Template design pattern

Intent

• Define the skeleton of an algorithm in an operation, deferring some steps to client subclasses.

• Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure.

• Base class declares algorithm 'placeholders', and derived classes implement the placeholders.

Motivation

• The component designer decides which steps of an algorithm are invariant (or standard), and which are variant (or customizable).

• The invariant steps are implemented in an abstract base class, while the variant steps are either given a default implementation, or no implementation at all.

• The variant steps represent "hooks", or "placeholders", that can, or must, be supplied by the component's client in a concrete derived class.

Structure

Participants

• AbstractClass- implements a template method defining the skeleton of an algorithm

• ConcreteClass - implements the primitive operations to carry out

subclass-specificsteps of the algorithm.

Structure

Collaborations

• ConcreteClass relies on AbstractClass to implement the invariant steps of the algorithm.

Applicability

• To implement the invariant parts of an algorithm once and leave it upto subclasses to implement the behavior that can vary.

Example

Consequences

• Template methods are a fundamental technique for code reuse.

• Template methods lead to an inverted control structure that is sometimes referred to as "the Hollywood principle," that is, "Don' t call us, we' ll call you“.• This refers to how a parent class calls the operations of a subclass and not the

other way around.

Known Uses

• Template methods are so fundamental that they can be found in almost every abstract class.

Related Patterns

• Factory Methods (121) are often called by template methods.

• Strategy (349) : Template methods use inheritance to vary part of an algorithm.