Applied Design Patterns with Java

Behavioral :: Template Method (325) {C ch 25}

Applicability

Use the Template Method pattern in either of the following cases:

• 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. This is a good example of "refactoring to generalize". First identify the differences in the existing code and then separate the differences into new operations. Finally, replace the differing code with a Template Method that calls one of these new operations.
• to control subclasses extensions. Define a Template Method that calls "hook" operations at specific points, permitting extensions only at those points.
  

Structure

Participants

• AbstractClass (Application) defines abstract primitive operations that concrete subclasses define to implement steps of an algorithm, and 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 (MyApplication) implements the primitive operations to carry out subclass-specific steps of the algorithm.

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