Scala | Final

In Scala, Final is a keyword and used to impose restriction on super class or parent class through various ways. We can use final keyword along with variables, methods and classes. Following are the ways of using final keyword in Scala

1. Scala Final Variable: Scale final variable initialized only once while declared and used as constant throughout the program. In below example, variable area is final which is declared as final and also initialized while declare in superclass shapes. if we want to access or modify the variable area from derived class Rectangle then it is not possible because the restriction on variable area is added by keyword final.

   Scala final variable initialized by following ways:

   • While declaring
   • In static block
   • In Constructor
   Scala

   // Scala program of using final variable
   class Shapes
   { 
       // define final variable
       final val area:Int = 60
   } 
   class Rectangle extends Shapes
   { 
       override val area:Int = 100
       def display()
       { 
           println(area) 
       } 
   } 
   
   // Creating object
   object GFG
   { 
       // Main method
       def main(args:Array[String])
       { 
           var b = new Rectangle() 
           b.display() 
       } 
   }
   

   Following error occurs while running above code Output :

    
   prog.scala:5: error: overriding value area in class Shapes of type Int;
    value area cannot override final member
   override val area:Int = 100
                ^
   one error found
   

2. Scala Final Methods: Final method CalArea in the parent class (Shapes) indicate that, method cannot override in a child class (Rectangle). Scala

   // Scala program of using final method
   class Shapes
   { 
       val height:Int = 0
       val width :Int =0
       
       // Define final method
       final def CalArea(){ 
       }
   } 
   class Rectangle extends Shapes
   { 
       override def CalArea()
       { 
           val area:Int = height * width 
           println(area) 
       } 
   } 
   
   // Creating object
   object GFG
   { 
       // Main method
       def main(args:Array[String])
       { 
           var b = new Rectangle() 
           b.CalArea() 
       } 
   }
   

   Following error occurs while running above code Output :

   prog.scala:8: error: overriding method CalArea in class Shapes of type ()Unit;
    method CalArea cannot override final member
       override def CalArea(){
                        ^
   one error found
   

3. Scala Final Classes If the class in Scala is final then it cannot inherit to derived class. Inheritance restriction will be added by final keyword. Here if class Shapes are final then its all members also final and cannot used in derived class. Scala

   // Scala program of using final class
   final class Shapes
   { 
       // Final variables and functions
       val height:Int = 0
       val width :Int =0
       final def CalArea()
       { 
       }
   } 
   class Rectangle extends Shapes
   { 
       // Cannot inherit Shapes class 
        override def CalArea()
        { 
           val area:Int = height * width 
           println(area) 
       } 
   } 
   
   // Creating Object
   object GFG
   { 
       // Main method
       def main(args:Array[String])
       { 
           var b = new Rectangle() 
           b.CalArea() 
           
       } 
   }
   

   Following error occurs while running above code Output :

    
   prog.scala:4: error: illegal inheritance from final class Shapes
   class Rectangle extends Shapes{
                           ^
   one error found