107. The Shapes demo - Inheritance

The class hierachy for the Shapes demo is provided below:

Inheritance is used in a number of places:

• The Square class is a subclass of Rectangle as squares are a special type of rectangle in which all four sides have the same length.
  • Square doesn’t need to do much over the already provided Rectangle class
• The Triangle class is extended by two subtypes: TriangleRightAngled and TriangleIsosceles
  • TriangleEquilateral is a subclass of TriangleIsosceles

The three Triangle subtypes encapsulate specific refinements such as the definition of the sides and refinement of the calculateArea calculation. By making calculateArea a static method, each subtype makes the calculation easily available to other code as well as tying the calculateArea instance method to the static method.

The Square, TriangleRightAngled and TriangleEquilateral classes are both declared as final because I decided they couldn’t be refined any further. This is an important aspect to designing/developing classes: only allow for inheritance if you will support it. By marking these classes as final I’ve indicated that the implementations can’t/shouldn’t be further enhanced. Naturally, another developer could extend Triangle with their own implementation as it isn’t marked as final because I believe Triangle to be a sound generic basis for describing triangles.