Introduction to Data Classes in Kotlin

While building any application, we often need to create classes whose primary purpose is to hold data/state. These classes generally contain the same old boilerplate code in the form of getters, setters, equals(), hashcode() and toString() methods.


Consider the following example of a Customer class in Java that just holds data about a Customer and doesn’t have any functionality whatsoever -

public class Customer {
    private String id;
    private String name;

    public Customer(String id, String name) { = id; = name;

    public String getId() {
        return id;

    public void setId(String id) { = id;

    public String getName() {
        return name;

    public void setName(String name) { = name;

    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        Customer customer = (Customer) o;

        if (id != null ? !id.equals( : != null) return false;
        return name != null ? name.equals( : == null;

    public int hashCode() {
        int result = id != null ? id.hashCode() : 0;
        result = 31 * result + (name != null ? name.hashCode() : 0);
        return result;

You see, for creating a Simple class with only two member fields, we had to write almost 50 lines of code.

Yes, I know that you don’t need to write that code yourself and any good IDE can generate all that boilerplate code for you.

But that code will still be there in your source file and clutter it. Moreover, whenever you add a new member field to the Class, you’ll need to regenerate/modify the constructors, getters/setters and equals()/hashcode() methods.

You can also use a third party library like Project Lombok to generate getters/setters, equals()/hashCode(), toString() methods and more. But there is no out of the box solution without any library that can help us avoid these boilerplate codes in our application.

Kotlin Data Classes

Kotlin has a better solution for classes that are used to hold data/state. It’s called a Data Class. A Data Class is like a regular class but with some additional functionalities.

With Kotlin’s data classes, you don’t need to write/generate all the lengthy boilerplate code yourself. The compiler automatically generates a default getter and setter for all the mutable properties, and a getter (only) for all the read-only properties of the data class. Moreover, It also derives the implementation of standard methods like equals(), hashCode() and toString() from the properties declared in the data class’s primary constructor.

For example, The Customer class that we wrote in the previous section in Java can be written in Kotlin in just one line -

data class Customer(val id: Long, val name: String)

Accessing the properties of the data class

The following example shows how you can access the properties of the data class -

val customer = Customer(1, "Sachin")

// Getting a property
val name =

Since all the properties of the Customer class are immutable, there is no default setter generated by the compiler. Therefore, If you try to set a property, the compiler will give an error -

// Setting a Property

// You cannot set read-only properties = 2	// Error: Val cannot be assigned

Let’s now see how we can use the equals(), hashCode(), and toString() methods of the data class-

1. Data class’s equals() method

val customer1 = Customer(1, "John")
val customer2 = Customer(1, "John")

println(customer1.equals(customer2))  // Prints true

You can also use Kotlin’s Structural equality operator == to check for equality. The == operator internally calls the equals() method -

println(customer1 == customer2)  // Prints true

2. Data class’s toString() method

The toString() method converts the object to a String in the form of "ClassName(field1=value1, field2=value)" -

val customer = Customer(2, "Robert")
println("Customer Details : $customer")
# Output
Customer Details : Customer(id=2, name=Robert)

3. Data class’s hashCode() method

val customer = Customer(2, "Robert")
println("Customer HashCode : ${customer.hashCode()}") // Prints -1841845792

Apart from the standard methods like equals(), hashCode() and toString(), Kotlin also generates a copy() function and componentN() functions for all the data classes. Let’s understand what these functions do and how to use them -

Data Classes and Immutability: The copy() function

Although the properties of a data class can be mutable (declared using var), It’s strongly recommended to use immutable properties (declared using val) so as to keep the instances of the data class immutable.

Immutable objects are easier to work with and reason about while working with multi-threaded applications. Since they can not be modified after creation, you don’t need to worry about concurrency issues that arise when multiple threads try to modify an object at the same time.

Kotlin makes working with immutable data objects easier by automatically generating a copy() function for all the data classes. You can use the copy() function to copy an existing object into a new object and modify some of the properties while keeping the existing object unchanged.

The following example shows how copy() function can be used -

val customer = Customer(3, "James")

   Copies the customer object into a separate Object and updates the name. 
   The existing customer object remains unchanged.
val updatedCustomer = customer.copy(name = "James Altucher")
println("Customer : $customer")
println("Updated Customer : $updatedCustomer")
# Output
Customer : Customer(id=3, name=James)
Updated Customer : Customer(id=3, name=James Altucher)

Data Classes and Destructuring Declarations: The componentN() functions

Kotlin also generates componentN() functions corresponding to all the properties declared in the primary constructor of the data class.

For the Customer data class that we defined in the previous section, Kotlin generates two componentN() functions - component1() and component2() corresponding to the id and name properties -

val customer = Customer(4, "Joseph")

println(customer.component1()) // Prints 4
println(customer.component2()) // Prints "Joseph"

The component functions enable us to use the so-called Destructuring Declaration in Kotlin. The Destructuring declaration syntax helps you destructure an object into a number of variables like this -

val customer = Customer(4, "Joseph")

// Destructuring Declaration
val (id, name) = customer
println("id = $id, name = $name") // Prints "id = 4, name = Joseph"

Requirements for Data Classes

Every Data Class in Kotlin needs to fulfill the following requirements -

  • The primary constructor must have at least one parameter
  • All the parameters declared in the primary constructor need to be marked as val or var.
  • Data classes cannot be abstract, open, sealed or inner.


Data classes help us avoid a lot of common boilerplate code and make the classes clean and concise. In this article, you learned how data classes work and how to use them. I hope you understood the all the concepts presented in this article.

Thank you for reading folks. See you in the next post!