CS 4773 Object Oriented Systems The Strategy Pattern

• From the Gang of Four:
  Design a family of algorithms, encapsulate each one, and make them interchangeable.
  Strategy lets the algorithm vary independently from clients that use them.

• Objects have responsibilities
• Different specific implementations of the responsibuilities are manifested through the use of polymorphism.
• You need to manage several different implementations of what is conceptually the same algorighm.
• Program to an interface, not an implementation
• Favor aggregation over inheritance
• Consider what should be variable in your design.

• One of the things that needs to be calculated is sales tax.
• Different types of items are taxed differently:
  food: no tax
  clothing: no tax on items under $100 one weekend a year
  others
  

Methods of handling variation

• Copy and Paste: Duplicate your code, and make the necessary changes in one copy.
  Have a different, independent bill of sale for each type of item
• switch statements or if-then-else
• function pointers (language must support this, e.g. C, C++, not Java)
  Use different functions of calculating each type of tax
  Set a function pointer to point to the appropriate function.
• inheritance: can be a good choice if used wisely.
  In this case, choose one of the types as the base bill of sale class.
  Others override the tax calculation method.
• delegation: find out what varies and include it its own class.
  Make one new class for each possibility.
  Each should implement a common interface.
  Create an instance of the appropriate one of these in a new class. (aggregation)
  This uses inheritance (each tax class implements the tax interface), but at a lower level.
  This approach is often referred to as the strategy pattern.

• Choose an EditorFile based on the contents of the file being edited.
  • If the file is very long, a virtual representation of the file may be appropriate.
  • This would keep most of the file on disk, and cache only those lines that have been recently requrested.
• Choose a GUI based on the contents of the file being edited.
  • If the file contains only short lines of printing characters (codes between ' ' and 127 and newline) the display could be simple.
  • If the lines are long, you might want a GUI that can wrap lines.
  • If the file contains non-printing characters, the GUI could display characters as two hex digits.
  • While all of this could be done with a single GUI, it might be appropriate to have different groups implement different versions.
  • Perhaps just a display class of the GUI would need to be modified.