Abstract class

An abstract class is a class that can have many abstract members. An abstract member defines only a signature for an attribute or a method, without providing any implementation. You cannot instantiate an abstract class: you must create a subclass that implements all the abstract members.

Replace the definition of Shape and Rectangle in the worksheet as follows:

abstract class Shape(val x: Int, val y: Int) {
  val area: Double
  def description: String
}

class Rectangle(x: Int, y: Int, val width: Int, val height: Int)
  extends Shape(x, y) {

  val area: Double = width * height

  def description: String =
    "Rectangle " + width + " * " + height
}

Our class Shape is now abstract. We cannot instantiate a Shape class directly anymore: we have to create an instance of Rectangle or any of the other subclasses of Shape. Shape defines two concrete members, x and y, and two abstract members, area and description. The subclass, Rectangle, implements the two abstract members.