Object-Oriented Programming


Michael L. Collard, Ph.D.

Department of Computer Science, The University of Akron

SOLID Principles for Object-Oriented Design

  • Five basic principles (guidelines) for Object-Oriented Design
  • Leads to systems that are:
    • Easy to maintain
    • Easy to extend
  • SOLID is a guide for:
    • Creating designs from scratch
    • Improving existing designs

SOLID Principles

  • Single Responsibility Principle (SRP)
  • Open/closed Principle (OCP)
  • Liskov Substitution Principle (LSP)
  • Interface Segregation Principle (ISP)
  • Dependency Inversion Principle (DIP)

Single Responsibility Principle

Every class should have a single responsibility

  • Responsibility - a reason to change
  • Responsibility should be entirely encapsulated by the class
  • All of the class should focus on that single responsibility
  • Why? More cohesive. Easier to understand. Easier to maintain.

SRP Examples

  • Multiple Responsibilities
  • Single Responsibility

Open/Closed Principle

Software entities (classes, function, etc.) should be open for extension but closed for modification

  • Closed - as can be compiled, stored in a library, and used by client classes
  • Open - as any new class can inherit and add new features
  • Why? Client code dependent on base (closed) class unaffected. Less testing. Less code to review.

Meyer’s Open/Closed Principle

Implementation is extended through inheritance

  • “Open” means available for extension (generalization/inheritance)
  • “Closed” because we do not change the original class
  • New functionality by adding new classes, not changing the current ones

Polymorphic Open/Closed Principle

Abstract base class and multiple implementations that we can substitute for each other

  • Base design on abstract base classes
  • Focus on sharing the interface, not the implementation
  • Reuse implementation via delegation

Liskov Substitution Principle

Object in a program should be replaceable with an instance of subtypes without affecting program correctness

  • Preconditions cannot be strengthened in a subtype
  • Postconditions cannot be weakened in a subtype
  • Invariants of supertype must be preserved in subtype
  • History constraint - new methods in subtype cannot introduce state changes in a way that is not permissible in the supertype
  • Why? Knowledge/assumptions about base class apply to the subclass. Easier to understand. Easier to maintain.

LSP Examples

Not substitutable


Interface Segregation Principle

Many client-specific interfaces are better than one general-purpose interface

A client should not be forced to depend on methods it does not use

  • More cohesive
  • Lower coupling
  • Easier to understand
  • Easier to maintain

ISP Examples

Original Interface

General Interface

Multiple Specific Interfaces

Dependency Inversion Principle

Depend upon abstractions, not concretions

  • Abstractions should not depend on details, but details on abstractions
  • High-level modules are independent and should not depend on low-level modules
  • Why? Lower coupling. Reuse. Easier to test. Easer to understand. Easier to maintain.

DIP Examples

Traditional Layered Architecture (violates principle)

Ownership Inversion (upholds principle)


  • Meant to be applied together
  • Make it more likely that the system is easy to maintain and extend over time
  • SOLID principles are guidelines
    • Do not guarantee success
    • Can be misused
  • Use in conjunction with other principles
    • Occam’s Razor (KISS)
    • GRASP
    • DRY