Kotlin Data Classes

Introduction

Data classes in Kotlin are a special kind of class designed to hold data. They provide a concise syntax for defining classes whose primary purpose is to store values. Data classes automatically provide methods like equals(), hashCode(), toString(), copy(), and component functions. This makes them very convenient for working with immutable data.

Defining a Data Class

To define a data class, use the data keyword before the class keyword. A data class must have at least one primary constructor parameter.

Syntax

data class ClassName(val property1: Type, val property2: Type)

Example

fun main() {
    val person = Person("Rahul", 25)
    println(person)

    val anotherPerson = person.copy(name = "Priya")
    println(anotherPerson)

    val (name, age) = person
    println("Name: $name, Age: $age")
}

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

Explanation:

• data class Person(val name: String, val age: Int): Defines a data class Person with properties name and age.
• println(person): Prints the string representation of the person object.
• val anotherPerson = person.copy(name = "Priya"): Creates a copy of the person object with a modified name property.
• val (name, age) = person: Destructures the person object into its properties.

Output:

Person(name=Rahul, age=25)
Person(name=Priya, age=25)
Name: Rahul, Age: 25

Properties of Data Classes

equals() and hashCode()

Data classes automatically generate equals() and hashCode() methods based on the properties defined in the primary constructor.

Example

fun main() {
    val person1 = Person("Rahul", 25)
    val person2 = Person("Rahul", 25)
    val person3 = Person("Priya", 30)

    println(person1 == person2) // True, because the properties are the same
    println(person1 == person3) // False, because the properties are different
}

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

Output:

true
false

toString()

The toString() method provides a string representation of the data class instance, including the class name and its properties.

Example

fun main() {
    val person = Person("Rahul", 25)
    println(person.toString())
}

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

Output:

Person(name=Rahul, age=25)

copy()

The copy() method creates a new instance of the data class with the same properties, allowing optional modifications to some properties.

Example

fun main() {
    val person = Person("Rahul", 25)
    val anotherPerson = person.copy(name = "Priya")
    println(anotherPerson)
}

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

Output:

Person(name=Priya, age=25)

Component Functions

Data classes automatically generate component functions for each property, which can be used for destructuring declarations.

Example

fun main() {
    val person = Person("Rahul", 25)
    val (name, age) = person
    println("Name: $name, Age: $age")
}

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

Output:

Name: Rahul, Age: 25

Example Program with Data Classes

Here is an example program that demonstrates various aspects of data classes in Kotlin:

fun main() {
    // Creating an instance of the data class
    val student = Student("Rahul", 21, "Computer Science")
    println(student)

    // Copying the data class instance
    val anotherStudent = student.copy(name = "Priya")
    println(anotherStudent)

    // Destructuring declarations
    val (name, age, major) = student
    println("Name: $name, Age: $age, Major: $major")

    // Comparing data class instances
    val student1 = Student("Rahul", 21, "Computer Science")
    val student2 = Student("Rahul", 21, "Computer Science")
    val student3 = Student("Amit", 22, "Mathematics")
    println(student1 == student2) // True
    println(student1 == student3) // False

    // Using toString method
    println(student1.toString())
}

data class Student(val name: String, val age: Int, val major: String)

Output:

Student(name=Rahul, age=21, major=Computer Science)
Student(name=Priya, age=21, major=Computer Science)
Name: Rahul, Age: 21, Major: Computer Science
true
false
Student(name=Rahul, age=21, major=Computer Science)

Conclusion

In this chapter, you learned about data classes in Kotlin, including how to define data classes, use properties, and leverage automatically generated methods such as equals(), hashCode(), toString(), copy(), and component functions. Data classes are a powerful feature in Kotlin that simplify the creation of classes intended to hold data, making your code more concise and readable.