Class Models

📘 Class Models (OOAD)

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🔹 1. Definition

A Class Model is a static representation of a system that shows:

• Classes
• Attributes (data)
• Methods (functions)
• Relationships between classes

👉 Simple idea: It describes the structure of the system (what it is made of), not how it behaves.

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🔹 2. What is a Class?

A class is a blueprint for creating objects.

Example:

Class: Student
- name
- rollNo
+ study()
+ attendClass()

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🔹 3. Components of a Class Model

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🔸 3.1 Class

Represented as a rectangle divided into 3 parts:

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

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🔸 3.2 Attributes

• Properties of a class
• Represented as variables

Example:

• name
• age

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🔸 3.3 Methods (Operations)

• Functions that define behavior

Example:

• display()
• calculate()

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🔹 4. Relationships in Class Models ⭐

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🔸 4.1 Association

Definition: A general relationship between two classes.

Example:

Teacher —— Student

👉 A teacher teaches students

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🔸 4.2 Multiplicity

Shows how many objects are involved.

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

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🔸 4.3 Aggregation

Definition: A weak “has-a” relationship (part can exist independently).

Example:

Classroom ◇—— Student

👉 Students can exist without classroom

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🔸 4.4 Composition

Definition: A strong “has-a” relationship (part cannot exist independently).

Example:

House ◆—— Room

👉 Room cannot exist without house

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🔸 4.5 Inheritance (Generalization)

Definition: One class inherits from another.

Example:

Vehicle
  ↑
Car

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🔸 4.6 Dependency

Definition: One class depends on another temporarily.

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🔹 5. Types of Class Models

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🔸 5.1 Conceptual Class Model

• High-level
• No implementation details

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🔸 5.2 Design Class Model

• Detailed
• Includes methods and data types

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🔸 5.3 Implementation Class Model

• Close to actual code

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🔹 6. Steps to Create a Class Model

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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: Establish Relationships

• Association, inheritance, etc.

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🔹 Step 5: Add Multiplicity

• Define how many objects relate

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🔹 7. Example (Library System)

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🔹 Classes:

• Book
• Member
• Librarian

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🔹 Class Diagram:

Book
- title
- author
+ issue()

Member
- name
+ borrow()

Relationship:
Member —— borrows —— Book

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🔹 8. Advantages of Class Models

• Clear system structure
• Easy to understand
• Supports code development
• Helps in maintenance

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🔹 9. Important Rules / Guidelines

• Keep classes simple and meaningful
• Avoid too many attributes
• Use proper relationships
• Maintain high cohesion & low coupling

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🔹 10. Likely Exam Questions

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

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🔹 11. Quick Revision Summary 🧠

• Class Model = Structure of system

• Shows:

  • Classes
  • Attributes
  • Methods
  • Relationships

Key Relationships:

• Association
• Aggregation
• Composition
• Inheritance

👉 Trick: "CAAMID" (C → Class, A → Attributes, A → Association, M → Methods, I → Inheritance, D → Dependency)

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