Data class definition rules

If any of the methods that were just presented are present in your class already, the compiler won't overwrite them with its own version. You can, therefore, take full control if the requirements are as such. When you define a data class, you need to follow the following rules:

• The primary constructor needs to have at least one parameter
• All primary constructor parameters need to be marked as val or var
• Data classes cannot be abstract, open, sealed, or inner
• Data classes cannot extend other classes (but may implement interfaces)

Many Java frameworks require your class to provide a default parameter less constructor. Imagine you are writing an e-mail application and you model the Email type like this (is the way it is for the sake of simplicity):

data class Email(var to:String = "",
                 var subject:String= "",
                 var content:String= "")

The key to having the empty constructor option is to provide a default...