Class diagrams in LLD - System Design Guide

Problem Statement

When designing software, developers often struggle to understand how different parts of the system relate to each other. Without a clear visual representation, it becomes hard to communicate structure, spot design flaws, or plan changes effectively.

Solution

Class diagrams provide a visual map of the system's classes, their attributes, methods, and relationships. By showing how classes connect and interact, they help developers and stakeholders understand the system's design and guide implementation.

Architecture

┌─────────────┐       ┌─────────────┐
│   Person    │       │   Address   │
├─────────────┤       ├─────────────┤
│ - name      │       │ - street    │
│ - age       │       │ - city      │
├─────────────┤       ├─────────────┤
│ + greet()   │       │ + full_addr()│
└─────┬───────┘       └─────┬───────┘
      │ 1                     │ 1..*
      │                       │
      └───────────── owns ─────┘

This diagram shows two classes, Person and Address, with their attributes and methods. It also shows a one-to-many ownership relationship where one Person owns multiple Addresses.

Trade-offs

✓ Pros

→

Helps visualize and communicate system structure clearly.

→

Identifies relationships and dependencies early in design.

→

Facilitates better planning and reduces misunderstandings.

→

Supports documentation and onboarding of new team members.

✗ Cons

→

Can become complex and hard to read for very large systems.

→

Requires effort to keep diagrams updated with code changes.

→

May oversimplify dynamic behaviors not captured in static diagrams.

Use class diagrams during design phases for systems with multiple interacting classes, especially when collaboration or documentation is important.

Avoid relying on class diagrams for very small or trivial programs where the overhead of creating diagrams outweighs the benefits.

Real World Examples

Amazon

Uses class diagrams to design their order processing system, clarifying how orders, customers, and payments relate before coding.

Uber

Employs class diagrams to model entities like drivers, riders, and trips, ensuring clear relationships and responsibilities.

LinkedIn

Uses class diagrams to represent user profiles, connections, and messaging components to maintain consistent design across teams.

Code Example

The before code shows two unrelated classes without any connection. The after code models a one-to-many relationship where a Person can have multiple Addresses, reflecting the class diagram structure.

LLD

### Before: No clear structure or relationships
class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

class Address:
    def __init__(self, street, city):
        self.street = street
        self.city = city

# No link between Person and Address

### After: Clear class design with relationships
class Address:
    def __init__(self, street: str, city: str):
        self.street = street
        self.city = city

    def full_addr(self) -> str:
        return f"{self.street}, {self.city}"

class Person:
    def __init__(self, name: str, age: int):
        self.name = name
        self.age = age
        self.addresses: list[Address] = []

    def add_address(self, address: Address) -> None:
        self.addresses.append(address)

    def greet(self) -> str:
        return f"Hello, my name is {self.name}."

Alternatives

Entity-Relationship Diagrams (ERD)

Focuses on database entities and their relationships rather than object-oriented classes and methods.

Use when: When designing or modeling database schemas specifically.

Sequence Diagrams

Shows object interactions over time rather than static class structure.

Use when: When you need to understand or document dynamic behavior and message flow.

Summary

Class diagrams visually represent classes, their attributes, methods, and relationships.

They help developers understand and communicate system structure before coding.

Class diagrams are best used for medium to large systems where design clarity is critical.

Practice

1. What does a class diagram primarily represent in system design?

easy

A. The structure of classes and their relationships

B. The flow of data between servers

C. The user interface layout

D. The sequence of method calls

Solution

1. Step 1: Understand the purpose of class diagrams

   Class diagrams show the static structure of a system by representing classes and their relationships.

2. Step 2: Differentiate from other diagrams

   Flow of data, UI layout, and method sequences are shown in other diagram types like sequence or activity diagrams.

