Master Essential Software Design Principles: From KISS to SOLID

Introduction

Software, like a living organism, evolves over time; understanding design principles helps developers create robust, adaptable systems.

General Design Principles

1. KISS

Keep It Simple, Stupid – design should stay as simple as possible without unnecessary complexity, making future modifications easy.

Everything should be made as simple as possible, but not simpler. – Albert Einstein

Practical ways to keep code simple include making it easy for other engineers to use, avoiding shortcuts, and ensuring understandability even as the system grows.

2. DRY

Don’t Repeat Yourself – eliminate duplicated information, not just duplicated lines of code. Repeated code, magic numbers, and similar logic increase maintenance effort.

Three‑step rule for removing duplication:

Write code the first time.

When you write similar logic elsewhere, feel the urge to refactor.

After the third repetition, extract the common part.

Eliminate duplication by cultivating good habits and refactoring; act immediately to improve code quality.

3. Maximize Cohesion, Minimize Coupling

High cohesion and low coupling are indicators of good design; modules should have a single, well‑defined purpose and minimal dependencies.

Coupling : degree of inter‑module dependence.

Cohesion : strength of internal relationships within a module.

Java‑specific recommendations include preferring interfaces over inheritance, keeping modules single‑purpose, using private instead of public, applying MVC, avoiding hard‑coded values, and minimizing direct inter‑module calls.

4. SOC (Separation of Concerns)

Different functional areas should be implemented in separate, minimally overlapping modules. MVC is a classic example that separates model, view, and controller.

A good architecture isolates concerns so that changes in one part do not affect others. – Ivar Jacobson

5. YAGNI

You Aren’t Gonna Need It – do not implement features until they are truly required, saving time and keeping code clean.

Only build what is needed now; premature features often become dead code. – Ron Jeffries

6. Boy‑Scout Rule

Always leave the codebase cleaner than you found it; continuously improve existing code to prevent technical debt.

Maintain clean code, fix bad smells early, and avoid the “broken‑window” effect.

Other Useful Principles

Avoid Premature Optimization – optimize only after measuring performance impact.

Principle of Least Astonishment – code should behave as expected without surprising the reader.

Code Reuse is Good – reuse reduces development time and improves readability.

Don’t Make Me Think – keep APIs simple and intuitive.

Write Code for the Maintainer – prioritize future maintainability over short‑term convenience.

Orthogonality – unrelated concepts should remain independent.

Do the Simplest Thing That Could Possibly Work – start with the minimal viable solution.

Hide Implementation Details – expose only necessary interfaces.

Curly’s Law – a module should do one thing.

Murphy’s Law – expect the worst and design defensively.

Object‑Oriented Design Principles (SOLID)

1. SRP – Single Responsibility Principle

A class should have only one reason to change, improving cohesion and reducing coupling.

There should never be more than one reason for a class to change.

Benefits include lower complexity, better readability, and reduced risk of change.

2. LSP – Liskov Substitution Principle

Derived classes must be usable wherever base classes are expected without altering behavior.

Functions that use base‑class references must work with derived‑class objects transparently.

Advantages: increased reuse and extensibility; drawbacks: tighter coupling and potential for breaking changes.

3. ISP – Interface Segregation Principle

Clients should depend only on the interfaces they actually use; keep interfaces small and focused.

Clients should not be forced to depend upon interfaces that they do not use.

Balance is needed: overly tiny interfaces can increase complexity.

4. OCP – Open/Closed Principle

Software entities should be open for extension but closed for modification, typically achieved through abstraction.

Software entities should be open for extension, but closed for modification.

Using abstractions allows new behavior without altering existing code.

5. DIP – Dependency Inversion Principle

High‑level modules should depend on abstractions, not concrete implementations; details depend on abstractions.

High‑level modules should not depend on low‑level modules. Both should depend on abstractions.

This promotes stable, interchangeable components.

6. LOD / LKP – Law of Demeter (Least Knowledge Principle)

Objects should only talk to their immediate friends, reducing coupling and increasing modularity.

Talk only to your immediate friends.

Over‑application can lead to many mediator classes, so balance is required.

7. CRP – Composite Reuse Principle

Prefer object composition over inheritance to achieve reuse, keeping systems flexible and loosely coupled.

Use composition rather than inheritance whenever possible.

Composition hides internal details (black‑box reuse) and allows dynamic behavior, though it may increase the number of objects to manage.