Vignesh Prabhu

Vignesh Prabhu

Kotlin Basics - Property

Kotlin Basics - Property

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Vignesh Prabhu

Published on Aug 7, 2021

3 min read

In this article , Lets understand how to use property getters and setters in kotlin

A simple student class as follows

class Student(var studentFirstName: String,
                                  var studentLastName: String = "No Name") {

    fun getStudentName(): String {
        return "$studentFirstName $studentLastName"
    }

    fun printStudentName() {
        println("$studentFirstName $studentLastName")
    }

    fun printStudentInfo() {
        println("Name:$studentFirstName $studentLastName)
    }

}

Now lets add a property to this class and understand the getters and setters behavior

Add a Property

Lets add a property age to the above class

var age : String= "0"

Now lets modify and print the age property from the main class

fun main(){
val student = Student("Vignesh", "Prabhu")
 println(student.age)
}
Console O/P:
0

Property Getter

Now lets add getter to this age property as follows

 var age = "0"
      get() {
            return "${getStudentName()}'s age is $field"
       }

In the above code , get() is called whenever we access this field . So we have manipulated the value of the property in a way that if we access the property age , then it will return as follows

fun main(){
val student = Student("Vignesh", "Prabhu")
println(student.age)
}
Console O/P:
Vignesh Prabhu's age is 0

In the above code we can observe that , we we accessed the property age and printed in the console shows Vignesh Prabhu's age is 0 because in the get method we have added the code return "${getStudentName()}'s age is $field".

Here $field is called backing field since the contains the value of property age

Property Setter

Now lets add setter to this age property as follows

 var age = "0"
  get() {...}
 set(value) {
            field = if (value == "0") {
                "unknown"
            } else {
                value
            }
        }

In the above code , set(value) is called whenever we want to modify the property in our own way . value is nothing but the passed value in the runtime. So lets see how it changes the property if we use this

fun main(){
  val student = StudentProperty("Vignesh", "Prabhu")
  println(student.age)
  student.age = "0"
  println(student.age)
}
Console O/P:
Vignesh Prabhu's age is 0
Vignesh Prabhu's age is unknown

As we can observe that when the value 0 is passed to age property , the set method takes over and manipulates the value wrt to the logic used.

Updated Student class:

class Student(var studentFirstName: String,
                                  var studentLastName: String = "No Name") {

    var age = "0"
        get() {
            return "${getStudentName()}'s age is $field"
        }
        set(value) {
            field = if (value == "0") {
                "unknown"
            } else {
                value
            }
        }


    fun getStudentName(): String {
        return "$studentFirstName $studentLastName"
    }

    fun printStudentName() {
        println("$studentFirstName $studentLastName")
    }

    fun printStudentInfo() {
        println("Name:$studentFirstName $studentLastName -------- Age:$age")
    }

}

So finally , I hope i made you understand property getter and setter in Kotlin.

Please leave your comments to improve.

Enjoy coding

 
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