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

• 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

• 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

• 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 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

• 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 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 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 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

• 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

• 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.