CSCI 370 - Lecture 6: UML Class Diagrams and Object-Oriented Relationships

Overview

This lecture focused heavily on UML (Unified Modeling Language) class diagrams and the key relationships between classes in object-oriented design. Concepts such as inheritance, association, aggregation, and composition were discussed in detail, including how these relationships translate into actual code. The lecture also highlighted the importance of testing in real-world environments and gave examples of software failures due to small code errors.

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🛰 Software Failure Case Study: Ariane 5

Summary:

• The Ariane 5 rocket failure (June 4, 1996) was due to a software bug.
• A 64-bit floating-point number was incorrectly cast to a 16-bit signed integer, causing an overflow.
• Both the primary and backup systems failed because they shared the same bug.
• Takeaways:
  • Even minor coding errors can lead to catastrophic results.
  • Importance of testing in real-world environments.
  • The need for robust exception handling.
  • Backup systems must be independently verified.

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📦 UML Class Diagrams

What is UML?

• UML (Unified Modeling Language) is a standardized way to visualize the design of a system.
• The Class Diagram is one of the most commonly used UML diagrams.

Class Diagram Basics

A UML class diagram typically includes:

• Class Name (top section)
• Attributes (middle section)
• Methods/Operations (bottom section)

Example:

For a class representing an animal:

+----------------+
|    Animal      |
+----------------+
| -name: String  |
| -age: int      |
+----------------+
| +getName(): String |
| +setName(String): void |
+----------------+

Symbols:

• + = public
• - = private

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📚 Object-Oriented Relationships in UML

1. Inheritance (Generalization)

• Represented by a hollow triangle pointing to the parent class.
• Example: Horse, Otter, and Slow Loris inherit from Animal.
• Benefit: Enables code reuse and polymorphism.

2. Association

• Represented by a solid line between classes.
• Denotes a general relationship, e.g., Person owns Dog.
• May include multiplicity (e.g., one-to-many, one-to-one).

3. Aggregation (“Has-a” Relationship)

• Represented by a hollow diamond.
• Whole-part relationship where the part can exist independently.
• Example: Class aggregates Student; students can exist without the class.

4. Composition

• Represented by a filled diamond.
• Strong whole-part relationship where the part cannot exist without the whole.
• Example: Order has OrderDetails; if Order is deleted, so is OrderDetails.

5. Dependency

• Represented by a dashed arrow.
• One class uses another, typically as a method parameter.
• Example: Dog depends on Food in eat(Food food).

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🔁 Multiplicity in Relationships

Indicates how many instances of one class relate to instances of another. Examples:

• 1 — exactly one
• 0..1 — zero or one
• 0..* — zero or many
• 1..* — at least one
• 0..5 — up to five

Example:

• Customer has 0..* Orders.
• Each Order belongs to 1 Customer.

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🧱 Abstraction and Abstract Classes

• Abstract classes cannot be instantiated.
• Serve as blueprints for other classes.
• UML typically uses italicized class names to denote abstract classes.

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🧪 Practical Tools & Application

• Tools like Visual Paradigm allow users to create UML diagrams easily.
• You can add attributes, methods, and relationships using GUI tools.
• Some software can generate code from diagrams or vice versa.

Code Examples:

Dependency Example:

class Food {}
class Dog {
    public void eat(Food food) {}
}

Inheritance Example:

class Animal {}
class Dog extends Animal {}

Composition Example:

class Tesla {
    Engine[] engines = new Engine[3];
    public Tesla() {
        engines[0] = new Engine();
    }
    class Engine {}
}

Aggregation Example:

class Person {
    Dog[] dogs = new Dog[5];
}

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✅ Key Takeaways

• Class diagrams model the structure of a system using classes and relationships.
• UML relationships clarify how objects interact: Inheritance, Association, Aggregation, Composition, Dependency.
• Proper diagramming helps with code understanding, communication, and design planning.
• Code examples reinforce the UML concepts.
• Knowing these concepts is highly useful in job interviews and real-world software engineering tasks.

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“UML diagrams aren’t meant to be perfect; they’re meant to communicate design ideas. Spend your time coding, not making perfect diagrams.” – Professor