Class Diagrams

📌 What is a Class Diagram?

A Class Diagram is a static structure diagram in UML (Unified Modeling Language) that represents the structure of a system by showing its classes, attributes, operations (methods), and relationships between classes.

It is primarily used for modeling object-oriented systems and describing the system's structure at a high level.

Purpose:

• Visualize system architecture and design.

• Define classes, their properties, and behaviors.

• Show relationships (association, inheritance, aggregation, composition) between classes.

• Provide a blueprint for coding and system development.

🎯 Key Elements of Class Diagrams

✅ When to Use Class Diagrams

• During system design and analysis.

• To define software architecture.

• For object-oriented modeling.

• To document system structure.

• To communicate system design with developers, stakeholders.

🧭 How to Create a Class Diagram (Step-by-Step)

1. Identify system entities (classes) relevant to your domain.

2. Define attributes and operations for each class.

3. Determine relationships between classes:

   • Association, Aggregation, Composition, Inheritance.

4. Specify multiplicities if necessary (e.g., one-to-many).

5. Draw classes as rectangles divided into three sections.

6. Connect classes with appropriate relationship lines.

7. Review for consistency, completeness, and clarity.

📊 Example of a Class Diagram

Scenario: Library Management System

Classes:

• Book

• Member

• Loan

• Librarian

Representation (Text version)

sqlCopyEdit+----------------------+
|        Book          |
+----------------------+
| - title: String      |
| - author: String     |
| - ISBN: String       |
| - available: Boolean |
+----------------------+
| + borrow(): Boolean  |
| + return(): Boolean  |
+----------------------+

+----------------------+
|       Member         |
+----------------------+
| - name: String       |
| - memberID: int      |
| - address: String    |
+----------------------+
| + register(): void   |
| + borrowBook(): void |
| + returnBook(): void |
+----------------------+

+----------------------+
|        Loan          |
+----------------------+
| - loanDate: Date     |
| - returnDate: Date   |
+----------------------+
| + issueLoan(): void  |
| + closeLoan(): void  |
+----------------------+

+----------------------+
|     Librarian        |
+----------------------+
| - employeeID: int    |
| - name: String       |
+----------------------+
| + manageLoan(): void |
| + addBook(): void    |
+----------------------+

Relationships:
- Member "borrows" Book (Association)
- Librarian "manages" Loan (Association)
- Loan "relates" Book and Member

Relationships Example

🚀 Benefits of Class Diagrams

🔑 Best Practices for Class Diagrams

• Use meaningful class names that reflect real-world entities.

• Keep attributes and methods relevant and concise.

• Indicate visibility:

  • + Public

  • - Private

  • # Protected

• Avoid unnecessary complexity — focus on core components.

• Clarify multiplicities (e.g., 1, 0..*) for precision.

• Organize related classes logically in the diagram.

• Use inheritance and composition appropriately to reflect real relationships.

🎯 Summary of Class Diagrams

💡 Class Diagram Example (Visual Summary)

pgsqlCopyEdit+---------------+         +------------+
|   Member      |<>------>|   Loan     |
+---------------+         +------------+
| +name         |         | +loanDate  |
| +memberID     |         | +returnDate|
+---------------+         +------------+
        |
        |
        v
+---------------+
|    Book       |
+---------------+
| +title        |
| +author       |
| +ISBN         |
+---------------+

+---------------+
| Librarian     |
+---------------+
| +employeeID   |
| +name         |
+---------------+

📚 Relation to Other UML Diagrams