What are sealed classes in Kotlin?

In KotlinKotlin is an open-source, modern programming language that is concise, expressive, and interoperable with Java., a sealed class is a type of class that restrict the inheritance of a class to a fixed number of classes. This means that any class outside of the declared hierarchy of classes cannot extend the sealed class. In Kotlin development, combining sealed classes with when expression is a popular approach to restricting object types.

Syntax

The syntax for declaring a sealed class in Kotlin is as follows:

// Creating a sealed class, Colour, and three subclasses, Red, Green, and Blue, each with a shade property.
sealed class Colour {
    class Red(val shade: String) : Colour() // SubClass 1
    class Green(val shade: String) : Colour() // SubClass 2
    class Blue(val shade: String) : Colour() // SubClass 3
}
fun main() {
    // Creating three instances of the Colour class: red, green, and blue.
    val red = Colour.Red("dark")
    val green = Colour.Green("light")
    val blue = Colour.Blue("medium")
    val colours = listOf(red, green, blue)
    colours.forEach { colour ->
        val message = when (colour) { // assigning a message to a variable based on the colour's type and its shade property.
            is Colour.Red -> "The shade of the red colour is ${colour.shade}."
            is Colour.Green -> "The shade of the green colour is ${colour.shade}."
            is Colour.Blue -> "The shade of the blue colour is ${colour.shade}."
        }
        println(message) // printing the message for each colour.
    }
}

Code explanation

Lines 1–5: We define a sealed class Colour with three subclasses: Red, Green, and Blue. Each subclass has a property named shade.

Lines 8–10: We create three instances of the Colour class (red, green, and blue) using the Colour.Subclass(value) method to create each instance, where the Subclass is one of the three subclasses (Red, Green, or Blue) and value is the value that we want to assign to the shade property.

Line 12: We create a list named colours containing the three instances of the Colour class.

Lines 14–19: We use a forEach loop to iterate over each color in the colours list. We use a when expression to determine the type of each color (Red, Green, or Blue) and assign a message to the message variable based on the type of color and its shade property.

Conclusion

Sealed classes in Kotlin are a powerful language feature that can be used to restrict the inheritance of a class to a fixed number of classes. They are often used with when expressions to restrict the types of objects that can be used in the when expression.

Unlock your potential: Kotlin series, all in one place!

To continue your exploration of Kotlin, check out our series of Answers below:

How to use companion objects in Kotlin
Learn how Kotlin's companion objects enable calling class members without creating an instance, similar to static methods in Java.
How to create a singleton class in Kotlin
Learn how to implement the singleton pattern in Kotlin using the object declaration for single-instance access to resources.
What are sealed classes in Kotlin?
Learn how sealed classes in Kotlin restrict inheritance to a predefined set, enhancing control and type safety in coding.
What is the purpose of Companion Object in Kotlin?
Learn how Kotlin's companion objects create static properties and functions, centralize shared attributes, and implement factory methods for cleaner, maintainable code.
Data types in Kotlin
Learn how Kotlin categorizes data types into primitive and reference types, supporting numbers, characters, booleans, arrays, strings, classes, functions, and nullable types.
Descending sort of 0s, 1s, and 2s in kotlin
Learn how to modify the Dutch National Flag algorithm in Kotlin to efficiently sort arrays of 0s, 1s, and 2s in descending order with O(N) time and O(1) space complexity.

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Free Resources

What are sealed classes in Kotlin?

Syntax

Code example

Code explanation

Conclusion