Kotlin Sealed Classes

Classes in Kotlin

Classes in kotlin define the format for a kotlin object. It works like any other class of object-oriented programming language where classes provide the blueprint for objects.

Kotlin objects consist of two things:

• State 
• Behavior
   

SYNTAX of classes in kotlin:

class <ClassName>
{
	< property> <member function>
}

Example

Let’s take the example of cars. A car is the kotlin object. It has wheels, tires, mirrors, etc., which are the vehicle's state. The behavior of the car is its mobility. 
All cars have the same state and behavior thus, we can create a class that contains all these details about an object.
Thus, classes in kotlin are a blueprint of Kotlin objects containing their state and behavior.
There are several types of classes like enum, abstract, and sealed class in kotlin.

We will learn about sealed class in kotlin in the next section.

Sealed Class in Kotlin

When we create subclasses in Kotlin with when and else branches, the compiler does not give the appropriate answer. The compiler does not detect the subclasses as the other branch exists. Thus all the conditions in when would not be executed.

Example

The code without sealed class is implemented below as an example with Output.

Implementation

class Arithmetics
class Const(Val value: Int) : Arithmetics
	class Sum(Val left: Arithmetics, Val right: Arithmetics) : Arithmetics

		fun evaluate(e: Arithmetics): Int =
        	when (x) 
        	{
            	is Const -> x.value
            	is Sum -> evaluate(x.right) + evaluate(x.left)
            	else ->
                	throw Exception("Unknown expression")
        	}

Output

Sealed class in kotlin resolves this problem. Let us see how it solves this in the next section. 

How to Use Sealed Class in Kotlin

Sealed class in Kotlin helps to represent the hierarchy of classes. We can define subclasses under the scope of the parent class or function. 

Syntax

Sealed class <classname>
{
	Class <classname>:<sealed classname>()
	Class <classname>:<sealed classname>()
}

A subclass under sealed classes in Kotlin can have multiple parameters and values. These subclasses can be of any type, a data class, object, sealed class in kotlin, or another class.
Each subclass extends to the parent class, but the parameters are different.
Sealed class in kotlin are restricted so that you can use the class extension only in the existing file. You can not extend a class to a sealed class in kotlin in another file.

Example

We have taken an example below to print the results in a subclass under a sealed class in kotlin. 

Implementation

sealed class Print {
    class One: Print()
{
        fun display() {
            println("This is the subclass One under sealed class Print ")
        }
    }
    class Two:Demo(){
        fun display(){
            println("This is the subclass Two under sealed class Print")
        }
    }
}
 
fun main(args: Array<String>){
    val object1 =Print.One()
    object1.display()
    val object2=Print.Two()
    object2.display()
}

Output

The above example shows that subclasses one and two are detected by the compiler, and the code is executed properly.
As discussed above, this sealed class in kotlin in the file “Print” can not be used in any other file. 

Sealed Class in Kotlin with When Expression

The “when” expression in kotlin replaces if, else-if, nested if, and switch in Java. It also has an “else” branch which has no use under a sealed class in kotlin.

Example

In the below given example, we will print output using the “when” expression by calling out the subclasses under the “colour” sealed class in kotlin.  

Implementation

sealed class Colour(Val a : String)
{
    // Subclass under the sealed class
    class Yellow : Colour("Yellow")
    class Blue : Colour("Blue")
}
 
// Subclass outside the sealed class
class Red: Colour("Red")
 
fun display(colour: colour)
{
   // Using sealed classes with when
    when(colour)
    {
        is Colour.Yellow -> println("${Colour.a} symbolises happiness")
        is Colour.Blue -> println("${Colour.a} symbolises freedom")
        is Red -> println("${Colour.a} symbolises love")
    }
}
fun main()
{
    val object1= Colour.Yellow()
    val object2 = Colour.Blue()
    val object3 = Red()
 
    display(object1)
    display(object2)
    display(object3)
}

Output

We can see in the above example that the “when” expression works for subclasses under sealed class in kotlin and also for subclasses outside sealed classes in kotlin.
If we notice, we will also see that the “else” branch is unnecessary, as there are no conditions under a sealed class in kotlin that are not caught or detected by the compiler.

Why Sealed Class Over Enum Class

Developers prefer sealed class in kotlin over enum classes because

• Subclasses can be defined outside the sealed class in kotlin, whereas in the enum class, subclasses are only defined within the enum class.
   
• Unlike enum classes, different states of subclasses can be contained in sealed class in kotlin. In enum classes, storing the different states of other subclasses is difficult.
   
• Unlimited subclasses can exist under sealed class in kotlin, whereas in enum classes, only a few subclasses can be executed properly.
   
• You can add custom constructors in a sealed class in kotlin, unlike in an enum class, where you can not define various functions under the enum class.
   

Must Read Elvis Operator Kotlin

Now it's time to discuss some FAQs.

Frequently Asked Questions

Is Kotlin better than Java?

Kotlin provides a lot more features than Java. It also makes some functions and statements easier to write, for example, the removal of semicolons. Kotlin code takes more time to run, but that is because of the large number of inbuilt functions and features.

What are enum classes in Kotlin?

The Enum class in Kotlin represents a group of constants. Only a few subclasses can be stored under an enum class. 

What is an abstract method?

An abstract method does not have a body, i.e. it can’t be defined inside an abstract class. It is defined inside the derived class of the same abstract class.

What is the visibility scope of a constructor?

By default, all constructors are visible as public. But we can also change the visibility of constructors by adding appropriate access modifiers.

What is the difference between private and protected access modifiers?

The protected access modifiers have the same functionality as the private access modifier. The only difference is that variables and functions declared protected also viewable in subclasses.

Conclusion

This article discussed what is sealed class in Kotlin. We also learned why we need to use sealed class in kotlin and how to create them. We discussed how to use a sealed class in kotlin with the “when” expression and why we use a sealed class in kotlin over an enum class.
To learn more about Kotlin, refer to:

• Flutter vs. kotlin
• Kotlin operators
• Kotlin arrays
• Kotlin interfaces
   

Please refer to our guided paths on Coding Ninjas Studio to learn more about DSA, Competitive Programming, JavaScript, System Design, etc. And also, enroll in our courses and refer to the mock test and problems available. Have a look at the interview experiences and interview bundle for placement preparations.

Happy Learning!