4 PILLARS OF LLD ???

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1)Strong in OOPS Concepts.

2) Design Principles (SOLID, DRY, KISS, YAGNI, Law of Demeter, Composition Over Inheritance)

3) Design Patterns.

4)  UML Diagrams.

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1)Strong in OOPS Concepts

4 PILLARS OF LLD ???

OOP Concept	Description	Example in C++
Class	A blueprint for creating objects that encapsulate data and methods.	class Car { int speed; void drive(); };
Object	An instance of a class with its values and behaviors.	Car myCar; myCar.drive();
Encapsulation	Wrapping data and methods into a single unit and restricting direct access.	private: int speed; public: void setSpeed(int s) { speed = s; }
Abstraction	Hiding implementation details and exposing only necessary parts.	class Vehicle { virtual void start() = 0; };
Inheritance	Enabling a new class to derive properties and behaviors from an existing class.	class SportsCar : public Car { void turboBoost(); };
Polymorphism	The ability to take multiple forms, such as function overloading and overriding.	class Shape { virtual void draw(); }; class Circle : public Shape { void draw() override; };
Dynamic Binding	Deciding at runtime which functions to execute using virtual functions.	virtual void display();
Message Passing	Objects communicate with each other via function calls.	obj1.sendMessage(obj2);
Constructor	A special function that initializes an object when it’s created.	Car() { speed = 0; }
Destructor	A special function is called when an object is destroyed.	~Car() { cout << "Car destroyed"; }

 

2) Design Principles

4 PILLARS OF LLD ???

Design Principle	Description	Example in C++
1. Single Responsibility Principle (SRP)	A class should have only one reason to change (one responsibility).	class Logger { void log(string msg); }; (Separating logging from business logic)
2. Open/Closed Principle (OCP)	A class should be open for extension but closed for modification.	class Shape { virtual double area() = 0; }; (New shapes can be added without modifying existing code)
3. Liskov Substitution Principle (LSP)	Derived classes should be replaceable for base classes without altering correctness.	class Bird { virtual void fly(); }; class Sparrow : public Bird {};
4. Interface Segregation Principle (ISP)	Clients should not be forced to depend on interfaces they do not use.	class Printer { virtual void print() = 0; }; class Scanner { virtual void scan() = 0; }; (Separate interfaces for different functionalities)
5. Dependency Inversion Principle (DIP)	High-level modules should not depend on low-level modules. Both should depend on abstractions.	class Database { virtual void connect() = 0; }; class MySQL : public Database { void connect() override; };
6. DRY (Don’t Repeat Yourself)	Avoid duplication by reusing code.	Using functions, templates, and inheritance to eliminate redundant code.
7. KISS (Keep It Simple, Stupid)	Write simple, clear, and maintainable code.	Avoid overly complex inheritance or nested logic.
8. YAGNI (You Ain’t Gonna Need It)	Don’t add features until they are necessary.	Avoiding premature optimizations and unnecessary abstractions.
9. Law of Demeter (LoD)	A class should have limited knowledge of other objects.	Using getters/setters instead of direct member access.
10. Composition Over Inheritance	Prefer composition (has-a) over inheritance (is-a).	class Engine {}; class Car { Engine engine; }; (Instead of class Car : public Engine {})

 

