Data Class Declaration in Kotlin

Introduction

Data Class Declaration 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.

Data classes are designed to hold data. 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

• Data classes are designed to hold data.
• Compiler automatically generates equals, hashCode, toString and copy.
• Primary constructor must have at least one parameter.

Syntax

data class User(val id: Int, val name: String)

Data Class Declaration in Kotlin Example Program in Kotlin

data class Student(val id: Int, val name: String)

fun main(args: Array<String>) {
    val s1 = Student(1, "Anu")
    println(s1)
}

Sample Output

Student(id=1, name=Anu)

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 s1 = Student(1, "Anu") assigns or updates a value used later in the program.

3. The println(s1) statement writes a line to the console — this produces part of the sample output below.

4. Data classes are designed to hold data.

5. 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: data classes are designed to hold data.
• 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

• Data classes are designed to hold data.
• Compiler automatically generates equals, hashCode, toString and copy.
• Primary constructor must have at least one parameter.
• 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.