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What is UML? ​

Overview ​

UML (Unified Modeling Language) is a standardized visual language used to model software systems. It helps developers, architects, and stakeholders understand the structure and behavior of a system through diagrams.

Think of UML as the blueprint of software: just like architects draw floor plans before constructing a building, engineers use UML diagrams before (and during) implementation.

Why UML Matters ​

  • Clarity: Converts abstract requirements into visual models.
  • Communication: Bridges developers, product managers, and non-technical stakeholders.
  • Design Aid: Makes it easier to spot flaws in architecture before coding.
  • Interview Relevance: Frequently used in LLD interviews to explain class design, API flow, and service interactions.

Types of UML Diagrams ​

UML defines many diagrams, but in practice, these are the most commonly used:

  1. Structural Diagrams (Static view)

    • Class Diagram: Shows classes, attributes, methods, and relationships.
    • Component Diagram: High-level modules or services and their interactions.

  2. Behavioral Diagrams (Dynamic view)

    • Sequence Diagram: How objects/services interact step-by-step.
    • Use Case Diagram: High-level user actions and system responses.
    • Activity Diagram: Workflow or process representation.

Example: Class Diagram (User Management) ​

+-------------------+       +---------------------+
|      User         |       |   UserRepository    |
+-------------------+       +---------------------+
| - id: String      |<>---->| + save(User): void  |
| - name: String    |       | + find(id): User    |
+-------------------+       +---------------------+
  • User is a domain entity.
  • UserRepository manages persistence.
  • The <> arrow shows association (UserRepository depends on User).

Example: Sequence Diagram (User Creation Flow) ​

Client -> UserController: createUser(name, email)
UserController -> UserService: createUser(name, email)
UserService -> UserRepository: save(User)
UserRepository --> UserService: success
UserService --> UserController: User created
UserController --> Client: Response 201 Created

This shows the flow of function calls when a client creates a user.

When to Use UML ​

  • Before coding: To clarify design in interviews or team discussions.
  • During coding: To validate class relationships and data flow.
  • After coding: For documentation and onboarding new team members.

Conclusion ​

UML is a visual toolkit for designing, explaining, and documenting software systems. By mastering a few key diagrams—class, sequence, and activity—you can communicate complex ideas clearly and perform better in both real-world design and system design interviews.

Next Step: Learn how to create a Class Diagram for LLD.