Java Developer Guide

📚 Overview

This repository is a comprehensive guide for Java Developers, covering a wide range of topics from Data Structures and Algorithms to backend technologies and cloud infrastructure. Currently featuring DSA problems organized from beginner to advanced levels, with plans to expand into Java frameworks, databases, microservices, and DevOps tools. Each module focuses on specific topics with detailed implementations, multiple approaches, complexity analysis, and test cases.

🎯 Repository Goals

• Provide clean, well-documented solutions to DSA problems and Java development concepts
• Cover multiple approaches (naive, optimized, alternative methods)
• Include time and space complexity analysis for algorithms
• Serve as a comprehensive learning resource for Java developers
• Cover modern Java technologies: Spring, Spring Boot, Microservices, JPA, Hibernate
• Include database concepts: SQL, NoSQL
• Provide DevOps and cloud infrastructure guidance: Docker, Kubernetes, Cloud platforms
• Support learning through progressive difficulty levels
• Maintain modular organization for easy navigation

📂 Project Structure

Java-Developer-Guide/
├── README.md                    # Main project documentation
├── Basics/                      # Fundamental programming concepts
│   ├── Basics-README.md         # Detailed module documentation
│   └── src/
│       ├── problems/
│       │   ├── numbers/         # Number manipulation (20 problems)
│       │   ├── strings/         # String manipulation (20 problems)
│       │   ├── operations/      # Mathematical operations
│       │   └── recursion/       # Recursion-based problems
│       └── samples/
├── Arrays/                      # Array-based problems
│   ├── Array-README.md          # Detailed module documentation
│   └── src/
│       ├── problems/            # Core array problems
│       │   └── level2/          # Advanced array problems
│       ├── samples/
│       └── sorting/             # Sorting algorithms
├── LeetCodeProblems/            # LeetCode problem collections
│   ├── LeetCodeProblems-README.md  # Detailed module documentation
│   └── src/
│       ├── problems/            # 94 LeetCode sorting problems
│       ├── patterns/            # Reusable patterns
│       ├── blind75/             # Blind 75 curated problems
│       │   ├── easy/            # Easy difficulty
│       │   ├── medium/          # Medium difficulty
│       │   └── hard/            # Hard difficulty
│       └── samples/             # Helper utilities
├── SQL/                         # SQL Module
│   ├── SQL50-README.md          # Detailed module documentation
│   └── src/
│       ├── problems/            # SQL solutions by difficulty
│       │   ├── easy/            # Easy SQL problems
│       │   ├── medium/          # Medium SQL problems
│       │   └── hard/            # Hard SQL problems
│       ├── schemas/             # Database schemas & test data
│       └── docs/                # Problem documentation & concepts
├── JavaCore/                    # Core Java Concepts ⭐ NEW
│   ├── JavaCore-README.md       # Detailed module documentation
│   ├── concepts/                # Theoretical explanations
│   └── src/
│       ├── oop/                 # OOP fundamentals
│       │   ├── encapsulation/   # Data hiding & access control
│       │   ├── inheritance/     # Code reusability & hierarchy
│       │   ├── polymorphism/    # Runtime & compile-time
│       │   └── abstraction/     # Abstract classes & interfaces
│       ├── solid/               # SOLID principles
│       ├── design-patterns/     # Gang of Four patterns
│       │   ├── creational/      # Singleton, Factory, Builder
│       │   ├── structural/      # Adapter, Decorator, Proxy
│       │   └── behavioral/      # Strategy, Observer, Template
│       ├── collections/         # Collections Framework
│       ├── streams/             # Java 8+ Streams API
│       ├── multithreading/      # Concurrency & Threading
│       ├── generics/            # Generic programming
│       └── exceptions/          # Exception handling
└── [Future Modules]
    ├── LinkedLists/
    ├── Stacks/
    ├── Queues/
    ├── Trees/
    ├── Graphs/
    ├── DynamicProgramming/
    ├── SpringBoot/             # Spring Boot concepts & examples
    ├── Microservices/          # Microservices architecture
    ├── Docker/                 # Containerization
    ├── Kubernetes/             # Container orchestration
    └── Cloud/                  # Cloud platforms (AWS, Azure, GCP)

📦 Modules

1. Basics Module

Status: ✅ Active - 20 Problems

Foundational programming problems covering fundamental concepts essential for any programmer.

