Design Patterns

Singleton pattern should be used when we must ensure that only one instance of a class is created and when the instance must be available through all the code. A special care should be taken in multi-threading environments when multiple threads must access the same resources through the same singleton object.

Factory pattern should be used when: - a framework delegate the creation of objects derived from a common superclass to the factory - we need flexibility in adding new types of objects that must be created by the class

Factory Method pattern should be used when:
- a framework delegate the creation of objects derived from a common superclass to the factory
- the base factory class does not know what concrete classes will be required to create - delegates to its subclasses the creation of concrete objects
- factory subclasses subclasses are aware of the concrete classes that must be instantiated

Factory method pattern, compared to Factory pattern replace the factory with an abstract class and a set of concrete factories subclasses. The subclasses are responsible for creating concrete product objects; for factory method is possible adding new product classes without changing the abstract factory. The same result can be achieved for simplified factory pattern if reflection is used.

Abstract Factory should be used when:

• A system should be configured with one of multiple families of products
• A system should be independent of how its products are created, composed and represented
• Products from the same family should be used all together, products from different families ahould not be used togheter and this constraint must be ensured.
• Only the product interfaces are revealed, the implementations remains hidden to the clients.

Basically, we'll use an object pool whenever there are several clients who needs the same stateless resource which is expensive to create.