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Prerequisite

Design pattern is based on the SOLID principles. So knowing SOLID principles is must to learn design pattern.

Strategy Pattern

  • The strategy pattern (also known as the policy pattern) is a software design pattern that enables an algorithm’s behavior to be selected at runtime
  • Separate the code that changes, from the code that remains same
  • Wrap the code with interface/abstraction that we will change

Observer Pattern

  • The Observer Pattern defines a one to many dependency between objects so that one object changes state, all of its dependents are notified and updated automatically
  • In this pattern, observer are those who are notified when changing of data occurs

Decorator Pattern

  • The decorator pattern attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality
  • The decorator pattern can be used to make it possible to extend (decorate) the functionality of a certain object at runtime. It is like wrapping a gift, putting it in a box, and wrapping the box

Adapter Pattern

  • It is like the problem of inserting a new three-prong electrical plug in an old two-prong wall outlet – some kind of adapter or intermediary is necessary
  • Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces
  • Wrap an existing class with a new interface
  • Impedance match an old component to a new system

Facade Pattern

  • Facade pattern presents a simplified interface to client
  • It Provides a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use
  • Facade pattern violates open closed principle.And to solve this problem we use command pattern

Command Pattern

  • The command pattern encapsulates a request as an object, thereby letting us parameterize other objects with different requests,queue or log requests, and support undoable operations
  • Command pattern is modified version of Facade pattern

Singleton Pattern

  • Singleton pattern is a design pattern which restricts a class to instantiate its multiple objects
  • It should have only one instance
  • Instance should be globally accessible

Prototype Pattern

  • Prototype pattern allows us to hide the complexity of making new instances from the client
  • The concept is to copy an existing object or make clone of an existing object rather than creating a new instance from scratch
  • This approach saves costly resources and time, especially when object creation is a heavy process

Template Pattern

  • Template design pattern is to define an algorithm as a skeleton of operations and leave the details to be implemented by the child classes
  • The overall structure and sequence of the algorithm are preserved by the parent class

Model View Controller Pattern

  • The Model View Controller(MVC) design pattern specifies that an application consist of a data model, view information, and control information
  • The pattern requires that each of these be separated into different objects
  • MVC is more of an architectural pattern, but not for complete application. MVC mostly relates to the UI / interaction layer of an application

Iterator Pattern

  • This pattern provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
  • Here the iterator interface provides a set of methods for traversing or modifying the collection that is in addition to next()/hasNext() it can also provide functions for search, remove etc.