UML Class Diagrams

UML (Unified Modeling Language) Class Diagrams are the foundation of object-oriented design. They help you visualize the structure of your system, understand relationships between classes, and communicate your design effectively.

What is a UML Class Diagram?

A Class Diagram shows:

• Classes - Blueprints for objects
• Attributes - Data/properties of classes
• Methods - Operations/behaviors of classes
• Relationships - How classes relate to each other

Why Learn UML Class Diagrams?

• Design - Think through your system structure before coding
• Communication - Visual representation is clearer than text
• Documentation - Living documentation of your system
• Interviews - Essential for LLD (Low-Level Design) interviews
• Refactoring - Understand existing codebases better

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Basic Class Diagram Elements

Class Structure

A class in UML has three compartments:

1. Name - The class name
2. Attributes - Variables/properties (with visibility: + public, - private, # protected)
3. Methods - Functions/operations

Python

user_example.py

class User:
    def __init__(self, username: str, email: str):
        self.username = username  # Public attribute
        self._email = email        # Protected attribute
        self.__password = None     # Private attribute

    def login(self):               # Public method
        pass

    def _validate(self):          # Protected method
        pass

Java

UserExample.java

public class User {
    public String username;     // Public attribute
    protected String email;      // Protected attribute
    private String password;     // Private attribute

    public void login() {        // Public method
        // ...
    }

    protected void validate() {  // Protected method
        // ...
    }
}

Basic Class Diagram

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Visibility Modifiers

UML uses symbols to indicate visibility:

• + Public - Accessible from anywhere
• - Private - Only accessible within the class
• # Protected - Accessible within class and subclasses
• ~ Package - Accessible within the same package (Java)

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Multiplicity in Relationships

Multiplicity shows how many instances participate in a relationship:

• 1 - Exactly one
• 0..1 - Zero or one (optional)
• 1..* or 1..n - One or more
• 0..* or 0..n - Zero or more
• * or n - Many (zero or more)
• m..n - Between m and n

Examples

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UML Relationship Symbols

For detailed explanations of each relationship type, see the Class Relationships section.

Quick Reference

Relationship	UML Symbol	Description
Inheritance	Solid line with hollow triangle	”Is-a” relationship
Association	Solid line with arrow	”Uses-a” or “Knows-a”
Aggregation	Solid line with hollow diamond	”Has-a” (weak ownership)
Composition	Solid line with filled diamond	”Has-a” (strong ownership)
Dependency	Dashed arrow	”Uses temporarily”

Visual Summary

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Reading UML Diagrams

When reading a UML class diagram:

1. Start with classes - Identify all classes and their names
2. Check attributes - Understand what data each class holds
3. Review methods - See what operations each class can perform
4. Analyze relationships - Understand how classes relate to each other
5. Check multiplicity - See how many instances are involved
6. Note visibility - Understand access levels

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Drawing UML Diagrams

Best Practices

1. Keep it simple - Don’t overcomplicate diagrams
2. Use consistent notation - Follow UML standards
3. Group related classes - Organize logically
4. Show only essential details - Hide implementation details
5. Use meaningful names - Clear, descriptive class and method names
6. Document relationships - Add labels to clarify relationships

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Complete Example: Simple E-Commerce System

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Key Takeaways

Remember

• UML Class Diagrams visualize class structure and relationships
• Visibility modifiers (+, -, #, ~) show access levels
• Multiplicity shows how many instances participate
• Relationship symbols indicate different types of relationships
• Use UML diagrams for design, communication, and documentation
• For detailed relationship explanations, see Class Relationships

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Next Steps

• Learn about Class Relationships - Detailed explanations of all relationship types
• Practice drawing UML diagrams for your own projects
• Use UML diagrams in LLD interviews to communicate your design
• Explore advanced UML features like interfaces, abstract classes, and packages

Remember: UML diagrams are a tool for communication and design thinking. Focus on clarity and correctness rather than perfect notation!