Kotlin extension function

03 Jan, 2022
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 functionsclass 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. 
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. 
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 inheritedopen class A(val a:Int, val b:Int){} // Class B inherits Aclass 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. 
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 nameclass 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
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 objectfun 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
abhishek0719kadiyan
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