Topics Covered:

• Numbers (9 problems): Reverse, Palindrome, Armstrong, Prime Check, Factorial, Fibonacci, GCD/LCM, Power Check, Sum of Digits
• Strings (11 problems): Reverse, Palindrome, Character Frequency, Vowel/Consonant Count, Anagram Check, Remove Duplicates, String Compression, Reverse Words, Digit Validation, String Rotation
• Recursion: Factorial, Fibonacci sequences
• Mathematical Operations: GCD, LCM, Power of 2/3

Key Features:

• Multiple implementation approaches for each problem
• Includes both iterative and recursive solutions
• Optimized algorithms with complexity analysis

📖 View Basics Module Documentation

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2. Arrays Module

Status: ✅ Active - 11 Problems

Comprehensive coverage of array manipulation, searching, and algorithmic problems.

Topics Covered:

• Basic Operations: Frequency count, Min/Max elements, Array reversal
• Rearrangement: Move negatives to end, Rotate array
• Searching: Peak element, Subarray with given sum, Missing number
• Advanced: Largest sum contiguous subarray (Kadane's Algorithm)
• Sorting Algorithms: Bubble Sort, Selection Sort

Key Features:

• Progressive difficulty levels (basic → level2)
• Multiple sorting algorithm implementations
• Efficient space and time optimized solutions

📖 View Arrays Module Documentation

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3. LeetCodeProblems Module

Status: ✅ Active - 94 LeetCode Problems + Blind 75

Comprehensive collection of LeetCode problems including sorting problems and curated Blind 75 interview problems, organized by difficulty.

Topics Covered:

• LeetCode Problems: All 94 easy-level sorting problems from LeetCode
• Blind 75: Curated collection organized by difficulty (easy/medium/hard)
  • 5 Easy problems (Two Sum, Linked List Cycle, Reverse Linked List, Merge Two Sorted Lists, Missing Number)
  • 9 Medium problems (3Sum, Reorder List, Word Break, and more)
  • 1 Hard problem (Merge k Sorted Lists)
  • Professional package structure for progressive learning
• Pattern Library: Reusable sorting patterns and techniques extracted from solutions
• Problem-Solving Strategies: Two-pointer, frequency counting, greedy, custom comparators, divide and conquer
• Helper Utilities: LinkedListUtils, testing and debugging tools

Key Features:

• Sequential problem solving (Problems 1-94)
• Blind 75 problems organized by difficulty with crisp algorithms
• Pattern documentation for reusable techniques
• Multiple approaches with complexity analysis
• Dedicated patterns package for high-level problem-solving strategies

📖 View LeetCodeProblems Module Documentation
🎯 View Blind 75 Problems

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4. SQL Module

Status: 🚀 Active - LeetCode SQL 50

Master SQL through LeetCode's curated SQL 50 problems, covering all essential database concepts from basic queries to advanced window functions.

Topics Covered:

• Basic Queries (1-5): SELECT, WHERE, AND/OR, DISTINCT, String functions
• JOINs (6-14): INNER JOIN, LEFT JOIN, SELF JOIN, CROSS JOIN
• Aggregation (15-20): COUNT, SUM, AVG, GROUP BY, HAVING, Date functions
• Subqueries (21-30): Scalar, correlated subqueries, IN, EXISTS
• Window Functions (31-40): ROW_NUMBER, RANK, DENSE_RANK, LAG, LEAD, partitioning
• Advanced Topics (41-50): CTEs, UNION, CASE, REGEX, DELETE, UPDATE, complex queries

Key Features:

• Complete LeetCode SQL 50 problem collection organized by difficulty
• Comprehensive documentation with detailed concept explanations
• Database schemas with test data for each problem
• Detailed documentation with multiple solution approaches
• Step-by-step query explanations and performance analysis
• MySQL 8.0 syntax with PostgreSQL compatibility notes

Module Structure:

• Problems: 50 total (20 Easy, 20 Medium, 10 Hard)
• Schemas: DDL and test data for all problems
• Documentation: Detailed README for each problem with concept explanations

📖 View SQL Module Documentation

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5. JavaCore Module ⭐ NEW

Status: 🚀 Active - OOP Fundamentals in Progress

Comprehensive coverage of Core Java concepts essential for technical interviews at top-tier companies. Focus on Object-Oriented Programming, SOLID principles, Design Patterns, Collections, Multithreading, and Java 8+ features.

