🚗 Parking Lot Management System

A robust, console-based Low-Level Design (LLD) application built in pure Java. This project simulates a fully functional parking lot system that handles vehicle entry, exit, and real-time status queries using an interactive command-line interface.

This project was intentionally engineered to move away from simple procedural scripts to enterprise-grade, object-oriented architecture.

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🧠 Architectural Principles & Rules Applied

SOLID Principles

• Single Responsibility Principle (SRP): * Where it's used: Every class has one specific job. The ParkingLotRepository solely manages data storage (the HashMap of slots). The Command classes only handle parsing terminal inputs. The ParkingService handles the core business logic.
• Open/Closed Principle (OCP): * Where it's used: The system is open for extension but closed for modification. We can add a new parking algorithm (like FarthestSlotStrategy) by creating a new class that implements SlotStrategy, without having to touch or rewrite the ParkingService engine.
• Dependency Inversion Principle (DIP):
  • Where it's used: High-level modules don't depend on low-level modules; both depend on abstractions. ParkingService depends on the SlotStrategy interface, not a concrete class. Similarly, Main.java treats all commands as the Command interface.

General Programming Rules

• DRY Principle (Don't Repeat Yourself): * Where it's used: Inside ParkingService.java. The getOccupiedSlots() method was created once and then reused inside getRegistrationNumbersByColor(), getSlotNumbersByColor(), and getSlotNumberByRegistrationNumber() to prevent rewriting the same loop and empty-check logic.
• Dependency Injection (DI): * Where it's used: Inside Main.java and the Command constructors. A single, central instance of ParkingService is created in Main and injected into the constructor of every single command (new ParkCommand(parkingService)). This ensures all commands share the exact same state and memory.

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📐 Design Patterns Implemented

• Command Pattern 🎮: * Where it's used: The com.Parkinglot.Commands package. We decoupled the terminal inputs from the execution logic by encapsulating every user action into discrete classes (ParkCommand, LeaveCommand, StatusCommand, etc.) that all implement a single Command interface.
• Strategy Pattern 🗺️: * Where it's used: The com.Parkinglot.Strategies package. The slot allocation logic is abstracted behind the SlotStrategy interface. The system uses NearestSlotStrategy (powered by a PriorityQueue for O(log n) retrieval) but can instantly swap to RandomSlotStrategy dynamically.
• Facade Pattern 🏛️: * Where it's used: ParkingService.java. This class acts as a unified front-facing interface. It hides the complex internal interactions between the data storage (ParkingLotRepository) and the algorithms (SlotStrategy) from the terminal commands.
• Registry / Dispatcher Pattern 🗂️:
  • Where it's used: Main.java. We avoided a massive, ugly if-else chain by using a HashMap<String, Command> as a registry. The program simply looks up the user's string input (e.g., "park") in the map and triggers the .execute() method of the corresponding Command object.

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💻 Commands & Examples

When the application is running, it acts as an interactive terminal. Below is the complete reference for all supported commands:

Category	Action	Command Syntax	Example Input	Expected Output
Setup	Create Lot	create_parking_lot <capacity>	create_parking_lot 6	Created a parking lot with 6 slots.
Core	Park Vehicle	park <reg_no> <color>	park KA-01-HH-1234 White	Allocated slot number: 1
Core	Leave Slot	leave <slot_number>	leave 4	Slot number 4 is free
Query	Lot Status	status	status	(Prints a formatted table of all occupied slots)
Query	Find Cars by Color	registration_numbers_for_cars_with_colour <color>	registration_numbers_for_cars_with_colour White	KA-01-HH-1234, KA-01-HH-9999
Query	Find Slots by Color	slot_numbers_for_cars_with_colour <color>	slot_numbers_for_cars_with_colour White	1, 2
Query	Find Slot by Reg No.	slot_number_for_registration_number <reg_no>	slot_number_for_registration_number MH-04-AY-1111	6 (or Not found)
System	Shutdown	exit	exit	(Safely terminates the application)

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🚀 How to Run

1. Clone this repository to your local machine.
2. Ensure you have Java installed.
3. Compile the source code from the root directory.
4. Run the Main.java file to start the interactive command-line interface.