Kotlin Data Class

In this tutorial, you'll learn about Fix Kotlin Data Class Copy Not Working. We cover key concepts, practical examples, and best practices.

The Problem

The data class copy() method does not create a properly modified copy.

Quick Fix

Use named arguments

Wrong:

data class User(val name: String, val age: Int)
val user = User("Alice", 30)
val copy = user.copy("Bob") // Positional argument

Output:

Wrong field modified

Right:

val copy = user.copy(name = "Bob") // Named argument

Output:

Correct field modified

Data class requirements

Wrong:

class User(val name: String) // Not a data class

Output:

No copy()

Right:

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

Output:

copy() method generated

Understand shallow copy

Wrong:

data class Address(val street: String)
data class Person(val name: String, val addr: Address)
val p1 = Person("Alice", Address("Main"))
val p2 = p1.copy()
p2.addr.street = "Second" // Both p1 and p2 affected

Output:

Shared reference

Right:

// Use immutable data classes or deep copy for nested data

Output:

Avoid mutable nested objects

Prevention

• Use named arguments with copy() for clarity
• Declare classes with data modifier for auto-generated methods
• Be aware that copy() performs shallow copy

Common Mistakes with data class

1. Using return to exit a function early instead of wrapping a pure value in the monad
2. Mixing let bindings with <- bindings in do notation, producing type errors
3. Overlapping type class instances that cause GHC to reject the program with ambiguous dispatch errors

These mistakes appear frequently in real-world KOTLIN code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

This quick fix is part of the DodaTech Spring & JVM ecosystem series. Built by the developers of Doda Browser, DodaZIP, and Durga Antivirus Pro.