Superclasses & Subclasses

• Often, an object of one class "is an" object of another class.
  • a complex number is a number;
  • a trapezoid is a quadrilateral;
  • a politician is a human (hmmm, well...the exception proves the rule!).
• These examples also suggest that a subclass represents a subset of the set that the superclass represents.
• Inheritance relationships induce a tree-like structure.

• Shapes:
  • Polygon
    • Triangle
      • Isosceles
        • Equalateral
      • Scalene
    • Quadrilateral
      • Rectangle
        • Square
      • Rhombus
        • Square
      • Trapezoid
  • Circle

   

• Where would you insert the class of right triangles? (life ain't that simple!)
• To specify that one class inherits from another we use the extends keyword:

  class Polygon extends Shapes { ... }

• Only private members (instance variables and methods) are not accessible from the subclass.
• All other members of the superclass become members of the subclass.
• Objects of all classes derived from a common superclass can all be treated as objects of that superclass.

  For example, we can treat Rectangles, Rhombuses, and Trapezoids all as Quadrilaterals.

  This sometimes can lead to much simpler code.