## Introduction Java is an incredibly powerful language, and it is pretty much the de-facto in Object-Oriented Programming. It is widely used across many industries and it sees a lot of usage in Enterprise settings. The JVM essentially allows it to run on any platform, hence why we can see Java in web applications, Android, gaming and even embedded systems. However, it is not without its flaws. Java is known for its verbosity and boilerplate code, which can be a pain, especially for beginners. And that is why JetBrains introduced Kotlin. ## The Appeal As we mentioned earlier, one huge selling point of Java is the Java Virtual Machine (JVM), which compiles Java code into bytecode that can run on any platform. Well, Kotlin is also powered by the JVM, which means it inherits those benefits. Kotlin is also fully interoperable with Java, which means you can use Kotlin and Java in the same project. This is a huge advantage, especially if you are migrating a Java codebase to Kotlin. You can do it incrementally, which means you can migrate one class at a time, or even one method at a time. This is a huge advantage, especially for large codebases. If they both run on the JVM, what is the big deal then? Well, Kotlin is a modern language, with a heavily improved syntax. It is much more concise than Java, which means you can write less code to achieve the same result. Additionally, while Kotlin fully supports Object-Oriented Programming, it also has a lot of functional programming features, which come in handy in multiple scenarios. It started gaining traction when Google announced that Kotlin would be the official language for Android development. This was a huge win for Kotlin, as it opened up a whole new market for the language. Functional programming allows **Jetpack Compose** to be written in Kotlin, which is a declarative UI toolkit for Android, meant to replace the traditional XML-based UI. ## Head-to-Head In this section, we will compare code snippets in both languages side by side, so that we can see a broad overview of the differences. ### Basics Let's start with a simple "Hello, World!" program. Here is how it looks in Java: ```java public class HelloWorld { public static void main(String[] args) { System.out.println("Hello, World!"); } } ``` And here is how it looks in Kotlin: ```kotlin fun main() { println("Hello, World!") } ``` As you can see, not everything in Kotlin has to be encapsulated in a class. In fact, you can write top-level functions, which is a huge advantage, especially for small scripts. #### Variables In Java, you would declare a variable like this: ```java String name = "John Doe"; //mutable final String name = "John Doe"; //immutable ``` or in newer versions: ```java var name = "John Doe"; //mutable final var name = "John Doe"; //immutable ``` In Kotlin, you can declare a variable like this: ```kotlin var name: String = "John Doe" // Mutable val name: String = "John Doe" // Immutable ``` but also like this: ```kotlin var name = "John Doe" // Mutable val name = "John Doe" // Immutable ``` That is because Kotlin has type inference, which means you don't have to specify the type of the variable if it can be inferred. Additionally, we can see that we do not need ";" at the end of each line in Kotlin. #### Functions In Java, you would declare a function like this: ```java public int sum(int a, int b) { return a + b; } ``` In Kotlin, you can declare a function like this: ```kotlin fun sum(a: Int, b: Int): Int { return a + b } ``` But you can also write it like this: ```kotlin fun sum(a: Int, b: Int) = a + b ``` The latter is called a single-expression function, and it infers the return type automatically. ##### Default Arguments In Java, if you want to have a function with default arguments, you would have to overload the function: ```java public double round(double number, int precision) { return new BigDecimal(number).setScale(precision, RoundingMode.HALF_UP).doubleValue(); } public double round(double number) { return round(number, 2); } ``` In Kotlin, you can write: ```kotlin fun round(number: Double, precision: Int = 2): Double { return BigDecimal(number).setScale(precision, RoundingMode.HALF_UP).toDouble() } ``` This is a huge advantage, especially when you have functions with multiple arguments, and you want to provide default values for some of them. #### Classes In Java, you would declare a class like this: ```java 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; } public boolean isAdult() { return age >= 18; } } ``` In Kotlin, you can