Object-Oriented Design Principles (SOLID and Package Principles)

This article, originally written by Robert C. Martin (Bob Uncle) and translated by Zhang Shuo, provides an overview of object‑oriented design (OOD) principles. It discusses why understanding OOD is crucial for developers who use object‑oriented languages, emphasizing the importance of managing dependencies and designing flexible, reusable code.

The author contrasts structured (procedural) programming with object‑oriented programming, noting that modern languages embody both paradigms. He argues that many programmers are unaware of the underlying design principles that make OOD effective.

The article introduces five core class‑level design principles, often referred to as the SOLID principles:

SRP – Single Responsibility Principle : a class/module/package should have only one reason to change.

OCP – Open/Closed Principle : software entities should be open for extension but closed for modification.

LSP – Liskov Substitution Principle : components must be replaceable with subtypes without altering the correctness of the program.

ISP – Interface Segregation Principle : clients should not be forced to depend on interfaces they do not use; keep interfaces fine‑grained.

DIP – Dependency Inversion Principle : depend on abstractions, not concrete implementations; high‑level modules should not depend on low‑level modules.

Beyond class design, the article presents six package‑level principles that guide the organization of binary deliverables (e.g., JAR, DLL):

REP – Reuse/Release Equivalence Principle : the granularity of reuse matches the granularity of release.

CCP – Common Closure Principle : classes that change together should be packaged together.

CRP – Common Reuse Principle : classes that are used together should be packaged together.

ADP – Acyclic Dependencies Principle : the package dependency graph must contain no cycles.

SDP – Stable Dependencies Principle : depend in the direction of stability; more volatile components should depend on stable abstractions.

SAP – Stable Abstractions Principle : the stability of a package should increase with its level of abstraction.

The author stresses that good dependency management is essential for architects to avoid fragile, hard‑to‑modify codebases. Proper application of these principles leads to flexible, extensible, robust, and reusable software systems.