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

This article is a conceptual explanation of class diagrams – including exam questions and tags.

In a Nutshell

Class diagrams show the structure of a software system through classes, their attributes, methods, and the relationships between them – a central component 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 as well as relationships such as association, inheritance, aggregation, and composition. Each class is represented as a rectangle with three sections: name, attributes, methods. Visibility levels (+ public, - private, # protected) and data types are explicitly specified. UML class diagrams are important for the design phase because they depict the later object-oriented design and reveal potential problems early on.

Exam-Relevant Key Points

• Component of UML for modeling system structure
• Contains classes with attributes and methods
• Represents relationships such as inheritance, association, aggregation, composition
• Allows visibility levels and data types (IHK-relevant)
• Supports object-oriented thinking
• Promotes early error detection through static structure
• Facilitates effort estimation through complexity analysis
• Must be documented and versioned

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
  +order(): void
}

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

Customer --> Order

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

Advantages and Disadvantages

Advantages

• High comprehensibility through standard notation
• Early visualization of software structure
• Support for object-oriented principles
• Helpful for team communication

Disadvantages

• No representation of temporal behavior
• Not suitable for dynamic processes
• Risk of over-complexity in large systems

Typical Exam Questions (with Short Answer)

1. Class diagram represents? Structure of an object-oriented system with classes, attributes, methods, and relationships.
2. Visibility levels in UML classes?
   • public, - private, # protected
3. Composition vs. aggregation? Composition means strict lifetime dependency, aggregation is a looser part-whole relationship.
4. Interfaces represented? Interface with stereotype «interface» and dashed lines to implementing classes.
5. Relationship types shown? Inheritance, association, aggregation, composition, dependency
6. Advantages during development? Structured design and improved maintainability.
7. Support testing? Clear delineation of responsibilities enables more targeted tests.
8. Interface in class diagram? Collection of abstract methods that must be implemented by classes.

Most Important Sources

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