W. Kandinsky, Improvisation VI(African), 1909

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Design Patterns : Elements of Reusable Object - Oriented Software

by Erich Gamma, Richard Helm, Ralh Johnson, John Vlissides

QA76.64.D47 1994

Template Method

Applicability

Use the Template Method when

• to implement the invariant parts of an algorithm once and leave it up to subclasses to implement the behavior that can vary.
• when common behavior among subclasses should be factored and localized in a common class to avoid code duplication. You first identify the differences in existing code and then separate the differences into new operations. Finally, you replace the differing code with a template method that calis one of these new operations.
• to control subclases extensions. You can define a template method that calls "hook" oyerations at specific points, thereby permitting extensions only at those points.

Structure

Participants

• AbstractClass
  • defines abstract primitive operations that concrete subclasses define to implement steps os an algorithm.
  • Implements a template method defining the skeleton of an algorithm. The template method calls primitive operations as well as operations defined in AbstractClass or those of other objects.
• ConcreteClass
  • implements the primitive operations to carry out subclass-specific steps of algorithm. ConcreteClass relies on AbsractClass to implement the invariant steps of the algorithm.