template

Template

Intent

Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Themplate Method lets subclasses redefine certain steps of an algorithm withouth changing the algorithms structure

When to Use

• 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
• To control subclasses extensions

Participants

• AbstractClass
  • Defines abstract primitive operations that concrete subclasses define to implement steps of 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 the algorithm

Collaborations

• ConcreteClass relies on Abstract Class to implement the invariant steps of the algorithm

Consequences

• Template methods are a fundamental technique for code reuse
• Template leads to an inverted control structure referred to as "The Hollywood Principle" - "Dont call us, well call you"
  • Refers to how a parent class call the operations of a subclass, and not the other way around
• Templates call the following kinds of operations
  • Concrete operations
  • Concrete AbstractClass operations
  • Primitive operations
  • Factory methods
  • Hook operations

Implementation

• Minimize primitive operations that a subclass must override
  • The more operations that need overridding, the more tedious things get for clients
• Keep a consistent naming convention

Related Patterns

• Factory Methods are often called by template methods
• Strategy: Template methods use inheritance to vary part of an algorithm. Strategies use delegation to vary the entire algorithm