Kotlin data class explained with easy to understand examples

Kotlin data class is a unique class that’s used to create a class whose main purpose is to hold data in your source code.

For example, if you have an e-commerce application like Amazon or Shopify, you may have a Product class that contains data for each of your products.

The following is a simplified example of a Product class that you may create in Java:

class Product {
    public int id;
    public String name;
    public int price;

    public Product(int id, String name, int price) {
        this.id = id;
        this.name = name;
        this.price = price;
    }
}

Once you have the Product class, you can create a new instance of the class in the main() method as follows:

public static void main(String[] args) {
    Product myProduct = new Product(1, "Apple", 5);
    myProduct.id = 5;
    System.out.println(myProduct.id); // 5
}

In Kotlin, the Product class can be simplified using the data class as follows:

data class Product(var id: Int, var name: String, var price: Int)

And that’s it. Once you create the data class, you can use that class to create a new instance as follows:

var myProduct = Product(1, "Apple", 5)
myProduct.id = 5
println(myProduct.id) // 5

By now you might be wondering what’s the difference between a regular class and a data class in Kotlin.

A data class in Kotlin will generate the following functions for you to use when you need it:

• toString()
• equals() / hashCode() pair
• componentN() function to allow destructuring of data class
• copy() function to return a copy of the data class instance.

The toString() function returns the string representation of the constructor. Consider the following example:

var myProduct = Product(1, "Apple", 5)

println(myProduct.toString()) 
// Product(id=1, name=Apple, price=5)

Without the data class, the toString() function will return the object itself instead of the content.

The equals() and hashCode() functions are the same as in the normal class:

• The equals() function allows you to compare the object instance equality
• The hashCode() function returns the hash number of the object instance

The componentN() function generated for the data class allows you to destructure the values of the object instance.

Consider the following example:

var myProduct = Product(1, "Apple", 5)
val (id, name) = myProduct
println(id) // 1
println(name) // Apple

Without the componentN() function, the highlighted line above will cause an error.

Finally, the copy() function allows you to copy an object instance from a data class.

You can also change the properties of the class as shown below:

var myProduct = Product(1, "Apple", 5)

val productCopy = myProduct.copy(id = 3, name = "Orange")

println(productCopy.toString()) // Product(id=3, name=Orange, price=5)

Those are the functions generated when you use the data class keyword in Kotlin.

A data class also has the following requirements:

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

And that’s how a data class works in Kotlin.

A data class is used when you need a class whose primary purpose is to hold data in your source code.