Kotlin vs Java: Key Differences Every Android Developer Must Know

Newkings September 30, 2025
Java vs Kotlin in Android Development: Key Differences with Examples

Java vs Kotlin in Android Development: Key Differences with Examples

When starting Android development, one of the biggest questions is: Should you use Java or Kotlin? Both languages are officially supported by Google, both run on the JVM, and both can be used in the same project, but they offer very different developer experiences. Choosing the right one can impact your productivity, code quality, and long‑term maintainability.

This detailed comparison explains how Java and Kotlin differ in syntax, features, ecosystem, performance, and real‑world usage. You will also see side‑by‑side code examples so you can visually understand why many Android developers are moving from Java to Kotlin, and in which situations Java might still make sense.



Quick Overview: Java and Kotlin

What Is Java?

Java is a mature, object‑oriented programming language created by Sun Microsystems (now owned by Oracle). It has been used for decades in backend systems, desktop applications, enterprise software, and, of course, Android development. Java was the original official language for Android apps and still powers a huge number of existing Android codebases.

Its main strengths are stability, a massive ecosystem of libraries and tools, and a large pool of developers. However, Java’s syntax can be verbose, and it lacks some modern language features that are helpful in mobile development, such as built‑in null safety and concise functional constructs.


What Is Kotlin?

Kotlin is a modern, statically typed language created by JetBrains, the company behind IntelliJ IDEA (which Android Studio is based on). Kotlin is fully interoperable with Java and runs on the JVM, which means it can use all existing Java libraries and frameworks.

Google announced Kotlin as an official language for Android in 2017 and later called it the preferred language for Android app development. Kotlin focuses on conciseness, safety, and developer productivity, offering features like null safety, data classes, coroutines, extension functions, and smart casts that reduce boilerplate and common runtime errors.


Java vs Kotlin: Key Language Differences

1. Syntax and Conciseness

One of the most noticeable differences between Java and Kotlin is how much code you need to write to accomplish the same task. Kotlin is designed to reduce boilerplate, which makes code shorter, cleaner, and less error‑prone.

Consider a simple data holder class.

Java version:

public class User {
    private String name;
    private int age;

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

    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; }

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

Kotlin version:

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

Kotlin’s data class automatically generates equals(), hashCode(), toString(), and copy methods, as well as getters (and setters for var properties), dramatically reducing boilerplate.


2. Null Safety

NullPointerException is one of the most common runtime crashes in Java. In Java, any reference type can be null, and it is easy to forget to perform null checks. Kotlin tackles this issue by making nullability part of the type system.

Java (manual null checks):

String name = getUserName(); // may be null
if (name != null) {
    int length = name.length();
}

Kotlin (nullable vs non‑nullable types):

val name: String? = getUserName()  // nullable
val length = name?.length          // safe call (length is Int?)

In Kotlin, String (non‑nullable) and String? (nullable) are different types. The compiler forces you to handle null values explicitly using safe calls (?.), the Elvis operator (?:), or other mechanisms, which significantly reduces runtime crashes.


3. Coroutines vs Threads

Asynchronous programming is critical in Android to keep the UI responsive. In Java, you typically work with threads, executors, callbacks, or libraries like RxJava to run tasks off the main thread. While powerful, these approaches can become complex and hard to read.

Kotlin introduces coroutines, a light‑weight concurrency framework that lets you write asynchronous code in a sequential style. Coroutines make it much easier to handle network calls, database operations, and other long‑running tasks without freezing the UI.


Java (using callbacks):

fetchDataAsync(new Callback() {
    @Override
    public void onSuccess(Result result) {
        updateUi(result);
    }

    @Override
    public void onError(Throwable t) {
        showError(t.getMessage());
    }
});

Kotlin (using coroutines):

lifecycleScope.launch {
    try {
        val result = fetchData()
        updateUi(result)
    } catch (e: Exception) {
        showError(e.message)
    }
}

The coroutine example looks like simple, sequential code, but it is non‑blocking. This leads to cleaner and more maintainable asynchronous logic.


4. Extension Functions

Kotlin allows you to add new functions to existing classes without modifying their source code or using inheritance, through extension functions. This is useful for creating utility methods that feel like native methods of a class.

Kotlin extension example:

fun String.trimAndLower(): String {
    return this.trim().lowercase()
}

val result = "  Hello Kotlin  ".trimAndLower()

In Java, you would usually create a utility class with static methods, which is more verbose and less expressive:

Java equivalent:

public class StringUtils {
    public static String trimAndLower(String input) {
        return input.trim().toLowerCase();
    }
}

String result = StringUtils.trimAndLower("  Hello Java  ");

5. Smart Casts and Type Inference

Kotlin provides smart casts: once you check a variable’s type or nullability, the compiler automatically treats it as that type inside the corresponding scope, eliminating extra casts. It also has strong type inference, which means you often don’t need to repeat the type on the right‑hand side.