Topics Covered:

• OOP Fundamentals:
  • ✅ Encapsulation (Completed - 3 examples: PasswordManager, UserProfile, BankAccount)
  • 🔜 Inheritance
  • 🔜 Polymorphism
  • 🔜 Abstraction
• SOLID Principles: Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, Dependency Inversion
• Design Patterns:
  • Creational (Singleton, Factory, Builder, Prototype)
  • Structural (Adapter, Decorator, Proxy, Facade)
  • Behavioral (Strategy, Observer, Template, Command)
• Collections Framework: List, Set, Map implementations, HashMap internals, ConcurrentHashMap
• Java 8+ Features: Lambda expressions, Streams API, Functional interfaces, Method references
• Multithreading: Thread creation, Synchronization, ExecutorService, Concurrent utilities
• Generics: Generic classes, Bounded types, Wildcards
• Exception Handling: Try-catch, Custom exceptions, Best practices

Key Features:

• Structured learning path for freshers to senior developers
• Real-world examples with practical applications
• Interview-focused preparation with Q&A sections
• Progressive difficulty levels
• Clean, well-documented code examples
• Defensive programming and best practices

Implementation Roadmap:

• Phase 1 (Current): OOP Fundamentals
  • ✅ Encapsulation: Data hiding, validation, defensive copying
  • 🔜 Inheritance: Class hierarchies, method overriding
  • 🔜 Polymorphism: Method overloading, dynamic dispatch
  • 🔜 Abstraction: Abstract classes, interfaces
• Phase 2: SOLID Principles
• Phase 3: Design Patterns
• Phase 4: Collections Framework
• Phase 5: Multithreading & Concurrency
• Phase 6: Java 8+ Features (Streams, Lambda)

Completed Components:

• Encapsulation: 3 practical examples with full documentation
  • PasswordManager: Secure password validation and management
  • UserProfile: User management with roles, preferences, timestamps
  • BankAccount: Banking operations with balance protection
  • Complete README with interview questions and best practices

Module Structure:

• Concepts: Theoretical explanations and documentation
• Code Examples: Practical implementations for each topic
• Interview Questions: Curated Q&A for interview preparation
• Best Practices: Industry-standard coding patterns

📖 View JavaCore Module Documentation

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6. Future Modules

Status: 🔜 Planned

Coming Soon - DSA Modules:

• Linked Lists: Singly, Doubly, Circular lists, reversal, cycle detection
• Stacks & Queues: Implementations, balanced parentheses, next greater element
• Trees: Binary trees, BST, traversals, height, diameter
• Graphs: BFS, DFS, shortest path, cycle detection
• Dynamic Programming: Knapsack, LCS, Matrix chain multiplication
• Hashing: Hash maps, collision handling, frequency problems
• Searching & Sorting: Binary search variants, Quick sort, Merge sort
• Greedy Algorithms: Activity selection, Huffman coding
• Backtracking: N-Queens, Sudoku solver, Permutations

Planned - Java & Backend Technologies:

• Core Java: OOP, Collections, Generics, Streams, Concurrency
• Spring Framework: IoC, DI, AOP, Spring MVC
• Spring Boot: Auto-configuration, REST APIs, Actuator, Profiles
• JPA & Hibernate: Entity mapping, Relationships, Queries, Caching
• Microservices: Service design, Communication patterns, API Gateway
• Spring Security: Authentication, Authorization, JWT, OAuth2
• Databases: SQL (MySQL, PostgreSQL), NoSQL (MongoDB, Redis)
• Docker: Containerization, Docker Compose, Multi-stage builds
• Kubernetes: Pods, Services, Deployments, ConfigMaps, Secrets
• Cloud Platforms: AWS, Azure, GCP essentials

🚀 Getting Started

Prerequisites

• Java Development Kit (JDK): Version 8 or higher
• IDE: IntelliJ IDEA (recommended), Eclipse, or any Java IDE
• Build Tool: Maven or Gradle (optional)

Running the Code

Option 1: Using IntelliJ IDEA

1. Clone the repository:

   git clone https://github.com/yourusername/Java-Developer-Guide.git

2. Open the project in IntelliJ IDEA
3. Navigate to any problem file (e.g., Basics/src/problems/numbers/ReverseNumber.java)
4. Right-click on the file → Run 'ClassName.main()'
5. View output in the console

Option 2: Command Line

# Navigate to module directory
cd Java-Developer-Guide/Basics

# Compile a Java file
javac -d out src/problems/numbers/ReverseNumber.java

# Run the compiled class
java -cp out problems.numbers.ReverseNumber

📝 Code Structure

Each problem implementation follows a consistent pattern:

