Kotlin extension function

Kotlin gives the programmer the ability to add more functionality to the existing classes, without inheriting them. This is achieved through a feature known as extensions. When a function is added to an existing class it is known as Extension Function. To add an extension function to a class, define a new function appended to the classname as shown in the following example: 

Kotlin

package kotlin1.com.programmingKotlin.chapter1

// A sample class to demonstrate extension functions
class Circle (val radius: Double){
    // member function of class
    fun area(): Double{
        return Math.PI * radius * radius;
    }
}
fun main(){
    // Extension function created for a class Circle
    fun Circle.perimeter(): Double{
        return 2*Math.PI*radius;
    }
    // create object for class Circle
    val newCircle = Circle(2.5);
    // invoke member function
    println("Area of the circle is ${newCircle.area()}")
    // invoke extension function
    println("Perimeter of the circle is ${newCircle.perimeter()}")
}

Output:

Area of the circle is 19.634954084936208
Perimeter of the circle is 15.707963267948966

Explanation:

Here, a new function is appended to the class using dot notation with class Circle.perimeter(), and its return type is Double. In the main function, an object is created to instantiate the class Circle and invoked the function in println() statement. When the member function is invoked it returns the area of a circle and similarly, the extension function returns the perimeter of the circle. 

Extended library class using extension function

Kotlin not only allows the user-defined classes to be extended but also the library classes can be extended. The extension function can be added to library classes and used in a similar way as for user-defined classes. The following example demonstrates an extension function created for a library class- 

Kotlin

fun main(){

    // Extension function defined for Int type
    fun Int.abs() : Int{
        return if(this < 0) -this else this
    }

    println((-4).abs())
    println(4.abs())
}

Output:

4
4

Explanation: Here, we have extended the library function using an extension function. We performed the modulus operation on an integer value. We have passed the integer value -4 and 4 and obtained a positive value for both. If the parameter value is less than 0 then returns -(value) and if the parameter value is greater than zero returns the same value. 

Extensions are resolved statically

One important point to note about the extension functions is that they are resolved statically i.e which extension function is executed depends totally on the type of the expression on which it is invoked, rather than on the type resolved on the final execution of the expression at runtime. The following example will make the above argument clear:  

Kotlin

// Open class created to be inherited
open class A(val a:Int, val b:Int){
}

// Class B inherits A
class B():A(5, 5){}

fun main(){

    // Extension function operate defined for A
    fun A.operate():Int{
        return a+b
    }

    // Extension function operate defined for B
    fun B.operate():Int{
        return a*b;
    }

    // Function to display static dispatch
    fun display(a: A){
        print(a.operate())
    }

    // Calling display function
    display(B())
}

Output:

10

Explanation:

If you are familiar with Java or any other object-oriented programming language, you might notice in the above program, that since class B inherits class A and the argument passed display function is an instance of class B. The output should be 25 according to the concept of the dynamic method dispatch, but since the extension functions are statically resolved, so the operation function is called on type A. Hence the output is 10. 

Nullable Receiver

Extension functions can also be defined with the class type that is nullable. In this case, when the check for null is added inside the extension function and the appropriate value is returned.
Example of an extension function as a nullable receiver –

Kotlin

// A sample class to display name name
class AB(val name: String){
    override fun toString(): String {
        return "Name is $name"
    }
}

fun main(){
    // An extension function as a nullable receiver
    fun AB?.output(){
        if(this == null){
            println("Null")
        }else{
            println(this.toString())
        }
    }

    val x = AB("Charchit")

    // Extension function called using an instance
    x.output()
    // Extension function called on null
    null.output()
}

Output:

Name is Charchit
Null

Companion Object Extensions

If a class contains a companion object, then we can also define extension functions and properties for the companion object. Companion object declaration –

Kotlin

class MyClass {
    // companion object declaration
    companion object {
        fun display(){
            println("Function declared in companion object")
        }
    }
}
fun main(args: Array<String>) {
   // invoking member function
   val ob = MyClass.display()
}

Output:

Function declared in companion object

Just like the calling of the regular member function of the companion object, the extension function can be called using only the class name as the qualifier. Companion object extension example –

Kotlin

class MyClass {
    companion object {
        // member function of companion object
        fun display(str :String) : String{
            return str
        }
    }
}
    // extension function of companion object
fun MyClass.Companion.abc(){
    println("Extension function of companion object")
}
fun main(args: Array<String>) {
    val ob = MyClass.display("Function declared in companion object")
    println(ob)
    // invoking the extension function
    val ob2 = MyClass.abc()
}

Output:

Function declared in companion object
Extension function of companion object