Introduction to Design Patterns

What are Design Patterns?

Design patterns are essentially a catalog common interactions between objects. These interactions have been found, tested / tried and optimized by experienced programmers to help others solve some of their programming problems very common in an efficient manner. Design Patterns illustrate just how objects can communicate with them without being much concerned about their individual data models and methods. This clear separation helps create solutions to design robust and scalable for different programming problems. If you've already made this separation in your code, then you already use some of the widely used Design Patterns.

Design patterns are mainly of three types.
Creational -
Creational patterns are ones that create objects for you, rather than having you instantiate objects directly. This gives your program more flexibility in deciding which objects need to be created for a given case.
Structural -
Structural patterns help you compose groups of objects into larger structures, such as complex user interfaces or accounting data.
Behavioral -
Behavioral patterns help you define the communication between objects in your system and how the flow is controlled in a complex program

Creational Design Patterns:Read More...

Singleton - Ensure that only one instance of a class is created and Provide a global access point to the object.

Factory(Simplified version of Factory Method) - Creates objects without exposing the instantiation logic to the client and Refers to the newly created object through a common interface.

Factory Method - Defines an interface for creating objects, but let subclasses to decide which class to instantiate and Refers to the newly created object through a common interface.

Abstract Factory - Offers the interface for creating a family of related objects, without explicitly specifying their classes.

Builder - Defines an instance for creating an object but letting subclasses decide which class to instantiate and Allows a finer control over the construction process.

Prototype - Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.

Object Pool - reuses and shares objects that are expensive to create..

Behavioral Design Patterns:Read More...

Chain of Responsibility - It avoids attaching the sender of a request to its receiver, giving this way other objects the possibility of handling the request too.

Command - Encapsulate a request in an object, Allows the parametrization of clients with different requests and Allows saving the requests in a queue.

Interpreter - Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language / Map a domain to a language, the language to a grammar, and the grammar to a hierarchical object-oriented design

Iterator - Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

Mediator - Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.

Observer - Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Strategy - Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

Template Method - Define the skeleton of an algorithm in an operation, deferring some steps to subclasses / Template Method lets subclasses redefine certain steps of an algorithm without letting them to change the algorithm's structure.

Visitor - Represents an operation to be performed on the elements of an object structure / Visitor lets you define a new operation without changing the classes of the elements on which it operates.

Null Object - Provide an object as a surrogate for the lack of an object of a given type. / The Null Object Pattern provides intelligent do nothing behavior, hiding the details from its collaborators.

Structural Design Patterns:Read More...

Adapter - Convert the interface of a class into another interface clients expect. / Adapter lets classes work together, that could not otherwise because of incompatible interfaces.

Bridge - Compose objects into tree structures to represent part-whole hierarchies. / Composite lets clients treat individual objects and compositions of objects uniformly.

Composite - Compose objects into tree structures to represent part-whole hierarchies. / Composite lets clients treat individual objects and compositions of objects uniformly.

Decorator - add additional responsibilities dynamically to an object.

Flyweight - use sharing to support a large number of objects that have part of their internal state in common where the other part of state can vary.

Memento - capture the internal state of an object without violating encapsulation and thus providing a mean for restoring the object into initial state when needed.

Proxy - provide a “Placeholder” for an object to control references to it.

Advantages And Disadvantages

Advantages:
1. A standard solution to a common programming problem
2. Enable large scale reuse of S/W
3. Helps in improve developer communication
4. Capture expert knowledge and design trade-offs and make expertise widely available
5. A technique for making code more flexible by making it meet certain criteria
6. A design or implementation structure that achieves a particular purpose
7. Speed up the development process by providing tested, proven development paradigms.
8. Reusing design patterns helps to prevent subtle issues that can cause major problems, and it also improves code readability for coders and architects who are familiar with the patterns.
9. Design patterns provide general solutions, documented in a format that doesn't require specifics tied to a particular problem.
10. In addition, patterns allow developers to communicate using well-known, well understood names for software interactions. Common design patterns can be improved over time, making them more robust than ad-hoc designs

Disadvantages:
1. Do not lead to direct code reuse
2. Complex in nature
3. They are deceptively simple
4. Design patterns may increase or decrease the understandability of a design or implementation. They can decrease understandability by adding indirection or increasing the amount of code.
5. They are validated by experience and discussion