Abstract Classes in Scala

Abstraction is the process to hide the internal details and showing only the functionality. In Scala, abstraction is achieved by using an abstract class. The working of the Scala abstract class is similar to Java abstract class. In Scala, an abstract class is constructed using the abstract keyword. It contains both abstract and non-abstract methods and cannot support multiple inheritances. A class can extend only one abstract class.
Syntax:

abstract class class_name
{
// code..
}

The abstract methods of abstract class are those methods which do not contain any implementation. Or in other words, the method which does not contain body is known as an abstract method.
Syntax:

def function_name()

Example:

// Scala program to illustrate how to
// create an abstract class

// Abstract class
abstract class myauthor
{

    // abstract method
    def details()
}

// GFG class extends abstract class
class GFG extends myauthor
{
    def details()
    {
        println("Author name: Ankita Saini")
        println("Topic name: Abstract class in Scala")
    }
}

object Main
{
    // Main method
    def main(args: Array[String])
    {
        // objects of GFG class
        var obj = new GFG()
        obj.details()
    }
}

Output:

Author name: Ankita Saini
Topic name: Abstract class in Scala

Following are some important observations about abstract classes in Scala.

• Like Java, in Scala, we are not allowed to create the instance of the abstract class. If we try to create objects of the abstract class, then the compiler will give an error as shown in the below program.
  Example:

  // Scala program to illustrate
  // the concept of abstract class
  
  // Abstract class
  abstract class myauthor{
  
      // abstract method
      def details()
  }
  
  
  object Main {
  
      // Main method
      def main(args: Array[String]) {
  
          // Object of myauthor class
      var obj = new myauthor()
      }
  }

  Output:

  prog.scala:18: error: class myauthor is abstract; cannot be instantiated
  var obj = new myauthor()
  ^
  one error found

• In Scala, an abstract class can also contain fields. These fields are accessed by the abstract class methods and by the methods of the class which inherit abstract class. As shown in the below program.
  Example:

  // Scala program to illustrate
  // the concept of abstract class
  
  // Abstract class with fields
  abstract class Geek
  {
      var name : String = "GeeksforGeeks"
      var tutorial: String = "Scala"
      def portal()
  }
  
  // GFG class extends abstract class
  class GFG extends Geek
  {
  
      // Abstract class method accessing
      // fields of the abstract class
      def portal()
      {
          println("Portal name: " + name)
  
      }
  
      // GFG class method accessing
      // fields of the abstract class
      def tutdetails()
      {
          println("Tutorial name: " + tutorial)
      }
  }
  
  object Main
  {
  
      // Main method
      def main(args: Array[String])
      {
  
          // objects of GFG class
          var obj = new GFG()
          obj.portal()
          obj.tutdetails()
      }
  }

  Output:

  Portal name: GeeksforGeeks
  Tutorial name: Scala

• Like Java, In Scala, an abstract class can also contain a constructor and a constructor of an abstract class is called when an instance of a inherited class is created. As shown in the below program.
  Example:

  // Scala program to illustrate
  // the concept of abstract class
  
  // Abstract class with constructor
  // And the constructor contain two arguments
  abstract class myauthor(name: String,
                          topic: String)
  {
      def details()
  }
  
  // GFG class extends abstract class
  class GFG(name: String, topic: String) extends
                              myauthor(name, topic)
  {
      def details()
      {
          println("Author name: " + name)
          println("Topic name: " + topic)
      }
  }
  
  object Main
  {
  
      // Main method
      def main(args: Array[String])
      {
  
          // objects of GFG class
          var obj = new GFG("Ankita", "Abstract class")
          obj.details()
      }
  }

  Output:

  Author name: Ankita
  Topic name: Abstract class
  

• An abstract class can also contain only non- abstract method. This allows us to create classes that cannot be instantiated, but can only be inherited. As shown in the below program.
  Example:

  // Scala program to illustrate
  // the concept of abstract class
  
  // Abstract class with
  // non-abstract method
  abstract class myauthor
  {
  
      // Non-abstract method
      def details()
      {
          println("Welcome to GeeksforGeeks")
      }
  }
  
  // GFG class extends abstract class
  class GFG extends myauthor{}
  
  object Main
  {
  
      // Main method
      def main(args: Array[String])
      {
  
          // objects of GFG class
          var obj = new GFG()
          obj.details()
      }
  }

  Output:

  Welcome to GeeksforGeeks
  

• In Scala, an abstract class can contain final methods (methods that cannot be overridden). For example, the following program compiles and runs without an error. In Scala, final method is created using final keyword.
  Example:

  // Scala program to illustrate
  // the concept of abstract class
  
  // Abstract class with the final method
  abstract class myauthor
  {
      final def mymethod()
      {
          println("Final method")
      }
  }
  
  // GFG class extends abstract class
  class GFG extends myauthor{}
  
  object Main
  {
  
      // Main method
      def main(args: Array[String])
      {
  
          // objects of GFG class
          var obj = new GFG()
          obj.mymethod()
      }
  }

  Output:

  Final method
  

When to use abstract class in Scala:
An abstract class is useful:

• When we want to construct a base class which needs constructor arguments.
• When our code will be called from Java code.

Note:Traits are also used to achieve abstraction.