Design patterns are formalized best practices that the programmer can use to solve common problems when designing an application or system. Each pattern is like a blueprint that you can customize to solve a particular design problem in your code.
Design patterns are divided into three fundamental groups:
1. Behavioral,
2. Creational,
3. Structural
Behavioral patterns
Behavioral patterns describe interactions between objects and focus on how objects communicate with each other. They can reduce complex flow charts to mere interconnections between objects of various classes. Behavioral patterns are also used to make the algorithm that a class uses simply another parameter that is adjustable at runtime.
Behavioral patterns are concerned with algorithms and the assignment of responsibilities between objects. Behavioral patterns describe not just patterns of objects or classes but also the patterns of communication between them.
These patterns characterize complex control flow that is difficult to follow at run-time. They shift your focus away from the flow of control to let you concentrate just on the way objects are interconnected. Behavioral class patterns use inheritance to distribute behavior between classes.
The Template Method is the simpler and more common of the two. A template method is an abstract definition of an algorithm. It defines the algorithm step by step. Each step invokes either an abstract operation or a primitive operation. A subclass fleshes out the algorithm by defining the abstract operations. The other behavioral class pattern is Interpreter pattern, which represents a grammar as a class hierarchy and implements an interpreter as an operation on instances of these classes.
Creational Patterns
Creational patterns are used to create objects for a suitable class that serves as a solution for a problem. Generally when instances of several different classes are available. They are particularly useful when you are taking advantage of polymorphism and need to choose between different classes at runtime rather than compile time.
These design patterns provide a way to create objects while hiding the creation logic, rather than instantiating objects directly using new operator. This gives program more flexibility in deciding which objects need to be created for a given use case.
Structural Patterns
Structural patterns form larger structures from individual parts, generally of different classes.
Structural patterns vary a great deal depending on what sort of structure is being created for what purpose.
Structural patterns are concerned with how classes and objects are composed to form larger structures.
Structural class patterns use inheritance to compose interfaces or implementations. As a simple example, consider how multiple inheritance mixes two or more classes into one. The result is a class that combines the properties of its parent classes. This pattern is particularly useful for making independently developed class libraries work together.
