| Hard Prerequisites |
| IMPORTANT: Please review these prerequisites, they include important information that will help you with this content. |
|
|
We frequently create a class to do nothing but hold data. In such a class some standard functionality is often mechanically derivable from the data. In Kotlin, this is called a data class and is marked as data:
data class User(val name: String, val age: Int)
The compiler automatically derives the following members from all properties declared in the primary constructor:
If any of these functions is explicitly defined in the class body or inherited from the base types, it will not be generated.
To ensure consistency and meaningful behavior of the generated code, data classes have to fulfil the following requirements:
On the JVM, if the generated class needs to have a parameterless constructor, default values for all properties have to be specified.
data class User(val name: String = "", val age: Int = 0)
It’s often the case that we need to copy an object altering some of its properties, but keeping the rest unchanged. This is what copy() function is generated for. For the User class above, its implementation would be as follows:
fun copy(name: String = this.name, age: Int = this.age) = User(name, age)
This allows us to write
val jack = User(name = "Jack", age = 1)
val olderJack = jack.copy(age = 2)
Component functions generated for data classes enable their use in destructuring declarations:
val jane = User("Jane", 35)
val (name, age) = jane
println("$name, $age years of age") // prints "Jane, 35 years of age"
The standard library provides Pair and Triple. In most cases, though, named data classes are a better design choice, because they make the code more readable by providing meaningful names for properties.