declare a class like this: ```kotlin class Person(var name: String, var age: Int) ``` That's it! Kotlin automatically generates getters and setters for you. If you want to customize them, you can write: ```kotlin class Person(val name: String, val age: Int) { fun isAdult() = age >= 18 } ``` This will only generate getters, because the properties are immutable. If we need a custom setter, we can write: ```kotlin class Person { var name: String = "" set(value) { field = value.replaceFirstChar { it.uppercase() } } var age: Int = 0 set(value) { field = if (value < 0) 0 else value } constructor(name: String, age: Int) { this.name = name this.age = age } } ``` Also classes in Kotlin are **public** and **final** by default, which means they are closed for inheritance. If you want to allow inheritance, you have to mark the class as **open**. ```kotlin open class Person(var name: String, var age: Int) ``` #### Static class members and methods In Java, you would declare a static member like this: ```java public class Utils { public static final String VERSION = "1.0"; public static void printVersion() { System.out.println(VERSION); } } //main public class Main { public static void main(String[] args) { Utils.printVersion(); // This will print "1.0" } } ``` The equivalent in Kotlin is what is called a companion object: ```kotlin class Utils { companion object { const val VERSION = "1.0" fun printVersion() { println(VERSION) } } } fun main() { Utils.printVersion() // This will print "1.0" } ``` Now you would probably ask if you can use Kotlin companion objects in Java. The answer is yes, you can. You can import Kotlin functions in Java and use them referencing the companion object. ```java System.out.println(Utils.Companion.VERSION); ``` #### Inheritance In Java, you would declare a subclass like this: ```java public class Student extends Person { private String school; public Student(String name, int age, String school) { super(name, age); this.school = school; } public String getSchool() { return school; } public void setSchool(String school) { this.school = school; } } ``` In Kotlin, you can declare a subclass like this: ```kotlin class Student(name: String, age: Int, var school: String) : Person(name, age) ``` #### Records/Data Classes In Java, if you want to create a class that automatically generates `equals()`, `hashCode()`, `toString()`, and `copy()` methods, you can use a **Record**. This feature was introduced in Java 14. ```java public record Person(String name, int age) {} ``` In Kotlin, you can write: ```kotlin data class Person(val name: String, val age: Int) ``` #### Primitive Types Kotlin does not have primitive types like Java. In Kotlin, everything is an object. However, Kotlin has a feature called "Platform Types", which means that the compiler can optimize the code to use primitive types when possible. This is done under the hood, and you don't have to worry about it. So that means that in Kotlin, you can write: ```kotlin val a: Int = 5 val b: Int = 10 val c = a + b ``` and the compiler will optimize it to use primitive types. ### Null Safety One of the biggest issues in Java is the infamous NullPointerException. Kotlin solves this issue by making null safety a first-class citizen. That created the need for a new type, called **Optional** in Java. ```java String name = null; // This is allowed Optional name = Optional.ofNullable(null); System.out.println(name.orElse("John Doe")); // This will print "John Doe" ``` As we can see, we have to explicitly check if the value is null, and provide a default value if it is. In Kotlin, you can write: ```kotlin val name: String? = null println(name ?: "John Doe") // This will print "John Doe" ``` If we want to perform an operation only if the value is not null, we can write: ```kotlin val name: String? = null name?.replace("John", "Jane") // This will not throw an exception ``` This is called the **Safe Call Operator**. If the value is null, the operation will not be performed, and the result will be null. ### Higher-Order Functions (Lambdas) Kotlin has first-class support for higher-order functions, which means that you can pass functions as arguments to other functions. In Java, you would have to use interfaces to achieve the same result. This is a huge advantage, especially when working with ```java public interface Operation { int execute(int a, int b); } public class Calculator { public int calculate(int a, int b, Operation operation) { return operation.execute(a, b); } } public class Main { public static void