Kotlin Extension Function

Kotlin Introduction

• Kotlin Hello World
• Kotlin Data Types
• Kotlin Operators
• Kotlin Type Conversion
• Kotlin Expression & Statement
• Kotlin Comments
• Kotlin Input/Output

Kotlin Flow Control

• Kotlin if expression
• Kotlin when Expression
• Kotlin while Loop
• Kotlin for Loop
• Kotlin break
• Kotlin continue

Kotlin Functions

• Kotlin function
• Infix Function Call
• Default and Named Arguments
• Recursion and Tail Recursion

Kotlin OOP

• Kotlin Class and Objects
• Kotlin Constructors
• Kotlin Getters and Setters
• Kotlin Inheritance
• Kotlin Visibility Modifiers
• Kotlin Abstract Class
• Kotlin Interfaces
• Kotlin Nested and Inner Classes
• Kotlin Data Class
• Kotlin Sealed Class
• Kotlin Object
• Kotlin Companion Objects
• Kotlin Extension Function
• Kotlin Operator Overloading

In this article, you will learn to extend a class with new functionality using extension functions.

Suppose, you need to extend a class with new functionality. In most programming languages, you either derive a new class or use some kind of design pattern to do this.

However, in Koltin, you can also use extension function to extend a class with new functionality. Basically, an extension function is a member function of a class that is defined outside the class.

For example, you need to use a method to the String class that returns a new string with first and last character removed; this method is not already available in String class. You can use extension function to accomplish this task.

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Example: Remove First and Last Character of String

fun String.removeFirstLastChar(): String =  this.substring(1, this.length - 1)

fun main(args: Array<String>) {
    val myString= "Hello Everyone"
    val result = myString.removeFirstLastChar()
    println("First character is: $result")
}

When you run the program, the output will be:

First character is: ello Everyon

Here, an extension function removeFirstLastChar() is added to the String class.

The class name is the receiver type (String class in our example). The this keyword inside the extension function refers the receiver object.

---

If you need to integrate Kotlin on the top of Java project, you do not need to modify the whole code to Koltin. Just use extension functions to add functionalities.

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Kotlin Getters and Setters

Kotlin Introduction

• Kotlin Hello World
• Kotlin Data Types
• Kotlin Operators
• Kotlin Type Conversion
• Kotlin Expression & Statement
• Kotlin Comments
• Kotlin Input/Output

Kotlin Flow Control

• Kotlin if expression
• Kotlin when Expression
• Kotlin while Loop
• Kotlin for Loop
• Kotlin break
• Kotlin continue

Kotlin Functions

• Kotlin function
• Infix Function Call
• Default and Named Arguments
• Recursion and Tail Recursion

Kotlin OOP

• Kotlin Class and Objects
• Kotlin Constructors
• Kotlin Getters and Setters
• Kotlin Inheritance
• Kotlin Visibility Modifiers
• Kotlin Abstract Class
• Kotlin Interfaces
• Kotlin Nested and Inner Classes
• Kotlin Data Class
• Kotlin Sealed Class
• Kotlin Object
• Kotlin Companion Objects
• Kotlin Extension Function
• Kotlin Operator Overloading

In this article, you will learn to use getters and setters in Kotlin with the help of an example.

Before you learn about getters and setter, be sure to check Kotlin class and objects.

In programming, getters are used for getting value of the property. Similarly, setters are used for setting value of the property.

In Kotlin, getters and setters are optional and are auto-generated if you do not create them in your program.

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How getters and setters work?

The following code in Kotlin

class Person {
    var name: String = "defaultValue"
}

is equivalent to

class Person {
    var name: String = "defaultValue"

    // getter
    get() = field

    // setter
    set(value) {
        field = value
    }
}

When you instantiate object of the Person class and initialize the name property, it is passed to the setters parameter value and sets field to value.

val p = Person()
p.name = "jack"

Now, when you access name property of the object, you will get field because of the code get() = field.

println("${p.name}")

Here’s an working example:

fun main(args: Array<String>) {

    val p = Person()
    p.name = "jack"
    println("${p.name}")
}

class Person {
    var name: String = "defaultValue"

    get() = field

    set(value) {
        field = value
    }
}

When you run the program, the output will be:

jack

This is how getters and setters work by default. However, you can change value of the property (modify value) using getters and setters.

---

Example: Changing value of the property

fun main(args: Array<String>) {

    val maria = Girl()
    maria.actualAge = 15
    maria.age = 15
    println("Maria: actual age = ${maria.actualAge}")
    println("Maria: pretended age = ${maria.age}")

    val angela = Girl()
    angela.actualAge = 35
    angela.age = 35
    println("Angela: actual age = ${angela.actualAge}")
    println("Angela: pretended age = ${angela.age}")
}

class Girl {
    var age: Int = 0
    get() = field
    set(value) {
        field = if (value < 18)
            18
        else if (value >= 18 && value <= 30)
            value
        else
            value-3
    }

    var actualAge: Int = 0
}

When you run the program, the output will be:

Maria: actual age = 15
Maria: pretended age = 18
Angela: actual age = 35
Angela: pretended age = 32

Here, the actualAge property works as expected.

However, there is additional logic is setters to modify value of the age property.

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