Kotlin Data Classes

In Kotlin, we often create classes just to hold data. These are called data classes, and they are marked with the data keyword. Kotlin automatically creates some useful functions for these classes, so you don’t have to write them yourself.

What Is a Data Class?

A data class is a class that holds data. Kotlin automatically provides useful methods like:

1. equals() – to check if two objects are equal
2. hashCode() – used when storing objects in hash-based collections
3. toString() – to get a string version of the object
4. copy() – to copy an object with some modified values

Example:

data class Student(val name: String, val rollNo: Int)

When you create this class, Kotlin automatically gives it the above functions using the primary constructor parameters (name and rollNo in this case).

Rules for Creating a Data Class -

To make sure data classes work correctly, Kotlin has some rules:

1. The primary constructor must have at least one parameter.
2. All primary constructor parameters must be marked with val or var.
3. A data class cannot be abstract, open, sealed, or inner.
4. A data class can only implement interfaces, not extend other classes.

Using toString()

The toString() function gives you a string showing all the values in the primary constructor.

Example:

data class Person(val name: String, val roll: Int, val height: Int)

fun main() {
    val man = Person("man", 1, 50)
    println(man)
}

Output:

Person(name=man, roll=1, height=50)

Note: But if you define properties inside the class body (not in the constructor), toString() won’t include them.

Example:

data class Person(val name: String) {
    var height: Int = 70
}

fun main() {
    val man = Person("manish")
    println(man)
    println(man.height)
}

Output:

Person(name=manish)
70

Here height is not used by the toString() function .

Using copy()

Sometimes, you want to make a copy of an object, but change just one or two values. That’s where the copy() function is useful.

Properties of copy()

• It copies all arguments or members defined in primary constructor
• Two objects can have same primary parameter values and different class body values if defined

Declaration of copy()

fun copy(name: String = this.x, age: Int = this.y) = user(x, y)

where user is a data class : user(String, Int) .

Example

data class Person(val name: String, val age: Int) {
    var height: Int = 100
}

fun main() {
    val man1 = Person("manish", 18)
    man1.height = 100

    val man2 = man1.copy(name = "rahul")
    man2.height = 90

    val man3 = man1.copy()
    man3.height = 110

    println("${man1.name}, ${man1.age} has ${man1.height} cm height")
    println("${man2.name}, ${man2.age} has ${man2.height} cm height")
    println("${man3.name}, ${man3.age} has ${man3.height} cm height")
}

Output:

manish, 18 has 100 cm height  
rahul, 18 has 90 cm height  
manish, 18 has 110 cm height

So, even though the primary constructor values are copied, the values in the class body like height can be changed separately.

Using equals() and hashCode()

• hashCode() function returns a hash code value for the object.
• equals() method return true if two objects have same contents and it works similar to "==", but works different for Float and Double values.

Declaration of hashCode() :

open fun hashCode(): Int

Properties of hashCode()

• Two hash codes declared two times on same object will be equal.
• If two objects are equal according to equals() method, then the hash codes returned will also be same

fun main() {
    val man1 = Person("manish", 18)
    val man2 = Person("rahul", 18)
    val man3 = Person("manish", 18)

    println(man1.hashCode())
    println(man2.hashCode())
    println(man3.hashCode())

    println("man1 == man2: ${man1 == man2}")
    println("man1 == man3: ${man1 == man3}")
}

Output:

835510190  
-938448478  
835510190  
man1 == man2: false  
man1 == man3: true  

Explanation:

man1 and man2 have same object contents, so they are equal, thus they have same hash code values.