main(String[] args) { Calculator calculator = new Calculator(); Operation addition = (a, b) -> a + b; Operation subtraction = (a, b) -> a - b; System.out.println(calculator.calculate(5, 3, addition)); // This will print 8 System.out.println(calculator.calculate(5, 3, subtraction)); // This will print 2 } } ``` In Kotlin, you can write: ```kotlin fun calculate(a: Int, b: Int, operation: (Int, Int) -> Int): Int { return operation(a, b) } fun main() { val r1 = calculate(5, 3) { a, b -> a + b } val r2 = calculate(5, 3) { a, b -> a - b } println(r1) // This will print 8 println(r2) // This will print 2 } ``` Also, single parameter lambdas can be written like this: ```kotlin fun calculate(a: Int, b: Int, operation: (Int) -> Int): Int { return operation(a) } fun main() { val r1 = calculate(5, 3) { it + 3 } val r2 = calculate(5, 3) { it - 3 } println(r1) // This will print 8 println(r2) // This will print 2 } ``` Kotlin auto-generates the parameter name `it` for single parameter lambdas and you can use it to refer to the parameter. #### Functions as types Functions behave similarly to types in Kotlin. The most basic function type is `() -> Unit`, which represents a function that takes no arguments and returns nothing. You can also define a function type that takes arguments and returns a value, like `(Int, Int) -> String`. ### Scope Functions Now that we have seen lambdas, let's talk about scope functions. These are functions that allow you to execute a block of code on an object. There are five scope functions in Kotlin: **let**, **run**, **with**, **apply** and **also**. #### let The **let** function allows you to execute a block of code on a non-null object. It is useful when you want to perform an operation on an object only if it is not null. ```kotlin val name: String? = "John Doe" name?.let { println(it) // This will print "John Doe" } ``` Why is it useful? Because you can chain multiple operations on the same object, without having to check for null each time. Also, if you want to use single expression functions, you can write: ```kotlin fun printName(name: String?) = name?.let { println(it) } ``` It lets you perform additional operations within the name scope. #### apply The **apply** function is useful when you want to perform operations on an object, and then return the object itself. Let's say we have a person object, and we want to print the name and age, and then return the object itself. Normally, you would write: ```kotlin data class Person(var name: String, var age: Int) val person = Person("John Doe", 30) println(person.name) println(person.age) ``` With **apply**, you can write: ```kotlin data class Person(var name: String, var age: Int) val person = Person("John Doe", 30).apply { println(name) println(age) } ``` As you can see, we can chain multiple operations on the same object, without explicitly referencing it, because apply puts us in the object's scope as if you were within the class itself. #### run The **run** function is similar to **apply**, but it does not return the object itself. It returns the result of the lambda. It is useful when you want to perform operations on an object, and then return a different value. ```kotlin data class Person(var name: String, var age: Int) val person = Person("John Doe", 30) val result = person.run { //value of the last expression in the lambda println(name) println(age) "Done" } println(result) // This will print "Done" ``` #### with The **with** function is similar to **run**, but it is not an extension function. It takes the object as a parameter. ```kotlin data class Person(var name: String, var age: Int) val person = Person("John Doe", 30) val result = with(person) { println(name) println(age) "Done" } println(result) // This will print "Done" ``` #### also The **also** function is useful when you want to perform an operation on an object, and then return the object itself. It is similar to **apply**, but it does not change the object itself. ```kotlin data class Person(var name: String, var age: Int) val person = Person("John Doe", 30).also { println(it.name) println(it.age) } ``` ### Extension Functions In Java, if you want to add a method to a class, you would have to create a new class that extends the original class. That works well if you have access to the source code of the original class, but what if you don't? Kotlin solves this issue by introducing **Extension Functions**. In Java, you would write: ```java public class StringUtils { public static String capitalize(String s) { return s.substring(0, 