Applying SOLID to real code in LLD - Deep Dive

Overview - Applying SOLID to real code

What is it?

Applying SOLID means using five key rules to write better code. These rules help make code easier to understand, change, and fix. Each rule focuses on one way to organize code so it works well alone and with other parts. Together, they guide developers to build strong, flexible software.

Why it matters

Without SOLID, code can become messy and hard to fix or add new features. This slows down projects and causes bugs. Using SOLID helps teams work faster and keeps software reliable. It makes sure changes don’t break other parts, saving time and money in the long run.

Where it fits

Before learning SOLID, you should know basic programming and how to write simple functions and classes. After SOLID, you can learn design patterns and architecture principles that build on these rules to create large, maintainable systems.

Mental Model

Core Idea

SOLID is a set of five simple rules that guide how to organize code so it is easy to change, understand, and reuse without breaking other parts.

Think of it like...

Imagine building with LEGO blocks where each block has a clear shape and purpose. SOLID rules help you design blocks that fit perfectly together but can also be swapped or fixed without rebuilding the whole model.

┌─────────────┐
│   SOLID     │
├─────────────┤
│ S - Single  │
│   Responsibility │
│ O - Open/Closed │
│ I - Interface │
│   Segregation │
│ L - Liskov   │
│   Substitution │
│ D - Dependency │
│   Inversion  │
└─────────────┘

Build-Up - 7 Steps

1

Foundation

Understanding Single Responsibility Principle

Concept: Learn that each part of code should do one clear job.

Single Responsibility Principle means a class or module should have only one reason to change. For example, a class that manages user data should not also handle sending emails. Keeping responsibilities separate makes code easier to fix and test.

Result

Code parts become simpler and focused, reducing bugs when changes happen.

Understanding that one part should do one job prevents tangled code and makes maintenance easier.

2

Foundation

Grasping Open/Closed Principle Basics

Concept: Code should be open to adding new features but closed to changing existing code.

Open/Closed Principle means you can add new behavior without changing existing code. For example, adding a new payment method should not require changing the payment processing class, but extending it instead.

Result

New features can be added safely without breaking old code.

Knowing how to extend code without modifying it protects working parts from accidental bugs.

3

Intermediate

Applying Interface Segregation Principle

🤔Before reading on: do you think one big interface is better than many small ones? Commit to your answer.

Concept: Use many small, specific interfaces instead of one large, general one.

Interface Segregation Principle says clients should only know about methods they use. For example, a printer interface should separate printing and scanning functions so devices that only print don’t need to implement scanning methods.

Result

Code becomes easier to implement and less error-prone because classes only handle what they need.

Understanding this prevents forcing classes to implement unused methods, reducing complexity.

4

Intermediate

Mastering Liskov Substitution Principle

🤔Before reading on: can a subclass change behavior so it breaks the parent’s expectations? Commit to yes or no.

Concept: Subclasses must behave like their parent classes without surprises.

Liskov Substitution Principle means you can replace a parent class with a child class without changing how the program works. For example, if a bird class can fly, a subclass penguin should not break this expectation by throwing errors when flying is called.

Result

Inheritance works safely, avoiding bugs from unexpected subclass behavior.

Knowing this keeps inheritance reliable and prevents subtle bugs in polymorphism.

5

Intermediate

Understanding Dependency Inversion Principle

🤔Before reading on: do you think high-level modules should depend on low-level modules directly? Commit to yes or no.

Concept: High-level code should not depend on low-level details but on abstractions.

Dependency Inversion Principle means both high-level and low-level modules depend on interfaces or abstract classes, not concrete implementations. For example, a payment processor depends on a payment interface, not a specific credit card class.

Result

Code becomes flexible and easier to change because implementations can swap without affecting high-level logic.

Understanding this decouples code layers, making systems more modular and testable.

6

Advanced

Refactoring Real Code with SOLID Principles

🤔Before reading on: do you think applying all SOLID principles at once is easy or challenging? Commit to your answer.

Concept: Learn how to identify code smells and refactor code step-by-step using SOLID rules.

Start by spotting classes doing too many jobs (violating SRP). Break them into focused classes. Then check if interfaces are too broad and split them. Replace direct dependencies with abstractions. Finally, ensure subclasses follow parent behavior. Refactor small parts iteratively to keep code stable.

Result

Messy code becomes clean, modular, and easier to maintain without breaking functionality.

Knowing how to apply SOLID gradually helps improve legacy code safely and effectively.

7

Expert

Balancing SOLID with Practical Constraints

🤔Before reading on: do you think strictly following SOLID always leads to better code? Commit to yes or no.

Concept: Understand when strict SOLID adherence may add unnecessary complexity and how to balance it with real-world needs.

In some cases, applying all SOLID principles strictly can lead to too many small classes and interfaces, making code harder to navigate. Experienced developers balance SOLID with simplicity, using it as a guide rather than a strict rulebook. They consider team size, project scope, and deadlines to decide the right level of design.

Result

Code remains maintainable without becoming over-engineered or confusing.

Knowing when to adapt SOLID principles prevents overcomplicating code and keeps development efficient.

