Val and Var in Kotlin

Question

What is the difference between var and val in Kotlin  I have gone through this link  KotlinLang  Properties and Fields As stated on this link   The full syntax of a read-only property declaration differs from a mutable one in two ways  it starts with val instead of var and does not allow a setter   But just before there is an example which uses a setter  fun copyAddress address  Address   Address       val result   Address      there s no  new  keyword in Kotlin     result name   address name    accessors are called     result street   address street                return result    What is the exact difference between var and val  Why do we need both  This is not a duplicate of Variables in Kotlin  differences with Java    39 var  39  vs    39 val  39   as I am asking about the doubt related to the particular example in the documentation and not just in general

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In short  val variable is final  not mutable  or constant value that won t be changed in future  and  var variable  mutable  can be changed in future    class DeliveryOrderEvent val d   Delivery     Only getter   See the above code  It is a model class  will be used for data passing  I have set val before the variable because this variable was used to get the data   class DeliveryOrderEvent var d   Delivery      setter and getter is fine here  No error   Also  if you need to set data later you need to use var keyword before a variable  if you only need to get the value once then use val keyword

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I get the exact answer from de-compiling Kotlin to Java   If you do this in Kotlin   data class UsingVarAndNoInit var name  String  data class UsingValAndNoInit val name  String    You will get UsingVarAndNoInit   package classesiiiandiiiobjects dataiiiclasses p04variiiandiiival   import kotlin jvm internal Intrinsics  import org jetbrains annotations NotNull   public final class UsingVarAndNoInit      NotNull private String name      NotNull   public final String getName         return this name         public final void setName  NotNull String string        Intrinsics checkParameterIsNotNull  Object  string   String    lt set-  gt         this name   string         public UsingVarAndNoInit  NotNull String name        Intrinsics checkParameterIsNotNull  Object  name   String   name        this name   name          NotNull   public final String component1         return this name          NotNull   public final UsingVarAndNoInit copy  NotNull String name        Intrinsics checkParameterIsNotNull  Object  name   String   name        return new UsingVarAndNoInit name           NotNull   public static    bridge       synthetic    UsingVarAndNoInit copy default        UsingVarAndNoInit usingVarAndNoInit  String string  int n  Object object        if   n  amp  1     0          string   usingVarAndNoInit name            return usingVarAndNoInit copy string          public String toString         return  UsingVarAndNoInit name     this name               public int hashCode         String string   this name      return string    null   string hashCode     0         public boolean equals Object object        block3              block2                  if  this    object  break block2          if    object instanceof UsingVarAndNoInit   break block3          UsingVarAndNoInit usingVarAndNoInit    UsingVarAndNoInit  object          if   Intrinsics areEqual  Object  this name   Object  usingVarAndNoInit name   break block3                return true            return false          You will also get UsingValAndNoInit   package classesiiiandiiiobjects dataiiiclasses p04variiiandiiival   import kotlin jvm internal Intrinsics  import org jetbrains annotations NotNull   public final class UsingValAndNoInit      NotNull private final String name      NotNull   public final String getName         return this name         public UsingValAndNoInit  NotNull String name        Intrinsics checkParameterIsNotNull  Object  name   String   name        this name   name          NotNull   public final String component1         return this name          NotNull   public final UsingValAndNoInit copy  NotNull String name        Intrinsics checkParameterIsNotNull  Object  name   String   name        return new UsingValAndNoInit name           NotNull   public static    bridge       synthetic    UsingValAndNoInit copy default        UsingValAndNoInit usingValAndNoInit  String string  int n  Object object        if   n  amp  1     0          string   usingValAndNoInit name            return usingValAndNoInit copy string          public String toString         return  UsingValAndNoInit name     this name               public int hashCode         String string   this name      return string    null   string hashCode     0         public boolean equals Object object        block3              block2                  if  this    object  break block2          if    object instanceof UsingValAndNoInit   break block3          UsingValAndNoInit usingValAndNoInit    UsingValAndNoInit  object          if   Intrinsics areEqual  Object  this name   Object  usingValAndNoInit name   break block3                return true            return false          There are more examples here  https   github com tomasbjerre yet-another-kotlin-vs-java-comparison

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VAR is used for creating those variable whose value will change over the course of time in your application  It is same as VAR of swift  whereas VAL is used for creating those variable whose value will not change over the course of time in your application It is same as LET of swift

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In Kotlin we use var to declare a variable  It is mutable  We can change  reassign variables  Example   fun main args   Array lt String gt        var x   10     println x       x   100    vars can reassign      println x      We use val to declare constants  They are immutable  Unable to change  reassign vals  val is something similar to final variables in java  Example   fun main args   Array lt String gt        val y   10     println y       y   100    vals can t reassign  COMPILE ERROR        println y