1).toUpperCase() + s.substring(1); } } ``` In Kotlin, you can write: ```kotlin fun String.capitalize() = substring(0, 1).uppercase() + substring(1) val name = "john doe" println(name.capitalize()) // This will print "John doe" ``` You can call the extension function on any object of that type, and it will behave as if it was a method of that class. ### Interfaces In Java, you would declare an interface like this: ```java public interface Animal { void makeSound(); } ``` In Kotlin, you can declare an interface like this: ```kotlin interface Animal { fun makeSound() } ``` In Java, you would implement an interface like this: ```java public class Dog implements Animal { @Override public void makeSound() { System.out.println("Woof!"); } } ``` In Kotlin, you can implement an interface like this: ```kotlin class Dog : Animal { override fun makeSound() { println("Woof!") } } ``` Now, here are some interesting things about interfaces in Kotlin: - They can have properties with default implementations - You an also have abstract properties - You can have private methods in interfaces ```kotlin interface Animal { val name: String get() = "Animal" fun makeSound() fun eat() { println("Eating...") } private fun sleep() { println("Sleeping...") } } ``` ### Sealed Classes Kotlin has a feature called **Sealed Classes**, which are used to represent restricted class hierarchies. A sealed class can have subclasses, but all of them must be declared in the same file as the sealed class itself. In Java, you would write: ```java public abstract class Result { private Result() {} public static final class Success extends Result { private final String message; public Success(String message) { this.message = message; } public String getMessage() { return message; } } public static final class Error extends Result { private final String message; public Error(String message) { this.message = message; } public String getMessage() { return message; } } } ``` In Kotlin, you can write: ```kotlin sealed class Result { data class Success(val message: String) : Result() data class Error(val message: String) : Result() } ``` This is much more concise and easier to read. Sealed classes are useful when you want to represent a restricted class hierarchy. They are commonly used in Jetpack Compose to represent application state. ### Collections Kotlin extends on the bulletproof Java Collections Framework by providing immutable collections and functions like `map, filter, reduce, fold, groupBy, partition, zip, unzip, associateBy, associateWith, and more`. To create an immutable list in Java, you would write: ```java List list = List.of("John", "Doe", "Jane"); ``` In Kotlin, you can write: ```kotlin val list = listOf("John", "Doe", "Jane") ``` To create a mutable ArrayList in Java, you would write: ```java List list = new ArrayList<>(List.of("John", "Doe", "Jane")); ``` In Kotlin, you can write: ```kotlin val list = mutableListOf("John", "Doe", "Jane") ``` To filter a list in Java, you would write: ```java List filteredList = list.stream() .filter(s -> s.startsWith("J")) .collect(Collectors.toList()); ``` In Kotlin, you can write: ```kotlin val filteredList = list.filter { it.startsWith("J") } ``` To map a list in Java, you would write: ```java List mappedList = list.stream() .map(s -> s.toUpperCase()) .collect(Collectors.toList()); ``` In Kotlin, you can write: ```kotlin val mappedList = list.map { it.toUpperCase() } ``` To reduce a list in Java, you would write: ```java String reduced = list.stream() .reduce("", (a, b) -> a + b); ``` In Kotlin, you can write: ```kotlin val reduced = list.reduce { a, b -> a + b } ``` To group a list in Java, you would write: ```java Map> grouped = list.stream() .collect(Collectors.groupingBy(String::length)); ``` In Kotlin, you can write: ```kotlin val grouped = list.groupBy { it.length } ``` To partition a list in Java, you would write: ```java Map> partitioned = list.stream() .collect(Collectors.partitioningBy(s -> s.length() > 3)); ``` In Kotlin, you can write: ```kotlin val partitioned = list.partition { it.length > 3 } ``` For more information on the incredibly rich Kotlin Collections API, you can check the official documentation [here](https://kotlinlang.org/docs/collections-overview.html#set) ### Traditional while and for loops In Java, you would write: ```java for (int i = 0; i < 10; i++) { System.out.println(i); } ``` In Kotlin, you can write: ```kotlin for (i in 0 until 10) { println(i) } ``` or even: ```kotlin (0 until 10).forEach { println(it) } ``` For while