SOLID Principles

// Example: Single Responsibility Principle
class Report {
  void generate() {}
}

class ReportSaver {
  void save() {}
}

// S - Single Responsibility Principle
class Report {
  public void generateReport() {
    System.out.println("Generating Report");
  }
}

class ReportSaver {
  public void saveReport() {
    System.out.println("Saving Report");
  }
}

// O - Open/Closed Principle
interface Shape {
  void draw();
}

class Circle implements Shape {
  public void draw() {
    System.out.println("Circle Drawn");
  }
}

class Square implements Shape {
  public void draw() {
    System.out.println("Square Drawn");
  }
}

// L - Liskov Substitution Principle
class Bird {
  void fly() {}
}

class Sparrow extends Bird {
  void fly() {
    System.out.println("Sparrow flying");
  }
}

// I - Interface Segregation Principle
interface Printer {
  void print();
}

interface Scanner {
  void scan();
}

class MultiFunctionPrinter implements Printer, Scanner {
  public void print() {
    System.out.println("Printing...");
  }

  public void scan() {
    System.out.println("Scanning...");
  }
}

// D - Dependency Inversion Principle
interface Keyboard {}
class WiredKeyboard implements Keyboard {}

class Computer {
  private Keyboard keyboard;

  public Computer(Keyboard keyboard) {
    this.keyboard = keyboard;
  }
}

• 1 Set of five OOP design principles.
• 2 Helps write maintainable code.
• 3 Improves scalability.
• 4 Used in professional software development.

• 1 S – Single Responsibility Principle
• 2 O – Open/Closed Principle
• 3 L – Liskov Substitution Principle
• 4 I – Interface Segregation Principle
• 5 D – Dependency Inversion Principle

• 1 Reduces code complexity.
• 2 Improves maintainability.
• 3 Enhances code reusability.
• 4 Supports clean architecture.

• 1 Spring Boot applications.
• 2 Enterprise software systems.
• 3 Microservices architecture.
• 4 Large-scale applications.

• 1 Used in Spring Framework architecture.
• 2 Used in enterprise application design.
• 3 Used in microservices systems.
• 4 Used in scalable backend systems.
• 5 SaaS products use SOLID Principles in Java in services, dashboards, background jobs, and API workflows.
• 6 ERP and banking systems apply SOLID Principles in Java with validation, logging, review, and rollback plans.
• 7 E-commerce and healthcare platforms use SOLID Principles in Java carefully because reliability and data correctness matter.

• 1 A Java program first evaluates the surrounding context, then applies the SOLID Principles in Java rules to the current data.
• 2 The important mental model is input, transformation, result, and failure path.
• 3 In production, the same flow usually sits inside a larger layer such as a controller, service, repository, job, or UI component.

• 1 Choose the simplest implementation first, then measure real workloads.
• 2 Watch for repeated work inside loops, unnecessary allocations, and slow I/O in hot paths.
• 3 Prefer clear data structures and stable APIs before micro-optimizing syntax.

• 1 Treat external input as untrusted until it is validated.
• 2 Avoid hardcoded secrets and never print sensitive values in examples or logs.
• 3 Use established libraries for authentication, encryption, parsing, and database access.

• 1 Ignoring SOLID principles in large projects.
• 2 Creating tightly coupled classes.
• 3 Mixing multiple responsibilities.
• 4 Not using interfaces properly.
• 5 Skipping the small working example before adding framework code.
• 6 Ignoring null, empty, duplicate, and boundary inputs.
• 7 Mixing business logic, input handling, and output formatting in one place.
• 8 Using broad error handling that hides the real failure.
• 9 Forgetting to test the behavior after refactoring.
• 10 Adding clever code that future maintainers will struggle to read.

• 1 Follow single responsibility strictly.
• 2 Use interfaces for abstraction.
• 3 Prefer dependency injection.
• 4 Write loosely coupled code.
• 5 Start with clear requirements and one minimal working example.
• 6 Use meaningful names that explain business intent.
• 7 Keep examples small enough to debug line by line.
• 8 Validate input at every trust boundary.
• 9 Handle errors explicitly and preserve useful context.
• 10 Prefer simple control flow over deeply nested logic.
• 11 Separate domain logic from I/O and framework code.
• 12 Write tests for normal, boundary, and failure cases.
• 13 Review security assumptions before production use.
• 14 Measure performance before optimizing.
• 15 Document non-obvious decisions close to the code or in project notes.
• 16 Use official documentation when behavior is version-specific.
• 17 Keep dependencies current and remove unused code.
• 18 Avoid hardcoded secrets, credentials, and environment-specific paths.
• 19 Log operational events without exposing sensitive data.
• 20 Design examples so learners can safely modify and rerun them.

• 1 Beginner: rewrite the example with different names and values.
• 2 Intermediate: add validation and handle one expected failure case.
• 3 Advanced: place SOLID Principles in Java inside a small service-style design with tests.

• 1 Build a small Java console feature that demonstrates SOLID Principles in Java.
• 2 Accept input, process it with the concept, print a clear result, and handle invalid input.
• 3 Add a README note explaining the design choice and two edge cases you tested.

• 1 If the program does not compile, check spelling, imports, braces, and file/class names first.
• 2 If output is unexpected, print intermediate values and verify each branch of the logic.
• 3 If the design feels complex, reduce it to the smallest working example and add pieces back one at a time.

• 1 Practice SOLID Principles in Java with a second example from a business domain such as inventory, payroll, banking, or e-commerce.
• 2 Review related Java topics that cover data flow, error handling, testing, and clean design.
• 3 Compare your solution with official documentation and simplify anything you cannot explain clearly.