Design Principles for OOP Brief Summary

If you are developing software using object oriented programming, design principles help you a lot, by increasing flexibility and reusability.
All design patterns follow design principles below.

Liskov Substitution Principle

• Description
  • An instance of superclass must be replaceable by an instance of subclass
  • Therefore, subclass's functionality should be general enough
• Benefit
  • It keeps client code away from being impacted
  • Open-Closed Principle can also be satisfied

Open-Closed Principle

• Description
  • Class should be open to extension, and closed to change
  • Class is recognized to be closed when design and implementation is completed in the following cases
    • It is tested to function properly
    • All the attributes and behaviors are encapsulated
    • Class and instance do not block the system, and they are confirmed to be stable
• Benefit
  • It keeps stability by preventing unexpected change
  • It allows adding functionality (e.g. by inheritance)

Dependency Inversion Principle

• Description
  • Access high level generalization
  • Do not directly access low level concrete class
• Example
  • Create an interface which generalizes the concrete class
  • Client accesses only the interface
• Benefit
  • It looses coupling, and realizes flexibility and reusability

Composing Object Principle (Composition over inheritance)

• Description
  • Extend functionality by composition(aggregation) rather than by inheritance
• Example
  • When you want to use some class's functionality, add a property of its instance, rather than inheriting it
• Benefit
  • It looses coupling, by avoiding inheriting all the properties and methods of the superclass

Interface Segregation Principle

• Description
  • Separate interface, and avoid enforcing to implement unwanted methods
• Benefit
  • It avoids unnecessary dependency, and looses coupling

Principle of Least Knowledge (Law of Demeter)

• Description
  • Method can only call other methods of the following
    • In the same object
    • Method parameter
    • Object instantiated within the method
    • Instance variable in the same object
• Benefit
  • It looses coupling, by avoiding unnecessary interaction over classes

Reference

Coursera Design Patterns