loops, in Java you would write: ```java int i = 0; while (i < 10) { System.out.println(i); i++; } ``` In Kotlin, you can write: ```kotlin var i = 0 while (i < 10) { println(i) i++ } ``` ### When / Switch In Java, you would write: ```java int day = 1; switch (day) { case 1: System.out.println("Monday"); break; case 2: System.out.println("Tuesday"); break; default: System.out.println("Unknown day"); } ``` In Kotlin, you can write: ```kotlin val day = 1 when (day) { 1 -> println("Monday") 2 -> println("Tuesday") else -> println("Unknown day") } ``` ### Pairs and Triples In Java, you would write: ```java Pair pair = new Pair<>("John", 30); Triple triple = new Triple<>("John", 30, "Doe"); ``` In Kotlin, you can write: ```kotlin val pair = "John" to 30 val triple = Triple("John", 30, "Doe") ``` ### Operator functions In Java, let's say you have a **Point** class with **x** and **y** properties, and you want to add two points together. ```java public class Point { private int x; private int y; public Point(int x, int y) { this.x = x; this.y = y; } public Point add(Point other) { return new Point(x + other.x, y + other.y); } } //main public class Main { public static void main(String[] args) { Point p1 = new Point(1, 2); Point p2 = new Point(3, 4); Point p3 = p1.add(p2); System.out.println(p3.x); // This will print 4 System.out.println(p3.y); // This will print 6 } } ``` In Kotlin, you can write: ```kotlin data class Point(val x: Int, val y: Int) { operator fun plus(other: Point) = Point(x + other.x, y + other.y) } //main fun main() { val p1 = Point(1, 2) val p2 = Point(3, 4) val p3 = p1 + p2 println(p3.x) // This will print 4 println(p3.y) // This will print 6 } ``` So, it features operator overloading, similar to Python or C++. ### Coroutines Java has a lot of ways to handle asynchronous programming, like **CompletableFuture**, **RxJava**, **ExecutorService**, etc. Kotlin introduces **Coroutines**, which are a way to handle asynchronous programming in a more concise and readable way. In Java, you would write: ```java ExecutorService executor = Executors.newFixedThreadPool(2); Future future = executor.submit(() -> { Thread.sleep(1000); return "Hello, World!"; }); try { String result = future.get(); System.out.println(result); // This will print "Hello, World!" } catch (InterruptedException | ExecutionException e) { e.printStackTrace(); } ``` In Kotlin, you can write: ```kotlin import kotlinx.coroutines.* fun main() { runBlocking { val result = async { delay(1000) "Hello, World!" } println(result.await()) // This will print "Hello, World!" } } ``` ### Singletons In Java, you would write: ```java public class Singleton { private static Singleton instance; private Singleton() {} public static Singleton getInstance() { if (instance == null) { instance = new Singleton(); } return instance; } } ``` In Kotlin, you can write: ```kotlin object Singleton ``` Yes, you can declare pure objects in Kotlin, similar to Scala or languages like JavaScript. You can even do it in place when you need it: ```kotlin fun main() { val singleton = object { var name = "John Doe" var age = 30 } println(singleton.name) // This will print "John Doe" println(singleton.age) // This will print 30 singleton.name = "Jane Doe" println(singleton.name) // This will print "Jane Doe" } ``` ### Generics Generics in OOP languages are absolutely essential to make sure that our creations can fit multiple types. The basics of generics are very similar in Java and Kotlin, but Kotlin has a few extra features. Let's say we have a simple **Box** class that can hold any type of object. In Java, you would write: ```java public class Box { private T value; public Box(T value) { this.value = value; } public T getValue() { return value; } } ``` In Kotlin, you can write: ```kotlin class Box(private val value: T) { fun getValue(): T { return value } } ``` or let type inference do its job: ```kotlin class Box(private val value: T) { fun getValue() = value } ``` Yes, Kotlin does create automatic getters for non-private properties, this was just done for demonstration purposes. What if we only want numbers in our box? In Java, you would write: ```java public class NumberBox { private T value; public NumberBox(T value) { this.value = value; } public T getValue() { return value; } } ``` In Kotlin, you can write: ```kotlin class NumberBox(private val value: T) { fun getValue(): T { return value } } ``` Now, some more advanced features of generics #### Star Projections/Wildcards In Java, you would write: ```java public void printList(List list) { for (Object o : list) { System.out.println(o); } } ``` The `?