Kotlin Crash Course for Programmers/

...

/

Extension Functions

Extension Functions

Understand Kotlin's extension functions, how to define them, and why they are a fundamental feature of Kotlin.

We'll cover the following...

Extension functions allow you to attach new functions to existing types. This includes ones that you don’t own, such as Int, String, any predefined Java class such as Date, Instant, and any type imported from a library.

Typically, this is done using utility classes that encapsulate a series of utility functions. You may be familiar with class names such as StringUtils or DateUtils. In Kotlin, missing capabilities of a class you want to use can be patched using extension functions instead.

Defining an Extension Function #

Let’s say you want to use Java’s old Date class (instead of the java.time package). In your application, you want to add a number of days to a given Date. Ideally, you’d want to write the following:

Press + to interact
// Goal
val now = Date()
val tomorrow = now.plusDays(1)  // No such method yet

However, Java’s Date class has no such plusDays method. Traditionally, you would create a helper function that allows you to write val tomorrow = plusDays(now, 1), but this call syntax has several drawbacks:

  • It doesn’t read as naturally
  • The function is not scoped to Date objects
    • Thus, autocomplete is a lot less helpful

Fortunately, Kotlin’s extensions can achieve what we want. A plusDays function that fulfills the requirements above can be written as follows:

Press + to interact
fun Date.plusDays(count: Int) = Date(this.time + count * 86400000L)

Note: The function signature contains the Date class ...