Software Design Patterns

Master the 15 essential design patterns every software engineer needs for interviews and production code. SOLID Principles — The Foundation of Clean Object-Oriented Design What Problem Does SOLID Solve? Without guiding principles, code becomes rigid (hard to change), fragile (changes break unrelated things), and immobile (impossible to reuse). SOLID gives you five rules that produce testable, extensible, maintainable code. --S — Single Responsibility Principle (SRP) A class should have only ONE reason to change. Every class owns exactly one job. If you can describe a class with "and" — it violates SRP. O — Open/Closed Principle (OCP) Open for extension, closed for modification. Add new behavior by writing new code (new classes), not by editing existing classes. L — Liskov Substitution Principle (LSP) Subtypes must be substitutable for their base types without altering correctness. If has , then breaks LSP because penguins cannot fly. Fix: split into and . I — Interface Segregation Principle (ISP) No client should be forced to depend on methods it does not use. Split fat interfaces into focused ones. A interface should not force a simple printer to implement and . D — Dependency Inversion Principle (DIP) High-level modules should not depend on low-level modules. Both should depend on abstractions. Don't instantiate dependencies directly — accept them via constructor injection. --UML Class Diagram (Mermaid) --Java Implementation (Compilable) --Python Implementation (Runnable) --When to Use Always. SOLID should guide every class you write. SRP and DIP are the most impactful in daily work. OCP is critical when designing libraries or frameworks. When NOT to Use Don't over-apply to trivial scripts or one-off utilities. YAGNI still applies: don't create 5 interfaces for code that will never change. --Real-World Examples Real-World Example Separate UserService, EmailService, LoggingService instead of one God class OCP Java Collections — ArrayList and LinkedList both implement List correctly ISP Spring Framework's dependency injection container | --Interview Questions Explain SRP with an example. A UserService that handles CRUD AND sends emails violates SRP. Split into UserService EmailService. What's the difference between SRP and ISP? SRP is about classes having one reason to change. ISP is about interfaces — don't force implementors to provide methods they don't need. How does DIP enable testing? By depending on abstractions (interfaces), you can inject mock/stub implementations in tests. Give an example of LSP violation. Square extending Rectangle breaks LSP — setting width on Square must also set height, breaking Rectangle's contract. How do OCP and Strategy pattern relate? Strategy implements OCP — add new algorithms by creating new strategy classes, never modifying the context.