Class Diagrams

📘 Class Diagrams (OOAD – UML)

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🔹 1. Definition

A Class Diagram is a static UML diagram that shows the structure of a system by representing:

• Classes
• Attributes
• Methods (operations)
• Relationships between classes

👉 Simple idea: It tells us what the system is made of and how parts are connected, not how they behave over time.

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🔹 2. Purpose of Class Diagrams

• Describe system structure
• Show relationships between objects
• Help in database and software design
• Act as a blueprint for coding
• Improve system understanding

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🔹 3. Basic Structure of a Class ⭐

A class is shown as a 3-part box:

----------------------
| Class Name         |
----------------------
| Attributes         |
----------------------
| Methods            |
----------------------

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🔹 4. Components of Class Diagram

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🔸 4.1 Class Name

• Top section of the box
• Represents entity (e.g., Student, Book)

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🔸 4.2 Attributes

• Data members (variables)
• Represent properties

Example:

• name
• age
• id

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🔸 4.3 Methods (Operations)

• Functions or behaviors

Example:

• login()
• calculateFee()

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🔹 5. Relationships in Class Diagrams ⭐

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🔸 5.1 Association

A general relationship between two classes.

Example:

Teacher —— Student

👉 A teacher teaches students

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🔸 5.2 Multiplicity

Defines how many objects participate.

Symbol	Meaning
1	One
0..1	Zero or one
*	Many
1..*	One or many

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🔸 5.3 Aggregation (Weak Has-A)

• Whole-part relationship
• Part can exist independently

Example:

Department ◇—— Teacher

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🔸 5.4 Composition (Strong Has-A)

• Strong ownership
• Part cannot exist without whole

Example:

House ◆—— Room

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🔸 5.5 Inheritance (Generalization)

• One class inherits another

Example:

Animal
  ↑
Dog

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🔸 5.6 Dependency

• Temporary relationship
• One class uses another

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🔹 6. Example (Library System)

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🔹 Classes:

• Book
• Member
• Librarian

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🔹 Class Diagram Representation:

Book
- title
- author
+ issue()

Member
- name
- id
+ borrowBook()

Relationship:
Member —— borrows —— Book

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🔹 7. Types of Class Diagrams

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🔸 7.1 Conceptual Class Diagram

• High-level view
• Used in analysis phase

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🔸 7.2 Specification Class Diagram

• More detailed
• Used in design phase

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🔸 7.3 Implementation Class Diagram

• Close to actual code
• Used in coding phase

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🔹 8. Steps to Create Class Diagram

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🔹 Step 1: Identify Classes

• Find nouns in problem statement

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🔹 Step 2: Define Attributes

• Identify properties

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🔹 Step 3: Define Methods

• Identify actions

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🔹 Step 4: Define Relationships

• Association, inheritance, etc.

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🔹 Step 5: Add Multiplicity

• Define quantity relationships

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🔹 9. Advantages of Class Diagrams

• Clear system structure
• Easy to convert into code
• Improves communication
• Helps database design
• Supports reuse

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🔹 10. Important Rules / Guidelines

• Keep classes simple and meaningful
• Avoid too many attributes/methods
• Maintain high cohesion & low coupling
• Use proper relationships
• Follow UML notation

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🔹 11. Likely Exam Questions

1. Define class diagram.
2. Explain components of a class diagram.
3. What is association?
4. Difference between aggregation and composition.
5. Explain inheritance in class diagrams.
6. What is multiplicity?
7. Draw class diagram for library system.
8. Types of class diagrams.
9. Steps to create class diagram.
10. Advantages of class diagrams.

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🔹 12. Quick Revision Summary 🧠

• Class Diagram = Structure of system

• Shows:

  • Classes
  • Attributes
  • Methods
  • Relationships

Key Relationships:

• Association
• Aggregation
• Composition
• Inheritance

👉 Trick to Remember: "CARM-I" (C → Class, A → Attributes, R → Relationships, M → Methods, I → Inheritance)

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