Use Data Class

Value Types Are data class

• Records, DTOs, request/response objects, domain values — these are data class
• A regular class with manually-written equals, hashCode, and toString is a data class that hasn't realised it yet
• The compiler-generated implementations are correct, fast, and impossible to forget to update when you add a field

Never Hand-Roll equals / hashCode / toString

• If you find yourself overriding all three, the class is a data class — convert it
• Hand-rolled equality silently desyncs when a field is added later and the override isn't updated. This is one of the most common Kotlin bug factories in code ported from Java
• The only legitimate reason to override these on a data class is to mask a sensitive field in toString — in which case override only toString, not all three

copy() Is the Mutation Pattern

• Update a data class instance via instance.copy(field = newValue) — preserves all other fields, gives you a new immutable instance
• This composes with val (the previous rule) — together they make immutable update painless
• Destructuring (val (id, name) = user) and componentN() come for free; lean on them when iterating collections of value objects

The Public-API Exception

• This rule targets application and internal code. On a published, binary-stable API a data class is the exception — its generated constructor / copy() / componentN() signatures break binary compatibility when properties change, so prefer a regular class there. Use the kotlin-api-review skill before converting library surface