3. Software Design Principles

Single Responsibility Principle (SRP)

SRP states that a class or module should have one reason to change. In other words, it should focus on one responsibility. This reduces coupling and makes code easier to modify without side effects.

Open/Closed, Liskov, Interface, Dependency

The other SOLID principles (Open/Closed, Liskov Substitution, Interface Segregation, Dependency Inversion) all aim to make code more flexible, testable, and maintainable. They encourage writing code that is open for extension but closed for modification, substitutable, focused on small interfaces, and decoupled from concrete dependencies.

SOLID is not about strict rules but about guiding good design. Applying these principles gradually leads to code that is easier to refactor, test, and evolve as requirements change.

DRY (Don’t Repeat Yourself)

DRY encourages avoiding duplication of knowledge. When the same logic appears in multiple places, changes become risky and error‑prone. Refactoring common behavior into shared functions or modules keeps code consistent.

KISS (Keep It Simple, Stupid)

KISS reminds developers to favor simplicity over cleverness. Over‑engineering and unnecessary abstraction can make code harder to understand and maintain. Simpler solutions are often more robust.

Together, DRY and KISS push you toward code that is both concise and understandable. You avoid copy‑paste duplication but also avoid abstraction for its own sake.

Design patterns are proven, reusable solutions to common design problems. They are not code templates but conceptual blueprints that help structure solutions.

Patterns are often grouped into creational (how objects are created), structural (how objects are composed), and behavioral (how objects interact). Examples include Singleton, Factory, Adapter, Observer, and Strategy.

Patterns are most helpful when you recognize a recurring design problem. Using a known pattern provides a shared vocabulary and a starting point for clean, extensible designs.

Code reusability means writing components that can be used in multiple places or projects. Functions, classes, modules, and libraries all support reuse by separating common logic from specific details.

Reusable code reduces duplication, saves time, and lowers bug counts because fixes are applied in one place. It also speeds up new feature development by allowing you to assemble solutions from existing building blocks.

There is a balance: chasing reusability too early can lead to overly generic code that is hard to read. The best practice is often to refactor for reuse when you see real duplication or recurring needs, not before.