Q 26 - SOLID

[HARSHITH-MV]

Explain SOLID principles ?

[AmjustGettingStarted]

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🧱 SOLID Principles in Java

SOLID is an acronym for five design principles that help developers write clean, scalable, and maintainable object-oriented code.

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1️⃣ S — Single Responsibility Principle (SRP)

A class should have only one reason to change.

• Meaning: Each class should do one thing and do it well.
• Example:

  class InvoicePrinter {
      void print(Invoice invoice) { /* printing logic */ }
  }
  
  class InvoiceCalculator {
      double calculateTotal(Invoice invoice) { /* calculation logic */ }
  }

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2️⃣ O — Open/Closed Principle (OCP)

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

• Meaning: You should be able to add new functionality without changing existing code.
• Example:

  interface Shape {
      double area();
  }
  
  class Circle implements Shape {
      public double area() { return Math.PI * r * r; }
  }
  
  class Rectangle implements Shape {
      public double area() { return width * height; }
  }

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3️⃣ L — Liskov Substitution Principle (LSP)

Subtypes must be substitutable for their base types.

• Meaning: Objects of a superclass should be replaceable with objects of a subclass without breaking the program.
• Example:

  class Bird {
      void fly() { /* flying logic */ }
  }
  
  class Ostrich extends Bird {
      // Ostrich can't fly — violates LSP
  }

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4️⃣ I — Interface Segregation Principle (ISP)

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

• Meaning: Split large interfaces into smaller, more specific ones.
• Example:

  interface Printer {
      void print();
  }
  
  interface Scanner {
      void scan();
  }
  
  class MultiFunctionPrinter implements Printer, Scanner { /* ... */ }

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5️⃣ D — Dependency Inversion Principle (DIP)

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

• Meaning: Use interfaces or abstract classes to decouple components.
• Example:

  interface Database {
      void save(String data);
  }
  
  class MySQLDatabase implements Database {
      public void save(String data) { /* save to MySQL */ }
  }
  
  class App {
      Database db;
      App(Database db) { this.db = db; }
  }

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