A Comparison of Swift and Kotlin Languages

Understanding Differences Between Swift and Kotlin

As you have seen, Swift and Kotlin are similar in many ways. However, they still have some differences.

Data Class vs. Struct
Swift has value types, such as structs, as well as reference types, while Kotlin only has reference types. Take a look at some examples.

Kotlin — Data Class
Consider the Person class from the previous examples in Kotlin:

val personOne = Person("Jon", "12 August")
val personTwo = personOne // assign to another variable 
personTwo.name = "Vanessa"
println(personOne.name) // prints Vanessa
println(personTwo.name) // prints Vanessa

Swift — Struct
Now, consider the above Person class as a Swift struct.

struct Person {
  let name: String
  let dob: String
}

let personOne = Person(name: "Jon", dob: "12 August")
var personTwo = personOne // assign to another variable
personTwo.name = "Vanessa"
print(personOne.name) // prints Jon
print(personTwo.name) // prints Vanessa

val vs. let

Other than not being able to re-assign the value, val and let do have a small difference in how they are handled across the two languages.

Kotlin — val
Consider the following persons list:

val persons = mutableListOf(Person("Jon", "12 August"),
  Person("Kim", "10 July"),
  Person("Vanessa", "12 August"),
  Person("Alisa", "26 March"))
persons.add(Person("Shaun", "11 Jan")) // can append a new value

Swift - let
In Swift:

let persons = [Person(name: "Jon", dob: "12 August"),
  Person(name: "Kim", dob: "10 July"),
  Person(name: "Vanessa", dob: "12 August"),
  Person(name: "Alisa", dob: "26 March")]
persons.append(Person(name: "Shaun", dob: "11 Jan")) // throws compile time error

The fundamental difference here is in the treatment of an array or list. Kotlin lets you append values to a mutable list even when it's declared with a val. Similarly, you can declare an array in Kotlin using val and afterward re-assign members of the array using subscript notation. In contrast, in Swift, an array must be declared as mutable using var in order to have members updated with subscript notation or to append values to it.

Where to Go From Here?

After reading this comparison between Swift and Kotlin, you must have gotten a pretty good idea of how much they have in common.

Both of these languages are not limited to mobile development, either. They both are used for server side development and cross-platform app development as well. To serve this purpose there are a number of frameworks out there to help you with it. Vapor and Kitura are used for server side Swift and video courses on these can be found on our website. For Kotlin there is Spring Boot. For cross-platform app development there are frameworks like Kotlin-Native and Scade.

Finally, here are some additional resources to help you learn about the similarities and differences between Swift and Kotlin:

• Swift Is Like Kotlin
• Kotlin vs. Swift: Are Android and iOS Moving Towards Creating a Universal Language?
• Swift vs. Kotlin
• Swift Vs. Kotlin: Similarities and Differences Every Developer Should Know
• Server Side Swift with Vapor
• Kotlin Apprentice
• Swift Apprentice
• My Transition From Swift to Kotlin by Hector Matos (KotlinConf 2017)

If you have any questions or comments, please let us know in the discussion below!