fun-interfaces.md

An interface with only one abstract method is called a functional interface, or a Single Abstract Method (SAM) interface. The functional interface can have several non-abstract members but only one abstract member.

To declare a functional interface in Kotlin, use the fun modifier.

fun interface KRunnable {
   fun invoke()
}

SAM conversions

For functional interfaces, you can use SAM conversions that help make your code more concise and readable by using lambda expressions.

Instead of creating a class that implements a functional interface manually, you can use a lambda expression. With a SAM conversion, Kotlin can convert any lambda expression whose signature matches the signature of the interface's single method into the code, which dynamically instantiates the interface implementation.

For example, consider the following Kotlin functional interface:

fun interface IntPredicate {
   fun accept(i: Int): Boolean
}

If you don't use a SAM conversion, you will need to write code like this:

// Creating an instance of a class
val isEven = object : IntPredicate {
   override fun accept(i: Int): Boolean {
       return i % 2 == 0
   }
}

By leveraging Kotlin's SAM conversion, you can write the following equivalent code instead:

// Creating an instance using lambda
val isEven = IntPredicate { it % 2 == 0 }

A short lambda expression replaces all the unnecessary code.

fun interface IntPredicate {
   fun accept(i: Int): Boolean
}

val isEven = IntPredicate { it % 2 == 0 }

fun main() {
   println("Is 7 even? - ${isEven.accept(7)}")
}

{kotlin-runnable="true" kotlin-min-compiler-version="1.4"}

In another example of a SAM conversion, the function process() uses a lambda instead of creating an anonymous object:

fun interface SuspendRunnable {
   suspend fun invoke()
}
class Listener {
   fun setOnClickListener(r: SuspendRunnable) {
       GlobalScope.launch { r.invoke() }
   }
}
class Button(private val listener: Listener) {
   fun process() {
       listener.setOnClickListener {
           addText()
       }
   }
   suspend fun addText() {
       // ...
   }
}

{kotlin-runnable="true" kotlin-min-compiler-version="1.5"}

You can also use SAM conversions for Java interfaces.

Functional interfaces vs. type aliases

Functional interfaces and type aliases serve different purposes. Type aliases are just names for existing types – they don't create a new type, while functional interfaces do.

Type aliases can have only one member, while functional interfaces can have multiple non-abstract members and one abstract member. Functional interfaces can also implement and extend other interfaces.

Considering the above, functional interfaces are more flexible and provide more capabilities than type aliases.