data class
- Kotlin course
- Intermediate
- Lesson 13/116
- 5 min
Learn to define data classes with data class.
data class
A data class is a special class designed to hold data. With the data modifier Kotlin generates equals, hashCode, toString, copy and componentN methods automatically.
Example: data class User(val name: String, val age: Int). This single line carries two properties and five helper methods. Data classes must have a primary constructor with at least one parameter. Parameters marked val or var become properties.
data class constraints
The primary constructor of a data class must have at least one parameter. All parameters must be marked val or var; otherwise they do not become properties.
Example: data class User(val name: String, val age: Int) is valid. data class User(name: String) lacks properties, so equals and toString do not work as expected. Data classes cannot be open, abstract or sealed; inheritance is closed.
Practice Define the User data class
Define a data class named User with a primary constructor taking val name: String, val age: Int. In main create a User("Alex", 30) instance and print it with println.
Starter code
// Define the User data class
// with name and age properties
// instantiate and print in mainSample solution
data class User(val name: String, val age: Int)
fun main() {
val u = User("Alex", 30)
println(u)
}Expected output
User(name=Alex, age=30)
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does <code>data class User(val name: String)</code> generate automatically?
- Only a constructor
- equals, hashCode, toString and copy Correct answer
- Only toString
- Nothing
Explanation: A data class generates equals, hashCode, toString, copy and componentN.
2 Complete the data class definition
data ____ User(val name: String, val age: Int)data class User(val name: String, val age: Int)Explanation: data class declares a data class.
3 Match the data class methods
| Term | Meaning |
|---|---|
equals |
Content comparison |
hashCode |
Hash value |
toString |
Readable string |
copy |
Clone production |