It is a description or template for how to solve a problem that can be used in many different situations. Design Patterns & Refactoring. Design Patterns. In software engineering, a design pattern is a general repeatable solution to a commonly occurring problem in software design.
A design pattern isn't a finished design that can be transformed directly into code. It is a description or template for how to solve a problem that can be used in many different situations. Uses of Design Patterns Design patterns can speed up the development process by providing tested, proven development paradigms.
Effective software design requires considering issues that may not become visible until later in the implementation. Often, people only understand how to apply certain software design techniques to certain problems. In addition, patterns allow developers to communicate using well-known, well understood names for software interactions. Object Oriented Design. Software design pattern. There are many types of design patterns, for instance Algorithm strategy patterns addressing concerns related to high-level strategies describing how to exploit application characteristics on a computing platform.Computational design patterns addressing concerns related to key computation identification.Execution patterns that address concerns related to supporting application execution, including strategies in executing streams of tasks and building blocks to support task synchronization.Implementation strategy patterns addressing concerns related to implementing source code to support program organization, andthe common data structures specific to parallel programming.Structural design patterns addressing concerns related to high-level structures of applications being developed.
Singleton pattern. There is criticism of the use of the singleton pattern, as some consider it an anti-pattern, judging that it is overused, introduces unnecessary restrictions in situations where a sole instance of a class is not actually required, and introduces global state into an application. In C++ it also serves to isolate from the unpredictability of the order of dynamic initialization, returning control to the programmer.
Common uses The Abstract Factory, Builder, and Prototype patterns can use Singletons in their implementation.Facade Objects are often Singletons because only one Facade object is required.State objects are often Singletons.Singletons are often preferred to global variables because: They do not pollute the global namespace (or, in languages with namespaces, their containing namespace) with unnecessary variables.They permit lazy allocation and initialization, whereas global variables in many languages will always consume resources. Abstract factory pattern. A factory is the location of a concrete class in the code at which objects are constructed.
The intent in employing the pattern is to insulate the creation of objects from their usage and to create families of related objects without having to depend on their concrete classes. This allows for new derived types to be introduced with no change to the code that uses the base class. Definition The essence of the Abstract Factory Pattern is to "Provide an interface for creating families of related or dependent objects without specifying their concrete classes. ". Usage This insulates client code from object creation by having clients ask a factory object to create an object of the desired abstract type and to return an abstract pointer to the object. As the factory only returns an abstract pointer, the client code (that requested the object from the factory) does not know — and is not burdened by — the actual concrete type of the object that was just created.
Builder pattern. The builder pattern has another benefit.
It can be used for objects that contain flat data (html code, SQL query, X.509 certificate...), that is to say, data that can't be easily edited. This type of data cannot be edited step by step and must be edited at once. The best way to construct such an object is to use a builder class. Builder often builds a Composite. Often, designs start out using Factory Method (less complicated, more customizable, subclasses proliferate) and evolve toward Abstract Factory, Prototype, or Builder (more flexible, more complex) as the designer discovers where more flexibility is needed. Prototype pattern. The client, instead of writing code that invokes the "new" operator on a hard-coded class name, calls the clone() method on the prototype, calls a factory method with a parameter designating the particular concrete derived class desired, or invokes the clone() method through some mechanism provided by another design pattern.
The mitotic division of a cell — resulting in two identical cells — is an example of a prototype that plays an active role in copying itself and thus, demonstrates the Prototype pattern. When a cell splits, two cells of identical genotype result. In other words, the cell clones itself. Structure UML class diagram describing the Prototype design pattern Rules of thumb Often, designs start out using Factory Method (less complicated, more customizable, subclasses proliferate) and evolve toward Abstract Factory, Prototype, or Builder (more flexible, more complex) as the designer discovers where more flexibility is needed.
Data access object. Although this design pattern is equally applicable to most programming languages, most types of software with persistence needs, and most types of databases, it is traditionally associated with Java EE applications and with relational databases accessed via the JDBC API because of its origin in Sun Microsystems' best practice guidelines ("Core J2EE Patterns") for that platform.
Advantages The advantage of using data access objects is the relatively simple and rigorous separation between two important parts of an application that can and should know almost nothing of each other, and which can be expected to evolve frequently and independently. Changing business logic can rely on the same DAO interface, while changes to persistence logic do not affect DAO clients as long as the interface remains correctly implemented. Tools and frameworks