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val property is similar to final property in Java  You are allowed to assign it a value only for one time  When you try to reassign it with a value for second time you will get a compilation error  Whereas var property is mutable which you are free to reassign it when you wish and for any times you want

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Value to val variable can be assigned only once   val address   Address  Bangalore   India   address   Address  Delhi   India      Error  Reassigning is not possible with val   Though you can t reassign the value but you can certainly modify the properties of the object     Given that city and country are not val address setCity  Delhi    address setCountry  India     That means you can t change the object reference to which the variable is pointing but the underlying properties of that variable can be changed   Value to var variable can be reassigned as many times as you want   var address   Address  Bangalore   India   address   Address  Delhi   India      No Error   Reassigning possible    Obviously  It s underlying properties can be changed as long as they are not declared val     Given that city and country are not val address setCity  Delhi   address setCountry  India

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In kotlin we can declare variable in two types  val and var  val cannot be reassigned  it works as a final variable   val x   2 x 3    cannot be reassigned   On the other side  var can be reassigned it is mutable   var x   2 x 3    can be reassigned

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val is immutable and var is mutable in Kotlin

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Val is immutable and its properties are set at run time  but you can use a const modifier to make it as a compile time constant  Val in kotlin is same as final in java   Var is mutable and its type is identified at compile time

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val like constant variable  itself cannot be changed  only can be read  but the properties of a val can be modified  var just like mutant variable in other programming languages

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Do you need to change a variable or set it permanently     A good example if it is something like val pi5places   3 14159 you would set it as val  Is there a possibility that you need to change that variable now or later  then you would set it as var   For example   The color of a car  can be var colorCar   green  Later you can change that colorCar   blue  where as a val  you can not  Responses here regarding mutable and immutable is fine  but may be scary if these terms are not well known or just getting into learning how to program

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val - Immutable once initialized can t be reassigned   var - Mutable can able to change value    Example   in Kotlin -  val n   20  amp  var n   20   In Java  - final int n   20   amp  int n   20

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Basically    var   variable  so it can change val   value  so it can not change

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val  from value   Immutable reference  A variable declared with val can   t be reassigned after it   s initialized  It corresponds to a final variable in Java   var  from variable   Mutable reference  The value of such a variable can be changed  This declaration corresponds to a regular  non-final  Java variable

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If we declare variable using val then it will be read-only variable  We cannot change it s value  It s like final variable of Java  It s immutable   But if we declare variable using var then it will be a variable which we can read or write  We can change it s value  It s mutable   data class Name val firstName  String  var lastName  String   fun printName name  Name   Name       val myName   Name  Avijit    Karmakar      myName variable is read only        firstName variable is read-only         You will get a compile time error  Val cannot be reassigned      myName firstName   myName firstName        lastName variable can be read and write as it s a var      myName lastName   myName lastName     return myName     val cannot be initialized lately by the keyword lateinit but non-primitive var can be initialized lately by the keyword lateinit

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Two ways to create variable in KOTLIN VAL and VAR  1 VAL stores constant values  Also called Final Variable  2 VAR stores Changeable Values  Click here for example

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Both are  variable the only difference is that Mutable variable and immutable variable and there is nothing more difference  var is Mutable variable and val is immutable In simple Language var can change is value after  initialization value val is constant and it cannot change it value after  initialization the value

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var is a mutable variable and can be assigned multiple times and val is immutable variable and can be intialized only single time

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Lets try this way   Val is a Immutable constant      val change  Unchange  println change        It will throw error because val is constant variable        change  Change          println change  Var is a Mutable constant     var name  String  Dummy      println name      name  Funny      println name

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Simply   var  mutable  and val  immutable values like in Java  final modifier    var x Int 3 x    x    gives compilation error  val cannot be re-assigned  val y  Int   6 y  y

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Var means Variable-If you stored any object using  var  it could change in time   For example    fun main args  Array lt String gt         var a 12     var b 13     var c 12     a c b     new object 25       print a      Val means value-It s like a  constant  in java  if you stored any object using  val  it could not change in time   For Example     fun main args  Array lt String gt         val a 12     var b 13     var c 12     a c b     You can t assign like that it s an error        print a

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val is immutable  final  the first assigned value cannot be changed   val name String    andy   name    thomas    Error  Val cannot be reassigned   var is mutable  reassignable  you can change the value over and over   val a Int   1 var b Int   1 println    a   b       output 2  b   4 println    a   b       output 5      I think the easiest way to remember it        val   variable final      var   variable reassignable  or the opposite of val

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val and var both are used to declare a variable   var is like general variable and it s known as a mutable variable in kotlin and can be assigned multiple times  val is like Final variable and it s known as immutable in kotlin and can be initialized only single time   For more information what is val and var please see below link  http   blog danlew net 2017 05 30 mutable-vals-in-kotlin

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Both variables are used as initialising    val like a constant variable  It can be readable  and the properties of a val can be modified  var just like a mutable variable  you can change the value at any time

