Data Class equals and hashCode in Kotlin

Introduction

Data Class equals and hashCode 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 compare values using equals instead of reference. 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 compare values using equals instead of reference.
• hashCode is generated consistently with equals.
• This is useful for collections like Set and Map keys.

Syntax

s1 == s2

Data Class equals and hashCode in Kotlin Example Program in Kotlin

data class Item(val code: String)

fun main(args: Array<String>) {
    val a = Item("A101")
    val b = Item("A101")
    println("Equal by value: ${a == b}")
}

Sample Output

Equal by value: true

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 a = Item("A101") assigns or updates a value used later in the program.

3. val b = Item("A101") assigns or updates a value used later in the program.

4. The println("Equal by value: ${a == b}") statement writes a line to the console — this produces part of the sample output below.

5. Data classes compare values using equals instead of reference.

6. 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 compare values using equals instead of reference.
• 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 compare values using equals instead of reference.
• hashCode is generated consistently with equals.
• This is useful for collections like Set and Map keys.
• 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.