Under the Hood

SOLID principles work by guiding how code dependencies and responsibilities are structured. Internally, this means classes have clear boundaries, interfaces define contracts, and inheritance hierarchies maintain consistent behavior. Dependency inversion uses abstraction layers to decouple modules, allowing flexible swapping of implementations without recompiling or changing high-level logic.

Why designed this way?

SOLID was created to solve common problems in software development like fragile code, tight coupling, and difficulty in adding features. Early software often became rigid and hard to maintain. SOLID principles emerged from decades of experience to provide a clear, practical framework that balances flexibility with simplicity.

┌───────────────┐       ┌───────────────┐
│ High-Level    │──────▶│ Abstraction   │
│ Module        │       │ (Interface)   │
└───────────────┘       └───────────────┘
         ▲                      │
         │                      ▼
┌───────────────┐       ┌───────────────┐
│ Low-Level     │◀─────│ Concrete      │
│ Module        │       │ Implementation│
└───────────────┘       └───────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Does applying SOLID mean writing more code always? Commit yes or no.

Common Belief: Applying SOLID always makes code longer and more complex.

Reality: SOLID encourages clear structure, which can add some code but reduces complexity and bugs overall.

Why it matters: Believing SOLID bloats code may discourage developers from using it, leading to messy, hard-to-maintain software.

Quick: Is it okay for subclasses to change parent behavior drastically? Commit yes or no.

Common Belief: Subclasses can change any behavior they want as long as they add features.

Reality: Subclasses must follow parent expectations to avoid breaking code that uses them polymorphically.

Why it matters: Ignoring this causes bugs when code expects certain behavior but gets something different.

Quick: Should interfaces always be large to cover all possible methods? Commit yes or no.

Common Belief: One big interface is better than many small ones for simplicity.

Reality: Many small, focused interfaces reduce unnecessary dependencies and make code easier to implement.

Why it matters: Using large interfaces forces classes to implement unused methods, increasing complexity and errors.

Quick: Does Dependency Inversion mean high-level modules depend on low-level modules? Commit yes or no.

Common Belief: High-level modules should directly depend on low-level modules for control.

Reality: Both should depend on abstractions to reduce coupling and increase flexibility.

Why it matters: Direct dependency makes code rigid and hard to change, increasing risk of bugs.

Expert Zone

1

Applying SOLID in small projects can sometimes add unnecessary complexity; knowing when to simplify is key.

2

Dependency Inversion often requires careful design of interfaces to avoid leaky abstractions that expose implementation details.

3

Liskov Substitution Principle violations are often subtle and cause bugs only in rare edge cases, making them hard to detect without thorough testing.

When NOT to use

SOLID is less useful in very small scripts or prototypes where speed matters more than maintainability. In such cases, simpler procedural code or minimal design is better. Also, overly strict SOLID adherence can lead to over-engineering in small teams or projects with tight deadlines.

Production Patterns

In real systems, SOLID guides modular design where services or components have clear roles. Dependency Injection frameworks implement Dependency Inversion automatically. Interface Segregation helps create microservices with focused APIs. Liskov Substitution ensures safe polymorphism in plugin architectures. Teams use SOLID to keep codebases clean and scalable.

Connections

Modular Design

SOLID principles build the foundation for modular design by defining clear boundaries and responsibilities.

Understanding SOLID helps grasp how to break systems into independent, interchangeable modules.

Lean Manufacturing

Both SOLID and lean manufacturing focus on reducing waste and improving efficiency through clear roles and processes.

Knowing lean principles clarifies why SOLID avoids unnecessary complexity and promotes simplicity.

Human Cognitive Load Theory

SOLID reduces cognitive load by limiting what each code part does, similar to how cognitive load theory advises breaking tasks into manageable chunks.

Recognizing this connection explains why SOLID makes code easier to understand and maintain.

Common Pitfalls

#1 Trying to apply all SOLID principles at once on messy legacy code.

Wrong approach:

Refactor entire codebase to separate all responsibilities, split all interfaces, and invert all dependencies in one go.

Correct approach:

Refactor incrementally, starting with the most problematic areas and applying SOLID principles step-by-step.

Root cause:

Misunderstanding that SOLID is a gradual improvement process, not a one-time rewrite.

#2 Creating too many tiny classes and interfaces that confuse rather than clarify.

Wrong approach:

Split every method into its own interface and class regardless of project size or team needs.

Correct approach:

Balance SOLID with practical simplicity, grouping related methods and classes logically.

Root cause:

Overzealous application of SOLID without considering project context.

#3 Ignoring Liskov Substitution and allowing subclasses to break parent contracts.

Wrong approach:

Subclass overrides a method to throw exceptions or change expected behavior drastically.

Correct approach:

Ensure subclass methods honor parent class contracts and expectations.

Root cause:

Lack of understanding of polymorphism and behavioral contracts.

Key Takeaways

SOLID is a set of five principles that guide writing clean, maintainable, and flexible code.

Each principle focuses on a specific aspect of code design, such as responsibility, extensibility, and dependencies.

Applying SOLID helps prevent common problems like tight coupling, fragile code, and difficult maintenance.

Real-world use of SOLID requires balancing strict rules with practical project needs to avoid over-engineering.

Understanding SOLID deeply improves your ability to design software that lasts and adapts over time.