Extension functions

The last feature we'll cover in this chapter before moving on is extension functions. Sometimes, you may want to extend the functionality of a class that is declared final. For example, you would like to have a string that has the hidePassword() function from the previous section.

One way to achieve that is to declare a class that wraps the string for us:

data class Password(val password: String) {
    fun hidePassword() = "*".repeat(password.length)
}

This solution is quite wasteful, though. It adds another level of indirection.

In Kotlin, there's a better way to implement this.

To extend a class without inheriting from it, we can prefix the function name with the name of the class we'd like to extend:

fun String.hidePassword() = "*".repeat(this.length)

This looks almost like a regular top-level function declaration, but with one crucial change – before the function name comes a class name. That class is called a method receiver.

Inside the function body, this will refer to any String class that the function was invoked on.

Now, let's declare a regular string and try to invoke this new function on it:

val password: String = "secretpassword"
println("Password: ${password.hidePassword()}")

This prints the following:

> Password: **************

What black magic is this? We managed to add a function to a final class, something that technically should be impossible.

This is another feature of the Kotlin compiler, one among many. This extension function will be compiled to the following code:

// This is not real Kotlin
fun hidePassword(this: String) {
    "*".repeat(this.length)
}

You can see that, in fact, this is a regular top-level function. Its first argument is an instance of the class that we extend. This also might remind you of how methods on structs in Go work.

The code that prints the masked password will be adapted accordingly:

val password: String = "secretpassword"
println("Password: ${hidePassword(password)}")

For that reason, the extension functions cannot override the member function of the class, or access its private or protected properties.