Data Class copy in Kotlin

Introduction

Data Class copy is a fundamental concept every Kotlin developer should understand. Data classes auto-generate equals, hashCode, toString, and copy for model types — ideal for DTOs, API payloads, and domain objects.

Copy() creates a new object with selected changed properties. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• copy() creates a new object with selected changed properties.
• Unchanged properties keep original values.
• Useful for immutable updates.

Syntax

val updated = user.copy(name = "New Name")

Data Class copy in Kotlin Example Program in Kotlin

data class Product(val id: Int, val price: Double)

fun main(args: Array<String>) {
    val p1 = Product(10, 99.0)
    val p2 = p1.copy(price = 89.0)
    println("Original: $p1")
    println("Updated: $p2")
}

Sample Output

Original: Product(id=10, price=99.0)
Updated: Product(id=10, price=89.0)

When to use

Use data classes for immutable value objects where structural equality and copy-with-modifications matter.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. val p1 = Product(10, 99.0) assigns or updates a value used later in the program.

3. val p2 = p1.copy(price = 89.0) assigns or updates a value used later in the program.

4. The println("Original: $p1") statement writes a line to the console — this produces part of the sample output below.

5. The println("Updated: $p2") statement writes a line to the console — this produces part of the sample output below.

6. Copy() creates a new object with selected changed properties.

7. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

• Understand the core idea: copy() creates a new object with selected changed properties.
• Prefer readable names and small functions so examples map directly to real projects.
• Run and modify the example — change values and observe how the output changes.

Common Mistakes

• Skipping the example and only reading the definition — hands-on practice cements the concept.
• Copying syntax without understanding nullable vs non-nullable types or scope rules.
• Ignoring compiler warnings that often point to safer alternatives.

Key Points

• copy() creates a new object with selected changed properties.
• Unchanged properties keep original values.
• Useful for immutable updates.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.