Low-Level Design (LLD)

A hands-on repository for Low-Level Design fundamentals: UML class diagrams, SOLID principles, design patterns, and classic LLD problems. Each topic includes Java and Python implementations where applicable, with READMEs and examples you can run locally.

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What this repo covers

• UML & class diagrams — How to model structure and relationships before coding.
• SOLID principles — Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, Dependency Inversion.
• Design patterns — Creational, structural, and behavioral patterns with code (with/without pattern comparisons where relevant).
• Problems — End-to-end designs (e.g. Parking Lot) that tie together patterns and principles.

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Repository structure

Section	Description
UML Diagrams	Notes and tutorials on UML, with focus on class diagrams.
SOLID Principles	Single Responsibility, Open/Closed, Liskov, Interface Segregation, Dependency Inversion.
Design Patterns	Pattern explanations and code (Java + Python).
Problems	Full LLD problems with Java and Python solutions.

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UML Diagrams

Intro to UML and class diagrams (elements, relationships, visibility, multiplicity).

Topic	Link
Class Diagram	uml-diagrams/class-diagram

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SOLID Principles

Java and Python examples (with and without violations) under SOLID-Principles/java/ and SOLID-Principles/python/.

Java	Python
SOLID-Principles/java	SOLID-Principles/python

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Design Patterns

Java and Python code live under design-patterns/java/<pattern>/ and design-patterns/python/<pattern>/. Each pattern folder has a README and runnable examples.

Pattern	Java	Python
Abstract Factory	design-patterns/java/AbstractFactory-DesignPattern	design-patterns/python/AbstractFactory-DesignPattern
Adapter	design-patterns/java/Adapter-DesignPattern	design-patterns/python/Adapter-DesignPattern
Builder	design-patterns/java/Builder-DesignPattern	design-patterns/python/Builder-DesignPattern
Decorator	design-patterns/java/Decorator-DesignPattern	design-patterns/python/Decorator-DesignPattern
Factory	design-patterns/java/Factory-DesignPattern	design-patterns/python/Factory-DesignPattern
Observer	design-patterns/java/Observer-DesignPattern	design-patterns/python/Observer-DesignPattern
Singleton	design-patterns/java/Singleton-DesignPattern	design-patterns/python/Singleton-DesignPattern
State	design-patterns/java/State-DesignPattern	design-patterns/python/State-DesignPattern
Strategy	design-patterns/java/Strategy-DesignPattern	design-patterns/python/Strategy-DesignPattern

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Problems

Java and Python implementations live under problems/java/<problem>/ and problems/python/<problem>/.

Problem	Java	Python
Parking Lot System	problems/java/ParkingLotSystem	problems/python/ParkingLotSystem

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Running the code

• Java: Open the project in an IDE or use javac/java from the relevant java/ (or java/src/...) directory.
• Python: From the pattern or problem folder (e.g. design-patterns/python/Adapter-DesignPattern), set PYTHONPATH=. so the src package is found, then run the module. Example:

  cd design-patterns/python/Adapter-DesignPattern && PYTHONPATH=. python3 -m src.demo

  For problems (e.g. Parking Lot), run from the problem folder with PYTHONPATH=src:

  cd problems/python/ParkingLotSystem && PYTHONPATH=src python3 src/parking_lot_demo.py

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Contributing

Contributions are welcome!

1. Fork the repository
2. Create a feature branch: git checkout -b feature/your-feature-name
3. Commit your changes: git commit -m 'Add some feature'
4. Push to the branch: git push origin feature/your-feature-name
5. Submit a pull request

Please make sure to update README files and documentation as appropriate.