Design Patterns
In 1994, four authors Erich Gamma, Richard Helm, Ralph Johnson and John
Vlissides published a book (Design Patterns: Elements of Reusable
Object-Oriented Software) for explaining the concept of Design Pattern
in Software development. These four authors are collectively known as
Gang of Four (GOF).
The 23 Design patterns are defined by the Gang of Four programmers. These 23 patterns are divided into three groups depending on the nature of the design problem they intend to solve.
What is Design Patterns:
Basically 3 types of Design Patterns:
The 23 Design patterns are defined by the Gang of Four programmers. These 23 patterns are divided into three groups depending on the nature of the design problem they intend to solve.
What is Design Patterns:
- Design Patterns in object oriented world is reusable solution to common software design problems which occur again and again in real world application development. It is a template or description for how to solve a problem which can be used in many different situations.
- "A pattern is a recurring solution to a problem in a context."
- "Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice."
- Patterns are used by developers to their particular design to solve their problems. Patterns usage and choice to choose among different design patterns is based on individual need and their problem.
- Design patterns are the most powerful tool for software developer. It is important to understand design patterns rather than memorizing its classes, methods and properties. It is also important to learn how to apply pattern to specific problem to get the desired result. This will be required continuous practice of using and applying design patterns in day to day software development. First, identify the software design problem, then see how to address these problems using design patterns and find out the best suited design problem to solve the problem.
- There are 23 design patterns also known as Gang of four design patterns (GoF). Gang of four are the authors of the book, “Design Patterns: Elements of Reusable Object Oriented Software”. These 23 patterns are grouped into three main categories based on their:
Basically 3 types of Design Patterns:
1. Creational Patterns:
2. Behavioral Patterns:
3. Structural Patterns:
Creational Patterns:
1.
Singleton:
Ensure that only one instance of a class is created
and Provide a global
access point to the object.
2.
Factory:
Creates objects without exposing the instantiation logic to
the client and Refers to the newly created object
through a common interface.
3.
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.
4.
Abstract
Factory: Offers the interface for creating a family of related objects,
without explicitly specifying their classes.
5.
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.
6.
Prototype:
Specify the kinds of objects to create using a prototypical instance, and
create new objects by copying this prototype.
7.
Object
Pool: Reuses and shares objects that are expensive to create.
Behavioral
Patterns:
1.
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. The objects become parts of a chain and the request is sent from one
object to another across the chain until one of the objects will handle it.
2.
Command:
Encapsulate a request in an object, Allows the parameterization of clients with
different requests and Allows saving the requests in a queue.
3.
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.
4.
Iterator:
Provide a way to access the elements of an aggregate object sequentially
without exposing its underlying representation.
5.
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.
6.
Observer:
Define a one-to-many dependency between objects so that when one object changes
state, all its dependents are notified and updated automatically.
7.
Strategy:
Define a family of algorithms, encapsulate each one, and make them
interchangeable. Strategy lets the algorithm vary independently from clients
that use it.
8.
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.
9.
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.
10.
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
Patterns:
1.
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.
2.
Bridge:
Compose objects into tree structures to represent part-whole hierarchies. /
Composite lets clients treat individual objects and compositions of objects
uniformly.
3.
Composite:
Compose objects into tree structures to represent part-whole hierarchies. /
Composite lets clients treat individual objects and compositions of objects
uniformly.
4.
Decorator:
add additional responsibilities dynamically to an object.
5.
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.
6.
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.
7.
Proxy:
provide a “Placeholder” for an object to control references to it.
