Understanding the Six SOLID Principles of Design Patterns

Programming is a process that starts with thinking before coding; thoughtful design improves reusability, reliability, and understandability, often using design patterns to engineer code.

Single Responsibility Principle (SRP) : A class should have only one reason to change. Benefits include reduced complexity, higher readability, easier maintenance, and lower risk when modifying interfaces.

Complexity reduction and clear responsibility definitions.

Improved readability.

Enhanced maintainability.

Reduced change‑induced risk, aiding extensibility and maintenance.

Interface Segregation Principle (ISP) : Clients should not be forced to depend on interfaces they do not use; dependencies should be on the smallest possible interface. An example shows how splitting a large interface into smaller ones prevents unnecessary coupling.

Dependency Inversion Principle (DIP) : High‑level modules should not depend on low‑level modules; both should depend on abstractions. Abstractions must not depend on details, while details depend on abstractions. Refactoring a direct dependency (Class A → Class B) into an interface reduces modification risk.

Liskov Substitution Principle (LSP) : Subtypes must be substitutable for their base types without altering program behavior. When extending functionality via inheritance, avoid overriding base methods that could break existing behavior. Proper use preserves code sharing, reusability, and system openness.

Code sharing reduces class creation effort.

Improves reusability and extensibility.

Enhances product/project openness.

Law of Demeter (LoD) : Each unit should know only its close collaborators and communicate only with its immediate friends. Example: a 手机 reads a 微信读书 APP which accesses a 书籍 ; direct calls between 手机 and 书籍 violate LoD.

Open/Closed Principle (OCP) : Software entities should be open for extension but closed for modification. When changes are needed, extend behavior rather than altering existing code to avoid introducing bugs and extensive retesting.

Summary :

SRP: keep responsibilities single.

ISP: design minimal, focused interfaces.

DIP: program to abstractions.

LSP: preserve inheritance contracts.

LoD: reduce coupling.

OCP: be open to extension, closed to modification.