Class Diagram (UML) – Classes, Attributes, Methods, Inheritance, Association, Aggregation & Composition

This article provides a conceptual overview of class diagrams, including exam questions and key terms.

In a Nutshell

Class diagrams represent the structure of a software system through classes, their attributes, methods, and the relationships between them—a cornerstone of object-oriented modeling with UML.

Compact Technical Description

A class diagram visualizes the static structure of an object-oriented system. It shows classes with their attributes and methods, along with relationships such as association, inheritance, aggregation, and composition. Each class appears as a rectangle divided into three sections: name, attributes, and methods. Visibility modifiers (+ public, - private, # protected) and data types are explicitly marked. UML class diagrams are crucial during the design phase because they reflect the eventual object-oriented architecture and can expose potential issues early.

Exam-Relevant Key Points

• Core UML construct for modeling system structure
• Contains classes with attributes and methods
• Represents relationships: inheritance, association, aggregation, composition
• Specifies visibility and data types (exam requirement)
• Supports object-oriented thinking
• Enables early problem detection through static structure analysis
• Assists effort estimation via complexity assessment
• Must be documented and version-controlled

Core Components

1. Class name
2. Attributes with data type and visibility
3. Methods with return type and parameters
4. Inheritance
5. Association
6. Composition
7. Aggregation
8. Interface
9. Dependency
10. Multiplicity in relationships

Practical Example

// Example: Customers and Orders
class Customer {
  -name: String
  -email: String
  +placeOrder(): void
}

class Order {
  -date: Date
  +calculateTotalPrice(): float
}

Customer --> Order

Explanation: A customer can place orders. The arrow direction shows an association from Customer to Order.

Strengths and Weaknesses

Strengths

• High clarity through standardized notation
• Early visualization of software structure
• Supports object-oriented principles
• Facilitates team communication

Weaknesses

• Does not represent temporal behavior
• Unsuitable for dynamic processes
• Risk of over-complexity in large systems

Common Exam Questions (with Brief Answers)

1. What does a class diagram represent? The structure of an object-oriented system including classes, attributes, methods, and their relationships.
2. What are visibility modifiers in UML classes?
   • public, - private, # protected
3. How do composition and aggregation differ? Composition implies strict lifetime dependency; aggregation is a looser part-to-whole relationship.
4. How are interfaces represented? Interface with «interface» stereotype and dashed lines to implementing classes.
5. What relationship types does it show? Inheritance, association, aggregation, composition, dependency
6. What benefits does it provide during development? Structured design and improved maintainability.
7. How does it support testing? Clear assignment of responsibilities enables more targeted test design.
8. What is an interface in a class diagram? A collection of abstract methods that implementing classes must provide.

Next in the UML Learning Path

The next article in the UML learning path covers Sequence Diagram: Message Flow, Lifelines, Activation Boxes, Alt, Opt, Loop — details on sequence diagrams and their notation.

Key References

1. https://plantuml.com/class-diagram
2. https://www.omg.org/spec/UML/
3. https://www.ihk-aka.de/