Software Design patterns

What are Design patterns?

Design patterns are optimized, reusable solutions to an everyday programming problem. A design pattern is not code and it is not language specific, it is a template that has to be implemented in the correct situation. You have to pick the right design pattern that fits your situation, otherwise it could lead to disastrous results.

There are three basic kinds of design patterns:

  • Structural
  • Creational
  • Behavioral

Why should we use them?

Design patterns are, by principle, well tough out solutions to programming problems. Many programmers have encountered these problems before, and have used these solutions to remedy them. These solutions are tested and proven to work well in the right conditions.

Structural design patterns

These design patterns are all about Class and Object composition. Structural class-creation patterns use inheritance to compose interfaces.


  • Adapter
    • Match interfaces of different classes
  • Bridge
    • Separates an object’s interface from its implementation
  • Composite
    • A tree structure of simple and composite objects


  • Decorator
    • Add responsabilities to objects dynamically
  • Facade
    • A single class that represents an entire subsystem
  • Flyweight
    • A fine-grained instance used for efficient sharing
  • Private Class Data
    • Restricts accessor/mutator access
  • Proxy
    • An object representing another object

Creational design patterns

These are about class instantiation. This pattern can be further divided into class-creation patterns and object-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
  • Object Pool
    • Avoid expensive acquisition and release of resources by recycling objects that are no longer in use
  • Prototype
    • A fully initialized instance to be copied or cloned


  • Singleton
    • A class of which only a single instance can exist.


These design patterns are about Class’s objects communication. Specifically concerned with communication between objects.

  • Chain of responsibility
    • A way of passing a request between a chain of objects
  • Command
    • Encapsulate a command request as an object
  • Interpreter
    • A way to include language elements in a program
  • Mediator
    • Defines simplified communication between classes
  • Memento
    • Capture and restore an object’s internal state
  • Null Object
    • Designed to act as a default value of an object
  • Observer
    • A way of notifying change to a number of classes
  • State
    • Alter an object’s behavior when its state changes


  • Strategy
    • Encapsulates an algorithm inside a class
  • Template method
    • Defer the exact steps of an algorithm to a subclass
  • Visitor
    • Defines a new operation to a class without change


Images from:–net-12752

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