3. Final Answer:

   The structure of classes and their relationships -> Option A

4. Quick Check:

   Class diagram = structure and relationships [OK]

Hint: Class diagrams show classes and links, not flow or UI [OK]

Common Mistakes:

• Confusing class diagrams with sequence diagrams
• Thinking class diagrams show UI design
• Assuming class diagrams show runtime behavior

2. Which of the following is the correct way to represent inheritance in a class diagram?

easy

A. A dashed line with a hollow circle pointing to the child class

B. A dashed line with an open arrow pointing to the child class

C. A solid line with a filled diamond pointing to the parent class

D. A solid line with a hollow triangle arrow pointing to the parent class

Solution

1. Step 1: Recall UML notation for inheritance

   Inheritance is shown by a solid line with a hollow triangle arrow pointing to the parent class.

2. Step 2: Identify incorrect options

   Filled diamonds represent composition, dashed lines with open arrows represent dependencies, and hollow circles are not standard UML symbols.

3. Final Answer:

   A solid line with a hollow triangle arrow pointing to the parent class -> Option D

4. Quick Check:

   Inheritance = solid line + hollow triangle [OK]

Hint: Inheritance uses solid line + hollow triangle arrow [OK]

Common Mistakes:

• Mixing composition and inheritance symbols
• Using dashed lines for inheritance
• Pointing arrow to child instead of parent

3. Given this class diagram snippet:
Class A - attr1: int + method1()
What does the '-' before attr1 and '+' before method1() mean?

medium

A. '-' means protected attribute; '+' means private method

B. '-' means private attribute; '+' means public method

C. '-' means public attribute; '+' means protected method

D. '-' means static attribute; '+' means abstract method

Solution

1. Step 1: Understand UML visibility symbols

   In UML, '-' means private visibility, '+' means public visibility.

2. Step 2: Apply to attributes and methods

   So, attr1 is private attribute, method1() is public method.

3. Final Answer:

   '-' means private attribute; '+' means public method -> Option B

4. Quick Check:

   Visibility: '-' private, '+' public [OK]

Hint: '-' = private, '+' = public in UML [OK]

Common Mistakes:

• Confusing visibility symbols with data types
• Assuming '-' means protected
• Mixing static/abstract with visibility

4. Identify the error in this class diagram description:
Class Car - engine: Engine - wheels: List<Wheel> -- engine "1" --- "4" wheels
What is wrong with the association notation?

medium

A. The association should use a hollow diamond for aggregation

B. The multiplicity numbers are reversed; wheels should be "4" and engine "1"

C. The association line uses double dashes instead of a single solid line

D. The class names should be connected with a dashed line

Solution

1. Step 1: Check association line style

   Associations use a single solid line, not double dashes.

2. Step 2: Verify multiplicity and line type

   Multiplicity "1" to "4" is correct; aggregation uses hollow diamond but not required here.

3. Final Answer:

   The association line uses double dashes instead of a single solid line -> Option C

4. Quick Check:

   Association = single solid line [OK]

Hint: Association lines are single solid lines, not double dashes [OK]

Common Mistakes:

• Using double dashes for associations
• Confusing aggregation with association
• Reversing multiplicity numbers

5. You need to design a class diagram for a library system where a Library has many Book objects, and each Book can have multiple Authors. Which of the following best represents this relationship?

hard

A. Library "1" --- "*" Book; Book "*" --- "*" Author with an association class for author details

B. Library "*" --- "1" Book; Book "1" --- "*" Author with inheritance

C. Library "1" --- "1" Book; Book "1" --- "1" Author with composition

D. Library "*" --- "*" Book; Book "*" --- "1" Author with aggregation

Solution

1. Step 1: Analyze multiplicity between Library and Book

   A library has many books, so multiplicity is "1" to "*".

2. Step 2: Analyze multiplicity between Book and Author

   A book can have multiple authors, and authors can write multiple books, so "*" to "*" is correct, often modeled with an association class for author details.

3. Step 3: Check other options for correctness

   Other options have incorrect multiplicities or wrong relationships like inheritance or composition.

4. Final Answer:

   Library "1" --- "*" Book; Book "*" --- "*" Author with an association class for author details -> Option A

5. Quick Check:

   Library to Book = 1 to many; Book to Author = many to many [OK]

Hint: Many-to-many needs association class; library has many books [OK]

Common Mistakes:

• Using inheritance instead of association
• Incorrect multiplicity numbers
• Confusing composition with aggregation