/**
 * Problem Title
 * Description and explanation
 */
public class ProblemName {
    
    /**
     * Method description
     * Time Complexity: O(...)
     * Space Complexity: O(...)
     */
    public static ReturnType methodName(Parameters params) {
        // Implementation
    }
    
    // Multiple approaches when applicable
    
    public static void main(String[] args) {
        // Test cases with formatted output
    }
}

📊 Progress Tracking

Module	Problems Solved	Status	Difficulty Range
Basics	20	✅ Active	Beginner
Arrays	11	✅ Active	Beginner → Medium
LeetCodeProblems	0 / 94	✅ Active	Easy
Blind 75	13 / 75	✅ Active	Easy → Hard
SQL	5 / 50	✅ Active	Easy → Hard
JavaCore	0 / 60+	✅ Active	Beginner → Advanced
Linked Lists	0	🔜 Planned	Beginner → Advanced
Stacks & Queues	0	🔜 Planned	Beginner → Medium
Trees	0	🔜 Planned	Medium → Advanced
Graphs	0	🔜 Planned	Medium → Advanced
Dynamic Programming	0	🔜 Planned	Medium → Advanced

Total Problems Solved: 49
LeetCode Problems: 0 / 94
Blind 75 Problems: 13 / 75 ⭐
SQL Problems: 5 / 50 (10%) ⭐
JavaCore Topics: Structure Ready ⭐ NEW

🎓 Learning Path

For Beginners:

1. Start with Basics Module - Numbers section
2. Move to Basics Module - Strings section
3. Begin JavaCore Module - OOP Fundamentals
4. Practice Arrays Module - Basic problems
5. Continue with Arrays Module - Sorting algorithms
6. Practice SQL Module - Easy problems (1-5)
7. Attempt LeetCodeProblems Module - Sequential problems

For Intermediate:

1. Complete Basics Module level1 string problems
2. Master JavaCore Module - SOLID principles and Collections
3. Work through Arrays Module level2 problems
4. Practice SQL Module - Medium problems with JOINs
5. Tackle LeetCodeProblems Module - Blind 75 problems
6. Study JavaCore Module - Design Patterns
7. Move to Linked Lists and Stacks/Queues (when available)

For Advanced:

1. Complete JavaCore Module - Multithreading and advanced topics
2. Master SQL Module - Window functions and complex queries
3. Focus on Trees and Graphs modules (when available)
4. Master Dynamic Programming patterns
5. Solve Backtracking and Greedy problems
6. Apply Design Patterns from JavaCore in real projects

🤝 Contributing

Contributions are welcome! To add new problems:

1. Fork the repository
2. Create a new branch: git checkout -b feature/new-problem
3. Add your problem file in the appropriate module
4. Follow the existing code structure and documentation style
5. Include:
   • Problem description and examples
   • Multiple approaches (when applicable)
   • Time and space complexity analysis
   • Test cases in main method
6. Update the module's README.md
7. Commit your changes: git commit -m 'Add: Problem Name'
8. Push to the branch: git push origin feature/new-problem
9. Submit a pull request

🔒 Branch Protection

The main branch is protected with 4 layers of security:

1. GitHub Branch Protection Rules - Prevents deletion, direct pushes, and force pushes
2. CODEOWNERS File - Requires @durgesh137 approval for all changes
3. Automated Workflow - Validates code compilation and file integrity
4. Pull Request Template - Ensures quality standards are met

What's Protected:

• ✅ Main branch cannot be deleted by anyone
• ✅ All changes must go through Pull Requests
• ✅ Code owner (@durgesh137) approval required before merging
• ✅ Automated checks must pass (compilation, file validation)
• ❌ Direct pushes, force pushes, and deletions are blocked

Your Control:

• Only you can approve Pull Requests
• Only you can merge approved PRs
• You control all changes to the main branch

📖 How Branch Protection Works (Detailed)
⚡ Quick Setup Guide
📚 Full Documentation

📖 Resources

• GeeksforGeeks - DSA
• LeetCode
• HackerRank
• Introduction to Algorithms (CLRS)

📄 License

This project is open source and available under the MIT License.

👤 Author

Durgesh Kumar

• GitHub: @durgesh137
• LinkedIn: Durgesh Kumar

⭐ Show Your Support

Give a ⭐️ if this project helped you learn DSA!

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Last Updated: December 18, 2025
Version: 1.2.0