Scala allows one to decide whether a variable is immutable or mutable. Immutable is Read only where as mutable is read-write. Immutable variables are declared with the keyword “val“.

scala> val age:Int=22

Here age has been initialized to a value during its declaration. As it has been declared as a val, age cannot be reassigned to a new value. Any attempt to do will result in al reassignment to val error. Let us consider an Array being declared as val.

scala> val names:Array[String]=new Array(6)

Here names reference cannot be changed to point to a different Array, but the array itself can be modified. In other words the contents/elements of the array can be modified.

scala> names= new Array(5) //This will give a reassignment to val error.

scala> names(0)="Scala" //This does not give an error.

Also it is to be noted that val variables must be initialized when they are declared.

Mutable variables are delcared with the keyword “var“. Unlike val, “var” can be reassigned to different values or point to different objects. But they have to be initialised at the time of declaration.

scala> var age:Int=22
age: Int=22

scala> age=35
age: Int=35

There’s an exception to the rule that one must initialize the val’s and var’s. When they are used as constructor parameters, the val’s and var’s will be initialised when the object is instantiated. Also, derived classes can override val’s declared inside the parent classes.

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Mohamed Sanaulla

4 Responses to "val" versus "var" Declarations in Scala

  1. […] So the fields can be declared as either- var’s and val’s (Read about their difference here). And the methods are declared using the keyword- def. Java programmers might note this difference […]

  2. […] as the field declared with val keyword would have only a getField generated. We already know from here the difference between val and […]

  3. […] a val. Variables declared as var can be reassigned where as those declared as val cannot be. Read here for more details on their difference. Lets make the name field in the above Person class to val and […]

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