, Java remains a valid choice in several situations.

Existing large codebases. If you have 500,000 lines of Java code, rewriting isn’t practical. You can add new features in Kotlin while maintaining Java code.

Team experience. A team fluent in Java but unfamiliar with Kotlin will be more productive sticking with what they know, at least initially.

Server-side development. While Kotlin works on the backend, Java has deeper ecosystem support for enterprise frameworks like Spring. The gap is closing, but Java still has more documentation and examples.

Learning fundamentals. Some educators prefer teaching Java first because it forces explicit understanding of concepts that Kotlin abstracts away. You learn what happens under the hood.

Learning Curve

Java developers can pick up Kotlin quickly. The syntax feels familiar. Most concepts map directly. JetBrains estimates a few weeks to become productive.

Going the other direction takes longer. Kotlin hides complexity that Java exposes. A developer who learned only Kotlin might struggle with Java’s verbosity and explicit type handling.

If you’re new to programming entirely, either language works as a starting point. Kotlin is more forgiving. Java teaches more fundamental concepts explicitly.

Job Market

Most Android job postings in 2025 list both Java and Kotlin. Employers expect familiarity with both, though new projects increasingly start in Kotlin.

Pure Java roles still exist for maintaining legacy apps. Pure Kotlin roles are growing, especially at startups and companies building new products.

Knowing both languages makes you more versatile. And since they interoperate so well, you’ll likely use both in practice.

The Verdict for Android Development

For new Android projects, Kotlin is the better choice. Google recommends it. The documentation assumes it. Modern Android libraries like Jetpack Compose are Kotlin-first.

For existing Java projects, there’s no rush to convert. Java works fine. The Android platform will support it for the foreseeable future.

And for your career? Learn both. Start with whichever matches your current job or interests, then pick up the other. They’re more similar than different, and the Android ecosystem rewards developers who can work in either.

Sources

Google. “Kotlin and Android.” developer.android.com, 2024
JetBrains. “Kotlin Language Documentation.” kotlinlang.org
Stack Overflow. “2024 Developer Survey.” stackoverflow.com
Android Developers Blog. “Kotlin First.” android-developers.googleblog.com, May 2019