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.

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🎯 Key Elements of Class Diagrams

Element	Description	Notation/Example
Class	Represents a system entity with attributes and methods	Rectangle divided into 3 parts: name, attributes, operations
Attributes	Properties or fields of a class	+ name: String (public attribute)
Operations (Methods)	Functions or behaviors of a class	+ calculateTotal(): double
Associations	Relationships between classes	Line connecting two classes
Multiplicity	How many instances are involved in a relationship	1, 0..*, *, 1..*
Generalization (Inheritance)	"Is-a" relationship between classes	Line with hollow triangle pointing to parent
Aggregation	"Has-a" relationship (whole-part, weak relation)	Line with empty diamond
Composition	Strong "contains" relationship (whole-part, strong relation)	Line with filled diamond
Dependency	One class depends on another	Dotted line with arrow
Interface	Specifies behavior a class must implement	Labeled with <<interface>>

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✅ 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.

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🧭 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.

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📊 Example of a Class Diagram

Scenario: Library Management System

Classes:

• Book

• Member

• Loan

• Librarian

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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

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Relationships Example

Relationship	Example
Association	Member --- borrows ---> Book
Aggregation	Library <>--- contains ---> Book (optional)
Composition	Loan ◆--- includes ---> Book (strong tie)
Inheritance	Staff ▷--- Librarian (Librarian is a Staff)

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🚀 Benefits of Class Diagrams

Benefit	Explanation
Clarifies system structure	Shows classes, attributes, and methods clearly.
Defines relationships	Visualizes how entities interact and relate.
Aids software design	Helps developers and designers plan system architecture.
Enhances communication	Bridges communication between technical and business teams.
Supports object-oriented principles	Models encapsulation, inheritance, and associations.

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🔑 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.

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🎯 Summary of Class Diagrams

Aspect	Details
Purpose	Define and visualize system structure and relationships
Key Elements	Classes, Attributes, Methods, Relationships (Association, Inheritance, Aggregation, Composition)
Use Cases	Software design, System modeling, Architecture planning
Benefits	Clarifies structure, improves communication, supports design and coding

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💡 Class Diagram Example (Visual Summary)

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

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

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📚 Relation to Other UML Diagrams

Diagram Type	Focus
Class Diagram	Static structure of system (classes, relationships)
Sequence Diagram	Dynamic interactions over time
Activity Diagram	Flow of activities and processes
Use Case Diagram	Functional requirements from user perspective