Kotlin Data Classes

Hey, welcome again to my blog! Today, we shall learn more about Kotlin Data Classes and if you have missed any of my previous tutorials in this series, kindly check them out here.

We learnt about Enum classes in the previous blog.

What are Data Classes?

Data classes, as the name suggests, are classes that are meant to hold data.

A data class helps create some boilerplate code for us as we shall see.

Initialisation

To make a data class, we simply add data in front of a normal class.

data class Person(val name: String, val id_no: Int)

Before we move on, there are rules considered in order to create data classes in Kotlin: These are;
🔹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.
🔹Data classes may only implement interfaces.

Example:

data class Movie(var name: String, var studio: String, var rating: Float)

As I said, when we create a data class in Kotlin, the following methods are automatically created by the compiler:

🔸equals()
🔸hashCode()
🔸toString()
🔸copy()
🔸componentN()

A data class is instantiated the same way as other classes: From our example, let's instantiate from it.

val movie = Movie("Free Guy", "Sony Pictures", 8.5F)

📌 toString() method
The toString() function returns a string representation of the object. Example:

println(movie.toString())

Output

Movie(name=Free Guy, studio=Sony Pictures, rating=9.0)

📌 copy() method
For a data class, you can create a copy of an object with some of its properties different using copy() function.

Here's how it works:

val movie2 = movie.copy(name = "Avengers")

When we print above:

print(movie2)

We get:

Movie(name=Avengers, studio=Sony Pictures, rating=8.5)

📌 hashCode()and equals() methods

The hasCode() method returns hash code for the object. If two objects are equal, hashCode() produces the same integer result. Recommended Reading: hashCode()

The equals() returns true if two objects are equal (has same hashCode()). If objects are not equal, equals() returns false. Recommended Reading: equals()

data class Movie(var name: String, var studio: String, var rating: Float)
val movie = Movie("Free Guy", "Sony Pictures", 8.5F)

fun main(){
    println(movie)
    println(movie.toString())
    val movie2 = movie.copy(name = "Avengers")
    val movie3 = movie.copy()

    println("movie hashcode = ${movie.hashCode()}")
    println("movie hashcode = ${movie2.hashCode()}")
    println("movie hashcode = ${movie3.hashCode()}")

    if (movie.equals(movie2) == true)
        println("movie is same as movie2")
    else
        println("movie is not same as movie2")

    if (movie.equals(movie3) == true)
        println("movie is same as movie3")
    else
        println("movie is not the same as movie3.")
}

The above is full code with the previous method examples and if we ran the code: The output for the above methods should be:

movie hashcode = 951067529
movie hashcode = 456663247
movie hashcode = 951067529
movie is not same as movie2
movie is same as movie3

📌 componentN() method
Component functions generated for data classes make it possible to use them in destructuring declarations:

movie.component1() //  Free Guy 
movie.component2() // Sony Pictures
movie.component3() // 8.5

If we added this code in our main function above to restructure declarations:

val movie4 = Movie("Dr Strange", "Marvel", 9.5F)
val (name, studio, rating) = movie
println(println("$name has a rating of $rating !!"))

Our Expected Output: Dr Strange has a rating of 9.5 !!

Python Data Classes

Data classes were introduced in Python 3.7 (and backported to Python 3.6), providing a handy way to make classes less verbose.

To install Data classes: pip install dataclasses Example:

from dataclasses import dataclass

@dataclass
class MyArticle():
    """A class for holding an article content"""

    # Attributes Declaration
    # using Type Hints

    title: str
    author: str
    language: str
    upvotes: int

# A DataClass object
article = MyArticle("Data Classes",
                    "Ronnie A",
                    "Kotlin", 1)
print(article)

Expected Output:

MyArticle(title='Data Classes', 
                 author='Ronnie A', 
                 language='Kotlin', 
                 upvotes=1)

Conclusion

Data classes are just regular classes that are geared towards storing state, rather than containing a lot of logic. Every time you create a class that mostly consists of attributes, you make a data class.

For More Reading, I will recommend:

🎗Programmiz Article
🎗kotlin Docs
🎗Geeks For Geeks Article
🎗Data Classes In Python: Official Docs

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Ronnie Atuhaire