Kotlin Data Class Copy Function: What It Is and How to Use

In Kotlin, the copy function is automatically generated for data class types and allows you to create a new object by copying an existing one while optionally changing some properties. It helps you make modified copies without altering the original object.

How It Works

The copy function in a Kotlin data class works like making a photocopy of a document but with the option to change some words on the copy. When you call copy, it creates a new instance of the data class with the same property values as the original.

You can also specify new values for some properties during copying. This means you get a new object that is mostly the same but with some differences you choose. This is useful because data classes are often used to hold data that should not be changed directly, so copying with changes keeps the original safe.

Example

This example shows a data class Person and how to use copy to create a new person with a changed age.

kotlin

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

fun main() {
    val original = Person("Alice", 25)
    val olderAlice = original.copy(age = 26)
    println(original)
    println(olderAlice)
}

Output

Person(name=Alice, age=25)
Person(name=Alice, age=26)

When to Use

Use the copy function when you want to create a new object based on an existing one but with some changes. This is common in situations where data is immutable or should not be changed directly, such as in state management or functional programming.

For example, in an app, you might have a user profile object and want to update just the email without changing the rest. Instead of modifying the original, you use copy to create a new profile with the updated email.

Key Points

• The copy function is automatically available in all Kotlin data class types.
• It creates a new object with the same property values by default.
• You can override any property value by passing it as a named argument.
• It helps keep data immutable and safe from unintended changes.

Key Takeaways

The copy function creates a new data class object by duplicating an existing one with optional changes.

It is automatically generated for all Kotlin data classes.

Use copy to safely modify data without changing the original object.

You can override any property by naming it in the copy call.

Copy supports immutability and clean data handling in Kotlin.