data class

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.

Exercise 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.

Show example solution

data class User(val name: String, val age: Int)

fun main() {
    val u = User("Alex", 30)
    println(u)
}

Test yourself

What does <code>data class User(val name: String)</code> generate automatically?
- Only a constructor
- equals, hashCode, toString and copy
- Only toString
- Nothing
Show answer

equals, hashCode, toString and copy — A data class generates equals, hashCode, toString, copy and componentN.

Complete the data class definition

data ____ User(val name: String, val age: Int)

Show answer

data class User(val name: String, val age: Int)

Match the data class methods

equals

Content comparison
hashCode

Hash value
toString

Readable string
copy

Clone production

Next lesson: copy

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