` essentially means that the list can hold any type of object but you do not care because you are not performing any operations that need to statically know the type. In Kotlin, this is achieved with `*`: ```kotlin fun printList(list: List<*>) { for (o in list) { println(o) } } ``` #### Reified Generics Now, here is the interesting thing. At runtime, the type of the generic is erased, which means that you cannot access it. To solve this issue, Kotlin introduced **reified generics**. In Java, the type information is erased at runtime, so we can only access the class information through the object itself. This is why we have to pass the class as an argument. ```java public void printClass(T t) { System.out.println(t.getClass().getName()); } ``` In Kotlin, you can write: ```kotlin inline fun printClass(t: T) { println(T::class.qualifiedName) } ``` In this case, we can access the class information at runtime, because Kotlin replaces the type with the actual class at compile time. This is incredibly powerful, especially when working with reflection. #### Inline Functions We saw the `inline` keyword in the previous example. What is this magic keyword? Well, in Kotlin, functions are first-class citizens, which means that you can pass them as arguments to other functions. However, this comes with a cost. Every time you pass a function as an argument, Kotlin creates a new object, which can be memory-intensive for large functions. To solve this issue, Kotlin introduced the `inline` keyword. The `inline` keyword tells the compiler to copy the function body to the call site, which means that you avoid creating a new object every time you pass a function as an argument. So if we have a function that takes a function as an argument: ```kotlin fun measureTimeMillis(block: () -> Unit) { val start = System.currentTimeMillis() block() val end = System.currentTimeMillis() println("Execution time: ${end - start} ms") } fun main() { measureTimeMillis { Thread.sleep(1000) } } ``` After compilation, the code will look like this: ```kotlin fun measureTimeMillis(block: () -> Unit) { val start = System.currentTimeMillis() Thread.sleep(1000) val end = System.currentTimeMillis() println("Execution time: ${end - start} ms") } ``` #### In/Out Projections (Covariance/Contravariance) Java relies on wildcards to achieve covariance and contravariance in the call site. This is a bit cumbersome and not very intuitive. Kotlin solves this issue by introducing `in` and `out` projections. In Java, you would write: ```java public class Box { private T value; public Box(T value) { this.value = value; } public T getValue() { return value; } } public void printBox(Box box) { // Covariance Number value = box.getValue(); System.out.println(value); } public void setBox(Box box) { // Contravariance box.setValue(5); } ``` In Kotlin, you can write: ```kotlin class Box(private var value: T) { fun getValue(): T { return value } } ``` or if we need to process T: ```kotlin class Box(private var value: T) { fun setValue(value: T) { this.value = value } } ``` The same functionality is achieved, but in different ways. ### Scoped Function Parameters What if we want to be inside a certain context when passing lambdas around? Since Java does not even support proepr lambdas, this is not possible. However, in Kotlin, it is possible. Let us say we have a **Person** class with a **print** function that takes a lambda as an argument. ```kotlin data class Person(val name: String, val age: Int) { fun print(block: Person.() -> Unit) { block() } } ``` Now, we can call the **print** function and be inside the **Person** context. ```kotlin fun main() { val person = Person("John Doe", 30) person.print { println(name) println(age) } } ``` This is incredibly powerful, especially when working with complex objects. ### Destructuring Declarations In Java, if you want to extract multiple values from an object, you would have to do it manually. ```java 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 int getAge() { return age; } } //main public class Main { public static void main(String[] args) { Person person = new Person("John Doe", 30); String name = person.getName(); int age = person.getAge(); System.out.println(name); // This will print "John Doe" System.out.println(age); // This will print 30 } } ``` In Kotlin, you can write: ```kotlin data class Person(val name: String, val age: Int) fun main() { val (name, age) = Person("John Doe", 30) println(name) // This