Value Class in Kotlin
2 min read

Value Class in Kotlin

Value Class in Kotlin

Kotlin's value classes(formerly known as inline classes) are a powerful feature that can help you write more efficient and expressive code. In this blog post, we'll take a look at what value classes are and how they differ from regular classes. We'll also explore some of the benefits and use cases for using value classes in your Kotlin projects.

What is a Value Class?

Sometimes it is necessary for business logic to create a wrapper around some type. However, it introduces runtime overhead due to additional heap allocations. Moreover, if the wrapped type is primitive, the performance hit is terrible, because primitive types are usually heavily optimized by the runtime, while their wrappers don’t get any special treatment.
To solve such issues, Kotlin introduces a special kind of class called an inline class. Inline classes are a subset of value-based classes. They don’t have an identity and can only hold values.

One of the key benefits of using value class is that they can help you improve the performance of your code. Because value classes do not have any identity, the compiler can optimize them at runtime, and the JVM does not need to allocate memory or perform any additional operations for a value class. This can lead to significant performance improvements, especially when working with large collections of data.

Another benefit of value classes is that they can be used to create type-safe, domain-specific types. For example, you might use a value class to create a type called PhoneNumber that is guaranteed to be a valid phone number. This can help you catch errors early on in your development process and reduce the number of runtime errors in your application.

The value class in practice

To use a value class in Kotlin, you need to define it using the value keyword. For example, here's an example of a value class called FirstName:

value class FirstName(val value: String)

If we are using Kotlin in the JVM, we have to use the @JvmInline annotation before the class declaration.

value class FirstName(val value: String)

In this example, the FirstName class is a value class that wraps a string value. You can use the FirstName class just like you would use a regular string, but it has a clear, meaningful name that makes your code more readable.

What can we use them for?

Since value classes support some functionality from regular classes we can enforce them as value objects.

In the init block we can add validation for the value so that the object is not created with invalid input.For example:

value class FirstName(
    val value: String,
) {
    init {
        require(value.isNotEmpty() && value.length > 2) { "Name should not be empty or less than 2 chars '$value'" }

In this case the FirstName object will always represent a no empty String greater than 2 chars. trying to create an object with empty name or less than two chars will through an IllegalArgumentException.

Exception in thread "main" java.lang.IllegalArgumentException: Name should not be empty or less than 2 chars ''

Another important thing to keep in mind is that value classes can only have one property, which is the property that holds the underlying value.


In conclusion, value classes are a powerful feature in Kotlin that can help you write more efficient and expressive code. They allow you to create type-safe, domain-specific types that can improve the performance of your code and make it easier to read and understand. If you're working on a Kotlin project, consider using value classes to improve the quality and maintainability of your code.