What are design patterns
Design patterns are documented tried and tested solutions for recurring problems in a given context. So basically you have a problem context and the proposed solution for the same. Design patterns existed in some or other form right from the inception stage of software development. Let’s say if you want to implement a sorting algorithm the first thing comes to mind is bubble sort. So the problem is sorting and solution is bubble sort. Same holds true for design patterns.
There are three basic classifications of patterns Creational, Structural, and Behavioral patterns.
Creational Patterns
• Abstract Factory:- Creates an instance of several families of classes
• Builder: - Separates object construction from its representation
• Factory Method:- Creates an instance of several derived classes
• Prototype:- A fully initialized instance to be copied or cloned
• Singleton:- A class in which only a single instance can exist
Note: - The best way to remember Creational pattern is by ABFPS (Abraham Became First President of States).
Structural Patterns
• Adapter:-Match interfaces of different classes.
• Bridge:-Separates an object’s abstraction from its implementation.
• Composite:-A tree structure of simple and composite objects.
• Decorator:-Add responsibilities to objects dynamically.
• Façade:-A single class that represents an entire subsystem.
• Flyweight:-A fine-grained instance used for efficient sharing.
• Proxy:-An object representing another object.
Note : To remember structural pattern best is (ABCDFFP)
Behavioral Patterns
• Mediator 
efines simplified communication between classes.
• Memento:-Capture and restore an object's internal state.
• Interpreter:- A way to include language elements in a program.
• Iterator:-Sequentially access the elements of a collection.
• Chain of Resp: - A way of passing a request between a chain of objects.
• Command:-Encapsulate a command request as an object.
• State:-Alter an object's behavior when its state changes.
• Strategy:-Encapsulates an algorithm inside a class.
• Observer: - A way of notifying change to a number of classes.
• Template Method efer the exact steps of an algorithm to a subclass.
• Visitor efines a new operation to a class without change.
What the difference between visitor and strategy pattern
Visitor and strategy look very much similar as they deal with encapsulating complex logic from data. We can say visitor is more general form of strategy.
In strategy we have one context or a single logical data on which multiple algorithms operate. In the previous questions we have explained the fundamentals of strategy and visitor. So let’s understand the same by using examples which we have understood previously. In strategy we have a single context and multiple algorithms work on it. Figure ‘Strategy’ shows how we have a one data context and multiple algorithm work on it.
Figure: - Strategy
In visitor we have multiple contexts and for every context we have an algorithm. If you remember the visitor example we had written parsing logic for every data context i.e. customer, address and phones object.
Figure: - Visitor
So in short strategy is a special kind of visitor. In strategy we have one data context and multiple algorithms while in visitor for every data context we have one algorithm associated. The basic criteria of choosing whether to implement strategy or visitor depends on the relationship between context and algorithm. If there is one context and multiple algorithms then we go for strategy. If we have multiple contexts and multiple algorithms then we implement visitor algorithm.
