Design Problems

• Code Reuse / Redefining Parts of a Behavior
  • How can the invariant parts of a behavior be implemented once
    so that subclasses can implement the variant parts?
  • How can subclasses redefine certain parts of a behavior
    without changing the behavior's structure?
• Extending Behavior at Specific Points
  • How can subclasses extend a behavior only at specific points (hooks)?
• Controlling Subclassing
  • How can a class control how it is subclassed?

Refactoring Problems

• Duplicated Code
  • How can common behavior among classes be factored out
    and localized (generalized) in a common class?
    Form Template Method (205) [JK05]
    "[…] common behavior among subclasses should be factored
    and localized in a common class to avoid code duplication." [GoF, p326]