Data class

How to make a simple class for storing data? In addition to storing information, it should be able to compare and copy objects. Also, it would be very convenient to output data immediately. Normally, for this functionality the class must have some methods: equals() and hashCode() for comparison, copy() for copying, toString() for the string representation of the object and componentN() functions corresponding to the properties in their order of declaration. But in Kotlin you don't need to implement all of these functions, you can simply use the data class. Let’s take a closer look at this type of class.

Data class

First of all, we need a class, so here is a nice Client class:

class Client(val name: String, val age: Int, val gender: String)

Right now it has 3 properties, so far so good! But in order to properly compare the objects (i.e., by their properties) we need to implement equals() and hashCode() functions:

class Client(val name: String, val age: Int, val gender: String) {
    override fun equals(other: Any?): Boolean {
        if (this === other) return true
        if (javaClass != other?.javaClass) return false

        other as Client

        if (name != other.name) return false
        if (age != other.age) return false
        if (gender != other.gender) return false

        return true
    }

    override fun hashCode(): Int {
        var result = name.hashCode()
        result = 31 * result + age
        result = 31 * result + gender.hashCode()
        return result
    }
}

Why do we need such a long piece of code just for standard stuff? That's the right question because with the data class we can simplify it like this:

data class Client(val name: String, val age: Int, val gender: String)

Wait! Where are all my functions now? Actually, with the data keyword, you don't need them anymore. It will magically work as if you have already implemented them all. This keyword will also provide toString() ,copy() and componentN() functions with default behavior (equals and hashCode). We'll look at copy() and componentN()a bit later, but right now you need to remember several rules here:

1. You can only count on properties that are inside the constructor. For example, this modified Client class:

data class Client(val name: String, val age: Int, val gender: String) {
    var balance: Int = 0
}

All those functions won't consider balance field, because it isn't inside the constructor.

2. You can override all those functions, except for copy():

data class Client(val name: String, val age: Int, val gender: String) {
    var balance: Int = 0

    override fun toString(): String {
        return "Client(name='$name', age=$age, gender='$gender', balance=$balance)"
    }

}

Now balance field is involved in the toString() function.

3. The primary constructor of a data class must have at least one parameter and all of those parameters must be val or var.

Copy and ComponentN

To be honest, there isn't really a convenient way to copy an object in Java, but Kotlin is different. For example, what if we have an instance of our Client class and we want the exact same client, just with a different name? Easy!

fun main() {
    val bob = Client("Bob", 29, "Male")
    val john = bob.copy(name = "John")
    println(bob)
    println(john)
}

As you may see, we just used our copy() function, which will be provided automatically with the data keyword. And the output will be the following:

Client(name='Bob', age=29, gender='Male', balance=0)
Client(name='John', age=29, gender='Male', balance=0)

The componentN() functions correspond to the properties in their order of declaration: component1(), component2(), ..., for all data class properties. The Component functions generated for data classes make it possible to use them in destructuring declarations

fun main() {
    val bob = Client("Bob", 29, "Male")
    println(bob.name) // Bob
    println(bob.component1()) // Bob
    println(bob.age)  // 29
    println(bob.component2()) // 29
    println(bob.gender) // Male
    println(bob.component3()) // Male
    
    // destructuring
    val (name, age, gender) = bob
    println(name) // Bob
    println(age)  // 29
    println(gender) // Male
}

Idiom

As we've demonstrated, the data class is a convenient way to organize data or to create DTOs (Data Transfer Objects). So use it, with community approval!

data class Customer(val name: String, val email: String)

Conclusion

Now you know how to simplify boilerplate code with the data keyword. It helps not only to shorten your code but also to save your time. Use it wisely!