Kotlin vs Java: The Real Differences That Matter in 2025


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


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

    public Person(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;
    }
}

A simple data class in Kotlin vs a traditional POJO in Java.

Kotlin vs Java: The Battle for Android Supremacy

Kotlin is basically Java if Java went to therapy and worked on itself. It's the newer, more streamlined alternative that's been gaining popularity since its release in 2011. But is it truly better? Let's explore the key differences and similarities between the two in this Kotlin vs Java showdown.

Syntax Comparison

Kotlin's syntax is often described as more concise and readable. But what does that mean in practice?


fun printName(name: String) {
    println("Hello, $name!")
}


public void printName(String name) {
    System.out.println("Hello, " + name + "!");
}

A simple function in Kotlin and its Java equivalent.

As you can see, Kotlin's version is a bit more straightforward. No need for explicit type declarations or verbose string concatenation.

Null Safety

One of Kotlin's most significant advantages is its built-in null safety features. In Java, null pointer exceptions can be a real pain.


var name: String? = "John"
println(name?.length) // prints 4


String name = "John";
System.out.println(name.length()); // prints 4, but would throw NPE if name was null

Kotlin's safe call operator in action.

By using the safe call operator (?.), Kotlin avoids the possibility of a null pointer exception.

Data Classes

Data classes are a staple of any programming language. In Java, creating a data class requires a lot of boilerplate code.


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


public class Person {
    // ... (see the first example)
}

A simple data class in Kotlin vs a traditional POJO in Java.

As you can see, Kotlin's data classes are much more concise. No need for getters, setters, or a toString method.

Coroutines vs Threads

When it comes to concurrency, Kotlin's coroutines are a more modern and efficient solution.


import kotlinx.coroutines.*

fun main() = runBlocking {
    launch { println("Coroutine 1") }
    launch { println("Coroutine 2") }
}


Thread thread1 = new Thread(() -> System.out.println("Thread 1"));
Thread thread2 = new Thread(() -> System.out.println("Thread 2"));
thread1.start();
thread2.start();

A simple coroutine example in Kotlin and its Java equivalent using threads.

Coroutines are much more lightweight than threads and provide a more structured way of handling concurrency.

Extension Functions

Extension functions are a unique feature in Kotlin that allows you to add functionality to existing classes.


fun String.hello() {
    println("Hello, $this!")
}

"John".hello() // prints "Hello, John!"

An example of an extension function in Kotlin.

This feature is not available in Java, making Kotlin a more flexible and expressive language.

Interoperability

One of the most significant advantages of Kotlin is its seamless interoperability with Java.


// Kotlin code
fun main() {
    val javaClass = JavaClass()
    println(javaClass.hello())
}


// Java code
public class JavaClass {
    public String hello() {
        return "Hello from Java!";
    }
}

A simple example of Kotlin and Java interoperability.

This means you can easily use Java libraries and frameworks in your Kotlin projects, making the transition to Kotlin much smoother.

When to Choose Kotlin vs Java

So, when should you choose Kotlin over Java? Here are a few scenarios:

New projects: If you're starting a new project, Kotlin is a great choice. Its concise syntax and modern features make it a more enjoyable and productive language to work with.
Android development: Kotlin is now the official language for Android development, making it a natural choice for Android projects.
Existing Java projects: If you're working on an existing Java project, you can easily integrate Kotlin into your codebase. This can help you take advantage of Kotlin's features and improve your overall development experience.

Kotlin vs Java: the choice is yours. But if you're looking for a more modern, concise, and expressive language, Kotlin is definitely worth considering.

Ready to make the switch? Try CodeConverter's Kotlin to Java converter (or vice versa!) to see how your code can be transformed.

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