Equality evaluation in Kotlin

Kotlin provides an additional feature of comparing the instances of a particular type in two different ways. This feature makes Kotlin different than the other programming languages.
The two types of equality are –

• Structural Equality
• Referential Equality

Structural Equality –

Structural equality is checked through the == operator and its inverse != operator. By default, the expression containing x==y is translated into the call of equals() function for that type.

x?.equals(y) ?: (y === null)

states that if x is not equal to null, it calls the function equals(y), otherwise if x is found to be null it will checks for y is referentially equal to null.
Note – When (x == null) then automatically it will be converted to referential equality (x === null) so no need of code optimization here.

Thus, to use == on instances of a type the type must override the equals() function. For a string, the structural equality compares the contents.

Referential Equality –

The referential equality in Kotlin is checked through the === operator and its inverse !== operator. This equality returns true only if both the instances of a type point to the same location in memory. When used on types that are converted into primitives type at runtime, the === check is converted into == check and !== check is converted into != check.

Kotlin program to demonstrate the structural and referential eqaulity –

filter_none

edit
close

play_arrow

link
brightness_4
code

class Square(val side: Int) {     override fun equals(other: Any?): Boolean {         if(other is Square){             return other.side == side         }         return false     } } // main function fun main(args :Array<String>) {     val square1 = Square(5)     val square2 = Square(5)     // structural equality     if(square1 == square2) {         println("Two squares are structurally equal")     }     // referential equality     if(square1 !== square2) {         println("Two squares are not referentially equal")     } }

chevron_right

filter_none

Output:

Two squares are structurally equal
Two squares are not referentially equal

My Personal Notes arrow_drop_up

Save

Recommended Posts:

• Kotlin Data Types
• Hello World program in Kotlin
• Kotlin | Retrieve Collection Parts
• Destructuring Declarations in Kotlin
• DatePicker in Kotlin
• Kotlin labeled continue
• Introduction to Kotlin
• Kotlin Type Conversion
• Kotlin Exception Handling | try, catch, throw and finally
• Kotlin if-else expression
• Kotlin Environment setup for Command Line
• Kotlin constructor
• Kotlin Environment setup with Intellij IDEA
• Kotlin Nested class and Inner class
• Kotlin Variables
• Kotlin Operators
• Kotlin Standard Input/Output
• Kotlin Expression, Statement and Block
• Kotlin when expression
• Kotlin for loop

---

CharchitKapoor

Check out this Author's contributed articles.

---

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.