Kotlin vs Java: A Comprehensive Comparison

Kotlin and Java are two popular programming languages that are widely used in the industry. While Java has been a long-standing powerhouse, Kotlin emerged as a modern alternative that sought to address some of Java's limitations. In this article, I'll compare Kotlin and Java across various dimensions to help you understand their similarities, differences, and strengths.

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Syntax and Readability

One of the most noticeable differences between Kotlin and Java is in the syntax. Kotlin offers a more concise and expressive syntax compared to Java. It eliminates boilerplate code, reduces verbosity, and enhances readability. Kotlin's modern syntax includes features like type inference, nullable types, lambda expressions, and extension functions, making code more compact and elegant. Java, on the other hand, has a more verbose syntax and requires explicit type declarations in most cases.

// Kotlin
fun calculateSum(numbers: List<Int>): Int {
    return numbers.sum()
}

// Java
public int calculateSum(List<Integer> numbers) {
    int sum = 0;
    for (int number : numbers) {
        sum += number;
    }
    return sum;
}

Null Safety

Null pointer exceptions (NPEs) have plagued Java developers for years. Kotlin addresses this issue by introducing a type system that differentiates between nullable and non-nullable types. With Kotlin's null safety features, developers are forced to handle nullable types explicitly, significantly reducing the risk of NPEs. In Java, null references are pervasive, and developers need to handle them manually, leading to potential bugs and crashes.

// Kotlin
fun printLength(text: String?) {
    if (text != null) {
        println(text.length)
    } else {
        println("Text is null")
    }
}

// Java
void printLength(String text) {
    if (text != null) {
        System.out.println(text.length());
    } else {
        System.out.println("Text is null");
    }
}

Interoperability

Java and Kotlin are fully interoperable, allowing developers to mix both languages in the same project seamlessly. Kotlin code can call Java code and vice versa without any issues. This means that developers can leverage existing Java libraries, frameworks, and tools while gradually introducing Kotlin into their codebase. The compatibility between the two languages makes it easier for teams to adopt Kotlin gradually and migrate existing Java projects to Kotlin.

// Java calling Kotlin
public class JavaClass {
    public static void main(String[] args) {
        KotlinClass.sayHello();
    }
}

// Kotlin calling Java
fun main() {
    JavaClass.sayHello()
}

Performance

Both Kotlin and Java are statically-typed languages that run on the Java Virtual Machine (JVM). Therefore, their performance is comparable, as they leverage the same underlying runtime environment. The execution speed and efficiency of the code primarily depend on how well it is written and optimized, rather than the choice of language. Thus, there is no significant performance advantage in choosing one language over the other.

Tooling and Ecosystem

Java has a mature and extensive ecosystem with a vast collection of libraries, frameworks, and tools. It has been around for decades and powers numerous enterprise-level applications. Kotlin, being a relatively newer language, has a smaller ecosystem but is rapidly growing. Many popular Java libraries and frameworks are Kotlin-friendly, and the Kotlin community actively contributes to building libraries specific to the language. The tooling support for both languages is excellent, with robust IDEs like IntelliJ IDEA providing support for Kotlin and Java development.

Learning Curve

For developers familiar with Java, learning Kotlin is generally straightforward. Kotlin builds upon Java's concepts and adds new features, making it easier to transition from Java to Kotlin. The cleaner syntax, enhanced null safety, and functional programming constructs may require some learning, but the overall transition is relatively smooth. However, developers new to Java or programming in general might find Java's syntax and concepts more daunting initially.

// Kotlin Functional Approach
fun filterEvenNumbers(numbers: List<Int>): List<Int> {
    return numbers.filter { it % 2 == 0 }
}

// Java's Imperative Approach
public List<Integer> filterEvenNumbers(List<Integer> numbers) {
    List<Integer> result = new ArrayList<>();
    for (Integer number : numbers) {
        if (number % 2 == 0) {
            result.add(number);
        }
    }
    return result;
}

Kotlin's functional features simplify common tasks, promoting concise and expressive code.

Conclusion

In conclusion, Kotlin and Java are powerful programming languages with their own strengths and characteristics. Kotlin offers a more concise syntax, improved null safety, and better expressiveness, making it an attractive choice for modern software development. Java, on the other hand, boasts a mature ecosystem, extensive tooling, and widespread adoption in enterprise environments. The choice between Kotlin and Java ultimately depends on the specific requirements of the project, the existing codebase, and the preferences and skill set of the development team. Both languages have their merits, and understanding their differences empowers developers to make informed decisions based on the context at hand.

Comments

[Andries]

A harder thing to learn is how to leverage idiomatic Kotlin for things. For example:

Handling of null:

val s:Boolean? = null
...
if (s == true) 

Instead of:

if ( s!= null && s == true)..

Or when and how to use things, such as:

• apply { }
• run { }
• let { }

These things are where Kotlin in my eyes out shines Java.

[Karl Heinz Marbaise]

It's interesting that all of such comparisons using Java from 2011 (JDK7)...

Create a sum at times of Java 7(2011) and before:

public int calculateSum(List<Integer> numbers) {
    int sum = 0;
    for (int number : numbers) {
        sum += number;
    }
    return sum;
}

So the Kotlin as well as the Java version are wrong because the return type is only int which is not sufficient for a sum of integers and even a long might not be sufficient, but that's a different story:

Ok let's switch to the present(2014+; JDK8+): Variant 1:

public long calculateSum(List<Integer> numbers) {
    return numbers.stream().mapToInt(__ -> __).sum();
}

or another variant:

public long calculateSum(List<Integer> numbers) {
    return numbers.stream().reduce(0, Integer::sum);
}

The other one to define a main method:

interface Main {
  static void main(String[] args) {
    System.out.println("Hello World.");
  }
}

And by the way JDK 21 (openjdk.org/projects/jdk/21/) will release in September; already has a preview (openjdk.org/jeps/445) where such thing looks like this:

void main() {
    System.out.println("Hello, World!");
}

My assumption is that this will become in JDK 22 default.. but we will see..

So the next one, as mentioned before JDK7 and before (2011!).

public List<Integer> filterEvenNumbers(List<Integer> numbers) {
    List<Integer> result = new ArrayList<>();
    for (Integer number : numbers) {
        if (number % 2 == 0) {
            result.add(number);
        }
    }
    return result;
}

Ok back to the present (JDK8 and 2014+):

  public List<Integer> filterEvenNumbers(List<Integer> numbers) {
    return numbers.stream()
        .filter(s -> s % 2 == 0)
        .collect(Collectors.toList());
  }

and another level of improvement with JDK16 (en.wikipedia.org/wiki/Java_version...

public List<Integer> filterEvenNumbers(List<Integer> numbers) {
  return numbers.stream()
      .filter(s -> s % 2 == 0)
      .toList();
}

[José Ramón (JR)]

As others pointed out, this article forgot Java has a Stream API since Java 8. For a more meaningful comparison, I wrote this post:
Java and Kotlin: a practical comparison
It's incomplete. I have many ideas for upcoming posts.