Strategy Pattern - Strategy and Open/closed Principle

Strategy and Open/closed Principle

According to the strategy pattern, the behaviors of a class should not be inherited, instead they should be encapsulated using interfaces. As an example, consider a car class. Two possible functionalities for car are brake and accelerate.

Since accelerate and brake behaviors change frequently between models, a common approach is to implement these behaviors in subclasses. This approach has significant drawbacks: accelerate and brake behaviors must be declared in each new Car model. The work of managing these behaviors increases greatly as the number of models increases, and requires code to be duplicated across models. Additionally, it is not easy to determine the exact nature of the behavior for each model without investigating the code in each.

The strategy pattern uses aggregation instead of inheritance. In the strategy pattern, behaviors are defined as separate interfaces and specific classes that implement these interfaces. Specific classes encapsulate these interfaces. This allows better decoupling between the behavior and the class that uses the behavior. The behavior can be changed without breaking the classes that use it, and the classes can switch between behaviors by changing the specific implementation used without requiring any significant code changes. Behaviors can also be changed at run-time as well as at design-time. For instance, a car object’s brake behavior can be changed from BrakeWithABS to Brake by changing the brakeBehavior member to:

This gives greater flexibility in design and is in harmony with the Open/closed principle (OCP) that states that classes should be open for extension but closed for modification.