Kotlin Setters and Getters

Properties in Kotlin –

Properties are important part in any programming language. In Kotlin, we can define properties in the same way as we declare another variable. Kotlin properties can declared either as mutable using var keyword, or as immutable using val keyword.

Syntax of property –

var <propertyName>[: <PropertyType>] [= <property_initializer>]
    [<getter>]
    [<setter>]

Here, property initializer, getter and setter are optional. We can also omit property type if it can be inferred from the initializer.
The syntax of a read-only or immutable property declaration differs from a mutable one in two ways:
starts with val instead of var, and does not allow a setter.



fun main(args : Array) {
    var x: Int = 0
    val y: Int = 1
    x = 2 // In can be assigned any number of times
    y = 0 // It can not be assigned again
    }

In the above code, we are trying to assign a value again to y but it gives compile time error.

Setters and Getters –

In Kotlin, setter is used to set the value of any variable and getter is used to get the value. Getters and setters are auto-generated in the code.
Let’s define a property name in a class company. The data type of name is String and initialize it with some default value.

class Company {
var name: String = "Defaultvalue"
}

The above code is equivalent to this code:

class Company {
    var name: String = "defaultvalue"
        get() = field                     // getter
        set(value) { field = value }      // setter
}

We instantiate an object c of the class Company {…} and, when we initialize the name property, it is passed to the setter’s parameter value and sets field to value.
When we are trying to access name property of the object, we get field because of this code get() = field.
We can get or set the properties of an object of the class using the dot(.) notation-

    val c = Company()
    c.name = "GeeksforGeeks"  // access setter
    println(c.name)           // access getter (Output: GeeksforGeeks)

Kotlin program of default setter and getter-

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class Company {
    var name: String = ""
        get() = field        // getter
        set(value) {         // setter
            field = value
        }
}
fun main(args: Array<String>) {
    val c = Company()
    c.name = "GeeksforGeeks"  // access setter
    println(c.name)           // access getter 
}

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Output:

GeeksforGeeks

value and field identifiers –

We have noticed these two identifiers in the above program.

  1. value
    Conventionally, we choose the name of the setter parameter is value, but we can choose a different name if we want. The value parameter contains the value that a property is assigned to.
    In above program, we have intialized the property name as c.name = “GeeksforGeeks”, the value parameter contains the assigned value “GeeksforGeeks”.
  2. Backing Field (field) –
    It helps in storing the property value in memory possible. When we initialize a property with a value, the initialized value is written to the backing field of that property.
    In above program, the value is assigned to field and then field is assigned to get().

private modifier –

If we only want get method in public access, we can write this code:

var name: String = ""
    private set

and, we can set the name only in a method inside the class because of private modifier near set accessor.

Kotlin program to set the value by using a method inside a class –

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class Company () {
    var name: String = "abc"
        private set
  
    fun myfunc(n: String) {
        name = n             // we set the name here
    }
}
  
fun main(args: Array<String>) {
    var c = Company()
    println("Name of the company is: ${c.name}")
    c.myfunc("GeeksforGeeks")
    println("Name of the new comapny is: ${c.name}")
}

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Output:

Name of the company is: abc
Name of the new comapny is: GeeksforGeeks

Explanation:
Here, we have used private modifier with set. First instantiate an object of class Company() and access the property name using ${c.name}. Then, we pass the name “GeeksforGeeks” as parameter in function defined inside class. The name property updates with new name and access again.

Custom Setter and Getter

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class Registration( email: String, pwd: String, age: Int , gender: Char) {
  
    var email_id: String = email
        // Custom Getter
        get() {
           return field.toLowerCase()
        }
    var password: String = pwd
        //Custom Setter
        set(value){
            field = if(value.length > 6) value else throw IllegalArgumentException("Passwords is too small")
        }
  
    var age: Int = age
        // Custom Setter
        set(value) {
            field = if(value > 18 ) value else throw IllegalArgumentException("Age must be 18+")
        }
    var gender : Char = gender
        //Custom Setter
        set (value){
            field = if(value == 'M') value else throw IllegalArgumentException("User should be male")
        }
  
  
}
  
fun main(args: Array<String>) {
    val geek = Registration("PRAVEENRUHIL1993@GMAIL.COM","Geeks@123",25,'M')
  
    println("${geek.email_id}")
    geek.email_id = "GEEKSFORGEEKS@CAREERS.ORG"
    println("${geek.email_id}")
    println("${geek.password}")
    println("${geek.age}")
    println("${geek.gender}")
      
    // throw IllegalArgumentException("Passwords is too small")
    geek.password = "abc"    
      
    // throw IllegalArgumentException("Age should be 18+") 
    geek.age= 5   
             
    // throw IllegalArgumentException("User should be male")
    geek.gender = 'F'         
}

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Output:

praveenruhil1993@gmail.com
geeksforgeeks@careers.org
Geeks@123
25
M


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