What variables can be used in a data class?

In a data class, the following can be used:

• val (read-only property): The property is initialized when the object is created and cannot be changed afterwards. It is part of the automatically generated methods (equals(), hashCode(), toString(), copy()).
• var (mutable property): The property can be changed after the object is created. It is also part of the generated methods.
• Properties declared in the class body: These properties are not part of the main properties of the data class and do not participate in the generation of standard methods.

Example:

data class User(
    val id: Int, // read-only, part of main properties
    var name: String, // mutable, part of main properties
    val creationTime: Long = System.currentTimeMillis() // read-only, with default value, part of main properties
) {
    val isAdmin: Boolean = false // read-only, not part of main properties, not included in equals/hashCode/toString
}