Kotlin Data Class | Android Introduction #23

However, objects created from custom classes behave somewhat differently. Let’s examine what happens when you compare or print these objects, and then discuss how this behavior can be altered using a special class modifier called data.

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fun main() {
  println("A" == "A") // true
  println("A" == "B") // false
}

Let’s say that you have a very simple class. You can compare it to itself, but this comparison will only return true, if exactly the same instance is on both sides of the comparison. This means that by default, all objects created with custom classes are considered unique, they are not equal to any other object.

class Dog(
  val name: String
)

fun main() {
  val pluto1 = Dog("Pluto")
  val pluto2 = Dog("Pluto")

  println(pluto1 == pluto2) // false
  println(pluto1 == pluto1) // true
}

Printing or transforming an object into a string is also not very useful. The result should be this class name, @ sign, and some numeric digits. The number is not really useful, it only helps us know if two objects are the same or not.

println(pluto1) // Dog@404b9385
println("Dog: $pluto1") // Dog: Dog@404b9385

So, what can you do to get more meaningful data from printing a string? This behavior can be altered if you use data modifier before a class. You use it before classes representing a bundle of data. Such a class is equal to a different instance of the same class if its constructor properties have the same value.

data class Dog(
  val name: String
)

fun main() {
  val pluto1 = Dog("Pluto")
  val pluto2 = Dog("Pluto")
  val rex = Dog("Rex")

  println(pluto1 == pluto2) // true
  println(pluto1 == pluto1) // true
  println(pluto1 == rex) // false
}

When you transform a data class into a string, you not only have this class name, but also values for each constructor property.

println(pluto1) // Dog(name=Pluto)
println("Dog: $pluto1") // Dog: Dog(name=Pluto)

That is not all. Data classes can be destructured, which means reading values from this class and assigning them to variables.

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

val dog = Dog("Pluto", 7)
val (name, age) = dog
println(name) // Pluto
println(age) // 7

Beware that destructuring in Kotlin is based on position, not name, so value names need to be placed at correct positions. For instance, if you place age at the position of name, and name at the position of age, then you will have age in a variable called name, and name in the variable called age.

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

fun main() {
  val dog = Dog("Pluto", 7)
  val (age, name) = dog
  println(age) // Pluto
  println(name) // 7
}

To prevent this, always check if your variables are assigned to the correct positions of constructor parameters.

Finally, data classes have a copy method, that creates a copy of an object. It also allows you to specify what modifications you would like to introduce into an object.

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

fun main() {
println(dog.copy()) // Dog(name=Pluto, age=7)
println(dog.copy(age = 8)) // Dog(name=Pluto, age=8)
println(dog.copy(name = "Neptune")) // Dog(name=Neptune, age=7)
}

You use data modifiers for classes that are used to represent a bundle of data. Such classes are quite common in programming.

İbrahim Can Erdoğan

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