What is a Kotlin data class?

In Java, we had to often make classes for our data models. A Kotlin equivalent would be something like this:

package com.example.firstgpslocation

class Person {
    var firstName: String = ""
    var lastName: String = ""
    var age: Int = 0
}

But now in Kotlin we have data classes, like this:

package com.example.firstgpslocation

data class Person(
    var firstName: String,
    var lastName: String,
    var age: Int
)

We can create it from MainActivity.kt with this:

package com.example.firstgpslocation

import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        setSupportActionBar(toolbar)

        val person = Person("Daniel", "Malone", 27)
    }
}

Accessing data is easy:

        // setup
        val person = Person("Daniel", "Malone", 27)
        
        // access firstName like this
        val firstName = person.firstName

No need for setFirstName, setLastName or getAge. Simply call person.firstName, person.age, etc.

A Kotlin data class can also be nested, as in the following example. Each person has Work, which has a Company.

package com.example.firstgpslocation

data class Person(
    var firstName: String,
    var lastName: String,
    var age: Int,
    var work: Work
) {

    data class Work(
        val company: Company,
        val startDate: Long,
        val endDate: Long,
        val jobTitle: String
    ) {

        data class Company(
            val name: String,
            val founded: Long
        )
    }
}

Data class tips

Although we can create mutable variables in data classes (for example, var name = "Daniel"), it's best to use val instead of var. This guarantees data integrity. Why make them var? You'd make them var if you need to change the variables. So generally it's a good idea to process the data before you create your data class not after.