Login :
Password :

Design patterns

Hidden switch statement

Static diagram :

Static diagram

Strong points :

  1. 1. Extensibility
    1. KO 1.1 States' protocol factorization
    2. KO 1.2 Addition or removal of a state does not need code modification
  2. 2. Simplified management
    1. OK 2.1 Possibility of state modification at execution time without deletion
    2. KO 2.2 State behavior decoupling

Spoiled pattern instantiations on different problems

Problem 1 :

Problem wording :
Design a DVD market place work. The DVD market place provides DVD to its clients with three categories: children, normal and new. A DVD is new during some weeks, and after change category. The DVD price depends on the category. It is probable that the system evolves in order to take into account the horror category.
Optimal solution :

Static diagram :

Static diagram

Thanks to the inheritance links, we are able to distinct the DVD to its states. In these case, the addition of a new state does not imply code modification.

Alternative solution :

Static diagram :

Static diagram

The DVD class manages its state thanks to an enumeration. In doing so, the solution imposes a ``switch'' statement, and so, the category changing is possible. The problem of this solution concerns the extensibility. Indeed, if a new category is added, the DVD class must be modified to manage the new type.

Les têtes de mule
Cédric BOUHOURS
Cédric BOUHOURS
Cédric BOUHOURS