Problem Statement
What are data classes and when should you use them? What are their requirements?
Explanation
Data classes marked with data keyword are specialized classes designed to hold data with automatically generated equals, hashCode, toString, copy, and destructuring component functions based on primary constructor properties. Use data classes for DTOs, API models, database entities, or any class primarily holding immutable or mutable data without significant behavior or business logic.
Data classes must have a primary constructor with at least one parameter marked as val or var, and they cannot be abstract, open, sealed, or inner. All the generated methods only consider properties declared in the primary constructor, so properties declared in the class body won't be included in equality checks or other generated methods.
The copy function is particularly useful for creating modified copies of immutable objects using named parameters like val updated equals original dot copy open paren age equals 30 close paren. Data classes eliminate dozens of lines of boilerplate compared to Java POJOs while ensuring correct implementations of equals, hashCode, and toString that respect all properties.
