Create README.md

Author: aayush-raj2

Advanced_OOP_Concepts/03_UML_Diagrams/README.md

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+# 📊 UML Diagrams in Java — Advanced OOP
+
+### 👨‍💻 Author: **Aayush Raj**
+
+This section explains **UML (Unified Modeling Language)** — the visual foundation of software design.  
+You’ll learn how to represent class relationships, runtime objects, and method interactions using **Class, Object, and Sequence diagrams** — demonstrated with an **Online Shopping System** example.
+
+---
+
+## 🧠 Objectives
+- Visualize system architecture before coding.  
+- Understand class relationships (association, composition).  
+- Model runtime instances and method call sequences.  
+
+---
+
+## 📘 Topics Covered
+
+| File | Concept | Description |
+|------|----------|-------------|
+| `ClassDiagramDemo.java` | Class Diagram | Static structure of system classes |
+| `ObjectDiagramDemo.java` | Object Diagram | Runtime snapshot of object instances |
+| `SequenceDiagramDemo.java` | Sequence Diagram | Step-by-step flow of messages between objects |
+| `UML_Basics.md` | Notes | Theory, comparisons, and key points |
+
+---
+
+## 🧩 Real-World Example
+🛍️ **Online Shopping System** — Customers browse products, add them to the cart, and place orders.  
+Used across all three UML representations to connect design to code.
+
+---
+
+## 🧠 Key Takeaways
+✅ UML bridges design and implementation.  
+✅ Improves communication between developers and stakeholders.  
+✅ Enhances maintainability and scalability of systems.
+
+---
+
+## 🏁 Run Instructions
+```bash
+javac ClassDiagramDemo.java
+java ClassDiagramDemo
+
+javac ObjectDiagramDemo.java
+java ObjectDiagramDemo
+
+javac SequenceDiagramDemo.java
+java SequenceDiagramDemo