Kotlin smart cast example:

fun printLength(obj: Any) {
    if (obj is String) {
        // smart cast: obj is treated as String
        println(obj.length)
    }
}

In Java, you must cast manually after an instanceof check: 

void printLength(Object obj) {
    if (obj instanceof String) {
        String s = (String) obj;
        System.out.println(s.length());
    }
}


Android-Specific Comparison

1. Official Support and Ecosystem

Both Java and Kotlin are officially supported languages for Android. However, Kotlin is now the preferred language promoted by Google. New Android libraries, documentation, and samples increasingly use Kotlin first, with Java sometimes being secondary or omitted.

That said, the existing Android ecosystem contains many Java libraries, and almost all of them can still be called from Kotlin thanks to full interoperability. This makes migrating from Java to Kotlin incremental and safe.


2. Interoperability

One major advantage of Kotlin is that you don’t have to rewrite your app from scratch. You can gradually introduce Kotlin files into a Java project, or vice versa, because both languages compile to JVM bytecode and can call each other’s classes and methods directly.

This mixed‑language approach is very common in real Android apps: legacy modules may remain in Java, while new features are written in Kotlin, allowing teams to modernize codebases step by step.


3. Performance and APK Size

In most scenarios, Java and Kotlin have very similar runtime performance because they both run on the JVM and are ultimately compiled to similar bytecode. For typical Android apps, the differences are usually negligible.

Kotlin may sometimes result in slightly larger bytecode or marginally longer compilation times due to features like coroutines and lambdas, but for most projects, these trade‑offs are worth the gains in readability and safety. Java may still have an edge when absolute minimal footprint and simplicity are the main priorities.


Code Examples: Java vs Kotlin in Android

Example 1: Simple Activity

Java:

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        TextView textView = findViewById(R.id.textView);
        textView.setText("Hello from Java");
    }
}

Kotlin:

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val textView: TextView = findViewById(R.id.textView)
        textView.text = "Hello from Kotlin"
}

With view binding or Jetpack Compose, Kotlin code often becomes even more concise compared to Java, especially as you add more UI logic.


Example 2: Listener / Lambda

Java (anonymous inner class):

button.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        showToast("Clicked in Java");
    }
});

Kotlin (lambda expression):

button.setOnClickListener {
    showToast("Clicked in Kotlin")
}

Kotlin’s support for lambdas and higher‑order functions makes event handling and callback‑based APIs significantly cleaner and easier to understand.


Pros and Cons: Java vs Kotlin for Android

Advantages of Java

  • Maturity and Stability: Java has been around for decades and is battle‑tested in enterprise and Android environments.
  • Large Talent Pool: Many developers already know Java, which can make hiring or onboarding easier in some teams.
  • Rich Ecosystem: A huge number of libraries, frameworks, and tools are written in Java and are well documented.
  • Simplicity for Beginners: Some newcomers find Java’s more explicit style easier to understand when first learning programming basics.

Disadvantages of Java

  • More verbose code, leading to more boilerplate and potential for bugs.
  • Lacks built‑in null safety, so NullPointerExceptions are more common.
  • Modern Android features and samples increasingly use Kotlin first.
  • Asynchronous programming can be more complex without coroutines and modern language constructs.

Advantages of Kotlin

  • Concise and Expressive: Less code to write and maintain, with features like data classes, extension functions, and type inference.
  • Null Safety: Type‑level nullability helps prevent many runtime crashes.
  • Coroutines: Modern, readable approach to asynchronous and concurrent programming on Android.
  • Officially Preferred for Android: First‑class support in Android Studio, with most new APIs and examples in Kotlin.
  • Interop with Java: Easy to use existing Java libraries and gradually migrate legacy code.

Disadvantages of Kotlin

  • Learning curve for teams coming from pure Java backgrounds.
  • Slightly more complex compilation pipeline, which can sometimes impact build times.
  • Some older tutorials and resources still primarily use Java, which may confuse beginners.

When to Choose Java vs Kotlin

Choose Java if:

  • You are maintaining a large, existing Android codebase written mostly in Java and cannot introduce Kotlin yet.
  • Your team has strong Java experience and very little exposure to Kotlin.
  • You are focusing on very lightweight libraries or components where you want to keep dependencies minimal and simple.

Choose Kotlin if:

  • You are starting a new Android project and want modern language features from day one.
  • You care about writing safer, more concise, and more maintainable code.
  • You want to use Jetpack libraries, coroutines, and modern Android patterns with the most up‑to‑date examples and documentation.
  • You plan to evolve your app over time and want a language that Google is actively pushing forward for Android.

Conclusion: Which Is Better for Android Development?

Both Java and Kotlin are powerful choices for Android development, and there is no single answer that fits every project. Java offers stability, a huge ecosystem, and familiarity, especially for developers with a traditional enterprise background. Kotlin, on the other hand, brings modern features, conciseness, safety, and is now the preferred choice for most new Android apps.

For new projects, Kotlin is generally the recommended language due to its productivity and deep integration with modern Android tools. For existing Java projects, a gradual migration strategy using Kotlin alongside Java often gives the best of both worlds. Ultimately, understanding the strengths and trade‑offs of each language will help you design Android apps that are easier to build, safer to run, and simpler to maintain over time.

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