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In Kotlin  we have two types of variables  var or val  The first one  var  is a mutable reference  read-write  that can be updated after initialization  The var keyword is used to define a variable in Kotlin  It is equivalent to a normal  nonfinal  Java variable  If our variable needs to change at some time  we should declare it using the var keyword  Let s look at an example of a variable declaration    fun main args  Array lt String gt          var fruit String    orange      1      fruit    banana                 2

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---------------- ----------------------------- ---------------------------                                 val                          var               ---------------- ----------------------------- ---------------------------    Reference type   Immutable once initialized    Mutable can able to change                     can t be reassigned           value                        ---------------- ----------------------------- ---------------------------    Example          val n   20                    var n   20                   ---------------- ----------------------------- ---------------------------    In Java          final int n   20              int n   20                   ---------------- ----------------------------- ---------------------------    Reference

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Simply think Val like final Variable in java

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val use to declare final variable  Characteristics of val variables   Must be initialized value can not be changed or reassign     var is as a general variable    We can initialize later by using lateinit modifier    lateinit also use for global variable  we can not use it for local variable  value can be changed or reassign but not in global scope     val in kotlin is like  final keyword in  java

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Both  val and var can be used for declaring variables  local and class properties    Local variables    val declares read-only variables that can only be assigned once  but cannot be reassigned    Example   val readonlyString      hello    readonlyString      c u       Not allowed for  val     var declares reassignable variables as you know them from Java  the keyword will be introduced in Java 10     local variable type inference        Example   var reasignableString      hello    reasignableString      c u       OK   It is always preferable to use val  Try to avoid var as often as possible   Class properties   Both keywords are also used in order to define properties inside classes  As an example  have a look at the following data class   data class Person  val name  String  var age  Int    The Person contains two fields  one of which is readonly  name   The age  on the other hand  may be reassigned after class instantiation  via the provided setter  Note that name won   t have a corresponding setter method

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In Kotlin val is a read-only property and it can be accessed by a getter only  val is immutable    val example    val piNumber  Double   3 1415926     get     field      However  var is a read-and-write property  so it can be accessed not only by a getter but a setter as well  var is mutable    var example    var gravity  Double   9 8     get     field     set value             field   value              If you try to change an immutable val  IDE will show you error    fun main             piNumber   3 14             ERROR     println piNumber        RESULT    Val cannot be reassigned    But a mutable var can be changed    fun main             gravity   0 0     println gravity        RESULT    0 0   Hope this helps

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Normal   Val is using for static field like in Java as Static Keyword Like Static in Java  Same as in kotlin And Var denotes Variable Field in Kotlin that  you can change it  Mostly Static is used when you want to save value in static memory at once    Example    if you assign   val a 1  a 3  You can not change it     You can not change  this is final value and Static  var b 2  b 4  U can change it

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You can easily think it as   var is used for setter  value will change    val is used for getter  read-only  value won t change

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In your code result is not changing  its var properties are changing  Refer comments below   fun copyAddress address  Address   Address       val result   Address      result is read only     result name   address name    but not their properties      result street   address street                return result     val is same as the final modifier in java  As you should probably know that we can not assign to a final variable again but can change its properties

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variables defined with var are mutable Read and Write   variables defined with val are immutable Read only   Kotlin can remove findViewById and reduce code for setOnClickListener in android studio  For full reference  Kotlin awesome features  value of mutable variables can be changed at anytime  while you can not change value of immutable variables   where should I use var and where val    use var where value is changing frequently  For example while getting location of android device  var integerVariable   Int    null   use val where there is no change in value in whole class  For example you want set textview or button s text programmatically   val stringVariables   String    Button s Constant or final Text

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Comparing val to a final is wrong  vars are mutable vals are read only  Yes val cannot be reassigned just like final variables from Java but they can return a different value over time  so saying that they are immutable is kind of wrong  Consider the following var a   10 a   11   Works as expected  val b   10 b   11   Cannot Reassign  as expected  So for so Good  Now consider the following for vals val d   get     System currentTimeMillis    println d    Wait a millisecond println d    Surprise   the value of d will be different both times  Hence  vars can correspond to nonfinal variables from Java  but val aren t exactly final variables either  Although there are const in kotlin which can be like final  as they are compile time constants and don t have a custom getter  but they only work on primitives

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val   must add or initialized value but can t change  var  it s variable can ba change in any line in code

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var is a variable like any other language  eg   var price  Double   On the other side  val provides you feature of referencing  eg    val CONTINENTS   7    You refer this to get constant value 7  In this case  val acts as access    specifier final in Java   and    val Int absolute  Int     get             return Math abs this           You refer to the newly create  method  which provides absolute value     of your integer  println -5 absolute     O P  5

[kotlin] Val and Var in Kotlin

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