will print "John Doe" println(age) // This will print 30 } ``` ### Type Aliases Let's say we have the Box class from before, that takes a generic type `T`. However, we notice that we use `String` and `Int` a lot and do not want to write `Box` and `Box` every time. In Java, you would have to create a new class for each type through inheritance. ```java public class StringBox extends Box { public StringBox(String value) { super(value); } } public class IntBox extends Box { public IntBox(Integer value) { super(value); } } ``` In Kotlin, you can write: ```kotlin typealias StringBox = Box typealias IntBox = Box ``` This is much more concise and easier to read. ### Constants In Java, you would write: ```java public class Constants { public static final String NAME = "John Doe"; public static final int AGE = 30; } ``` In Kotlin, you can write: ```kotlin object Constants { const val NAME = "John Doe" const val AGE = 30 } ``` ### Libraries #### Server Side Kotlin is supported by all major Java frameworks, like Spring, Micronaut, Quarkus, Http4k. Additionally, it has its own framework, called **Ktor**, which is a modern and asynchronous web framework. The Java frameworks are way more mature, but Ktor's API fits incredibly well in Kotlin's design paradigm, because it is developed by JetBrains with Kotlin in mind. #### Object-Relational Mapping (ORM) Kotlin has a few libraries for Object-Relational Mapping, like **Exposed** and **Ktorm** which are Kotlin Native. Additionally, you can use Java libraries like **Hibernate** and **JPA**. However, they are not as idiomatic as the Kotlin ones, but are more mature and have a larger community. ##### SQL Builders Both **Exposed** and **Ktorm** have their own SQL builders, which have pretty good support for most SQL databases. However, for more complex queries, you might want to use a Java library like **jOOQ**. It is not as idiomatic as the Kotlin ones, but it is incredibly powerful and has a lot of features. #### Serialization Kotlin has its own serialization library, called **Kotlinx Serialization**, which is a multiplatform serialization library. It is incredibly powerful and has a lot of features. Additionally, you can use Java libraries like **Jackson** and **Gson**. #### Testing Kotlin has a few testing libraries, like **Kotest** and **Spek**. Additionally, you can use Java libraries like **JUnit** and **TestNG**. ##### Mocking Kotlin has a few mocking libraries, like **Mockk** and **Kotest**. Additionally, you can use Java libraries like **Mockito** and **EasyMock**. #### Dependency Injection Kotlin has a few dependency injection libraries, like **Koin** and **Dagger**. Additionally, you can use Java libraries like **Spring** and **Guice**. #### Logging Kotlin has a few logging libraries, like **Kotlin Logging** and **Logback**. Additionally, you can use Java libraries like **Log4j** and **SLF4J**. ### Kotlin Multiplatform Now, while we just talked about Kotlin on the JVM, Kotlin can now do so much more through Kotlin Multiplatform. Google recently announced support for the **KMP** project, which can target multiple platforms, like **JVM**, **JS**, **iOS**, **Android**, and **Native**. This means that you can write your business logic once, and then share it across all platforms. This is incredibly powerful, especially for companies that have multiple platforms. That said, it is still in its early stages, and it is not as mature as frameworks like **React Native** or **Flutter**. ### Negatives While Kotlin is an incredibly powerful language, it is not without its flaws. Here are a few negatives: - Heavy reliance on Gradle: Kotlin added syntax sugar cannot be achieved without Gradle, which adds more complexity to the build process. - It does not have as strong of a language server as Java, which means that it only works well with IntelliJ IDEA. - The job market is still dominated by Java, which means that it might be harder to find a job as a Kotlin developer. ### Conclusion Kotlin is an incredibly powerful language, and it is a great alternative to Java. It has very rich, beautiful syntax, and awesome libraries for almost everything. #### More Resources - [Kotlin Official Documentation](https://kotlinlang.org/docs/home.html) - [Kotlin Koans](https://play.kotlinlang.org/koans/overview) - [Kotlin Playground](https://play.kotlinlang.org/) - [Kotlin Multiplatform](https://kotlinlang.org/lp/mobile/) - [Kotlin for Android Developers](https://developer.android.com/kotlin)

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