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.
Read more about this topic: Strategy Pattern
Famous quotes containing the words strategy and, strategy, open, closed and/or principle:
“Do you think that mere words are strategy and power for war?”
—Bible: Hebrew, 2 Kings 18:20.
“To a first approximation, the intentional strategy consists of treating the object whose behavior you want to predict as a rational agent with beliefs and desires and other mental states exhibiting what Brentano and others call intentionality.”
—Daniel Clement Dennett (b. 1942)
“...there are important considerations in the world beyond plain sewing and teaching dull little boys the alphabet. Any woman who has brains and willing hands finds twenty remunerative occupations open to her where formerly she would have found merely the inevitable two—plain sewing, or the dull little boys. All she had to do is to make her choice and then buckle on her armor of perseverance, while the world applauds.”
—Clara (Marquise)
“No domain of nature is quite closed to man at all times.”
—Henry David Thoreau (1817–1862)
“What is an atheist, but one who does not, or will not, see in the universe a ruling principle of love; and what a misanthrope, but one who does not, or will not, see in man a ruling principle of kindness?”
—Herman Melville (1819–1891)