What are Class Diagrams in LLD?

Definition
Class Diagrams are UML (Unified Modeling Language) structural diagrams that visualize the static structure of a system by showing its classes, attributes, methods, and relationships between classes.
Class diagrams solve the problem of communicating object-oriented design decisions without ambiguity. They serve as blueprints that bridge the gap between requirements and implementation, allowing designers to visualize inheritance hierarchies, composition relationships, and dependencies before writing code.
Core Components:
Classes: Rectangles divided into three sections: name, attributes, methods
Attributes: Data members with visibility (+ public, - private, # protected) and type
Methods: Operations with parameters and return types
Relationships: Lines connecting classes showing inheritance, composition, aggregation, or association
Interview Tip: In machine coding rounds, start by sketching a class diagram on paper or whiteboard. This demonstrates structured thinking and catches design flaws early, before you write 500 lines of code.
Problem it Solves: Without class diagrams, teams often discover design conflicts late (during implementation or code review), leading to costly refactors. For example, if two developers implement a parking lot system without a shared diagram, one might make ParkingSpot inherit from Vehicle (incorrect) while another creates proper composition. A class diagram establishes a single source of truth.

💡 Key Takeaways

✓Class diagrams are UML structural diagrams that show static relationships between classes

✓They consist of classes (with attributes and methods) and relationships (inheritance, composition, aggregation, association)

✓Used to communicate design decisions before implementation, reducing ambiguity and costly refactors

✓Critical skill for LLD interviews where you must justify design choices, not just write code

✓Visibility markers: + (public), - (private), # (protected), ~ (package-private)

📌 Examples

1Drawing a Parking Lot system class diagram to show Vehicle, ParkingSpot, and ParkingLot relationships before coding

2Using a class diagram in a design review to explain why PaymentProcessor uses Strategy pattern for different payment methods

3Sketching a Library Management System diagram to clarify that Book and Member have a borrowing association, not inheritance

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