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Reflection and annotations in Kotlin

By David Li on Fri, 14 December 2024

Reflection and annotations in Kotlin

Reflection and annotations are powerful tools in Kotlin for introspection and metaprogramming.

Reflection is the ability of a program to inspect its own structure at runtime. Kotlin’s reflection API allows you to retrieve information about classes, properties, and functions at runtime. You can also use reflection to dynamically create instances of classes, invoke methods, and get and set properties.

Annotations are metadata that you can attach to classes, properties, and functions to provide additional information about them. Kotlin provides several built-in annotations that you can use, such as @Deprecated and @JvmName. You can also create your own annotations to add custom metadata to your code.

Here’s an example of using reflection to get the properties of a class:

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

fun main() {
 val person = Person("Alice", 29)
 val properties = person.javaClass.declaredFields
 properties.forEach { property ->
 property.isAccessible = true
 val value = property.get(person)
 println("${property.name} = $value")

In this example, we create a Person class with two properties, name and age. We then use reflection to get the declared fields of the Person class and print out their names and values. We set isAccessible to true to allow us to access private properties.

Here’s an example of using an annotation to mark a deprecated function:

@Deprecated("Use the 'greet' function instead")
fun hello(name: String) {
 println("Hello, $name!")

fun greet(name: String) {
 println("Hi, $name!")

fun main() {

In this example, we have a deprecated hello function that we mark with the @Deprecated annotation. We provide a message that tells the user to use the greet function instead. When we call the hello function in our main function, we get a warning that the function is deprecated.

Overall, reflection and annotations are powerful tools that you can use to add flexibility and customization to your Kotlin code.

Destructuring declarations is a feature in Kotlin that allows you to break down an object into multiple variables. This feature is particularly useful when working with data classes or other types of objects with multiple properties.

To use destructuring declarations, you simply create a new variable for each property you want to extract and assign them using the object you want to destructure. For example:

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

fun main() {
 val person = Person("John Doe", 30)
 val (name, age) = person
 println("Name: $name, Age: $age")

In this example, we create a Person object and then use destructuring declarations to extract the name and age properties into separate variables. We then print out the values of these variables.

Destructuring declarations can also be used with other types of objects, such as arrays and maps. For example:

fun main() {
 val array = arrayOf("John", "Doe")
 val (firstName, lastName) = array
 println("First name: $firstName, Last name: $lastName")
 val map = mapOf("name" to "John Doe", "age" to 30)
 val (name, age) = map
 println("Name: $name, Age: $age")

In this example, we use destructuring declarations to extract the values from an array and a map. We create new variables for each value we want to extract and then assign them using destructuring declarations. We then print out the values of these variables.

Sealed classes are a special type of class in Kotlin that restricts the inheritance of the class hierarchy. In other words, sealed classes are used when we have a limited set of possible subclasses that we want to restrict.

To define a sealed class, we use the “sealed” keyword followed by the name of the class. Then, we define the subclasses of the sealed class within the same file as the sealed class using the “class” keyword.

The main benefit of using sealed classes is that they allow us to use “when” expressions (similar to switch statements in other languages) to handle a limited set of possible subclasses. This can make our code more concise and easier to read.

Here’s an example of how to define a sealed class in Kotlin:

sealed class Shape {
 class Circle(val radius: Double) : Shape()
 class Rectangle(val width: Double, val height: Double) : Shape()

In this example, we define a sealed class called “Shape” with two subclasses: “Circle” and “Rectangle”. Now, we can use “when” expressions to handle instances of these subclasses:

fun calculateArea(shape: Shape): Double {
 return when (shape) {
 is Shape.Circle -> Math.PI * shape.radius * shape.radius
 is Shape.Rectangle -> shape.width * shape.height

In this example, we define a function called “calculateArea” that takes an instance of “Shape” as an argument and returns the area of the shape. We use a “when” expression to handle each of the possible subclasses of “Shape” and calculate the area accordingly.

Type aliases in Kotlin allow developers to create custom names for existing types in the code. This feature can be helpful in creating more concise and expressive code, and can also improve readability and maintainability. Type aliases are especially useful when dealing with complex types, such as function types or nested classes.

To define a type alias in Kotlin, use the “typealias” keyword followed by the desired alias name and the existing type. Here is an example:

typealias EmployeeSet = Set<Employee>

In this example, we are creating an alias called “EmployeeSet” for the existing type “Set”. Once this alias is defined, it can be used in place of the original type in the code:

fun addEmployee(employeeSet: EmployeeSet, employee: Employee) {

Here, the “EmployeeSet” alias is used as the parameter type for the “addEmployee” function.

Type aliases can also be used with function types:

typealias Operation = (Int, Int) -> Int

val add: Operation = { a, b -> a + b }
val subtract: Operation = { a, b -> a - b }

fun calculate(op: Operation, a: Int, b: Int): Int {
 return op(a, b)

In this example, we define a type alias called “Operation” for a function type that takes two integers and returns an integer. We then define two variables of type “Operation”, which represent the addition and subtraction operations. Finally, we define a function called “calculate” that takes an operation and two integers as parameters, and returns the result of applying the operation to the integers.

Type aliases can make code more readable and concise by allowing developers to define custom names for complex types. They are especially useful when working with function types and nested classes.

Operator overloading is a feature in Kotlin that allows operators such as +, -, *, /, %, and many others to be defined with custom behavior when used with custom types. It allows for more expressive and readable code when working with custom types.

To overload an operator in Kotlin, the operator keyword is used followed by the operator being overloaded. For example, to overload the plus operator (+), the function signature would be:

operator fun plus(other: CustomType): CustomType {
 // custom implementation

The function can then be called using the plus operator with instances of the custom type:

val result = customType1 + customType2

It is important to note that not all operators can be overloaded and some have predefined behavior that cannot be changed. Additionally, it is recommended to use operator overloading sparingly and only when it adds clarity to the code.

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