3) Design Patterns

4 PILLARS OF LLD ???

Category	Design Pattern	Description	Example in C++
Creational	Singleton	Ensures that a class has only one instance and provides a global access point.	class Singleton { static Singleton* instance; public: static Singleton* getInstance(); };
	Factory Method	Provides an interface for creating objects but lets subclasses alter the type of objects that will be created.	class Shape { public: virtual void draw() = 0; }; class ShapeFactory { Shape* createShape(string type); };
	Abstract Factory	Creates families of related objects without specifying their concrete classes.	class GUIFactory { virtual Button* createButton() = 0; };
	Builder	Separates the construction of a complex object from its representation.	class CarBuilder { void setEngine(); void setWheels(); }; class CarDirector { CarBuilder* builder; };
	Prototype	Creates new objects by copying an existing object.	class Prototype { virtual Prototype* clone() = 0; };
Structural	Adapter	Converts the interface of a class into another interface clients expect.	class USB { void connect(); }; class USBAdapter { USB usb; void connectToLaptop(); };
	Bridge	Separates an abstraction from its implementation so that both can be modified independently.	class Device { virtual void turnOn() = 0; }; class Remote { Device* device; };
	Composite	Composes objects into tree structures to represent part-whole hierarchies.	class Component { virtual void operation() = 0; }; class Composite : public Component {};
	Decorator	Attaches additional responsibilities to an object dynamically.	class Coffee { virtual string description() = 0; }; class MilkDecorator : public Coffee {};
	Facade	Provides a simplified interface to a complex system.	class Facade { Subsystem1 s1; Subsystem2 s2; void operation(); };
	Flyweight	Reduces memory usage by sharing common objects instead of creating new ones.	class Character { char symbol; }; class CharacterFactory { Character* getCharacter(char c); };
	Proxy	Provides a surrogate or placeholder for another object.	class Image { virtual void display() = 0; }; class ProxyImage : public Image {};
Behavioral	Chain of Responsibility	Passes requests along a chain of handlers.	class Handler { Handler* next; void handleRequest(); };
	Command	Encapsulates a request as an object.	class Command { virtual void execute() = 0; }; class ConcreteCommand : public Command {};
	Interpreter	Defines a grammatical representation for a language and an interpreter to interpret sentences in the language.	class Expression { virtual int interpret() = 0; };
	Iterator	Provides a way to access elements of an aggregate object sequentially without exposing its underlying representation.	class Iterator { virtual bool hasNext() = 0; virtual Object* next() = 0; };
	Mediator	Defines an object that encapsulates how a set of objects interact.	class Mediator { virtual void notify(); }; class Component { Mediator* mediator; };
	Memento	Captures and restores an object’s internal state.	class Memento { private: string state; }; class Originator { Memento save(); void restore(Memento m); };
	Observer	Defines a dependency between objects so that when one changes state, all its dependents are notified.	class Observer { virtual void update() = 0; }; class Subject { vector<Observer*> observers; void notify(); };
	State	Allows an object to change its behavior when its internal state changes.	class State { virtual void handle() = 0; }; class Context { State* state; };
	Strategy	Defines a family of algorithms, encapsulates each one, and makes them interchangeable.	class Strategy { virtual void execute() = 0; }; class ConcreteStrategy : public Strategy {};
	Template Method	Defines the skeleton of an algorithm but allows subclasses to redefine certain steps.	class Template { void algorithm(); };
	Visitor	Allows adding new operations to objects without modifying their structure.	class Visitor { virtual void visit(Element* e) = 0; }; class Element { virtual void accept(Visitor* v) = 0; };

 

4)  UML Diagrams

UML Diagram Type	Description	Usage Example
Structural Diagrams	Depict static aspects of the system (classes, objects, relationships).	–
Class Diagram	Represents classes, attributes, methods, and relationships between them.	Designing object-oriented systems (e.g., defining class relationships in C++).
Object Diagram	Shows instances of classes and their relationships at a specific point in time.	Debugging object interactions during runtime.
Component Diagram	Describes the physical structure of software components and their dependencies.	Microservices architecture, modular design in large applications.
Deployment Diagram	Represents the physical deployment of artifacts (software) on hardware.	Cloud infrastructure, system architecture planning.
Package Diagram	Groups related classes or components into packages.	Organizing large codebases into modules.
Composite Structure Diagram	Shows the internal structure of a class and collaborations between its components.	Modeling complex components with sub-elements.
Behavioral Diagrams	Depict dynamic aspects of the system (interactions, workflows, state changes).	–
Use Case Diagram	Represents user interactions with the system.	Understanding system functionality from a user’s perspective.
Sequence Diagram	Depicts the flow of messages between objects over time.	Designing API interactions, and process flow in a feature.
Activity Diagram	Illustrates workflows, processes, and decision-making logic.	Modeling business processes, and login workflows.
State Machine Diagram	Represents states of an object and transitions between them.	Designing state-based behavior, like vending machines.
Communication Diagram	Similar to sequence diagrams, it focuses on object interactions.	Analyzing real-time communication between components.
Timing Diagram	Shows changes in state or behavior over time.	Embedded systems, real-time system modeling.
Interaction Overview Diagram	Combines activity and sequence diagrams for a high-level view of system interactions.	Representing complex workflows at a high level.

 

Important UML Diagrams for Low-Level Design (LLD) Interviews

UML (Unified Modeling Language) diagrams help visualize, specify, construct, and document software systems. For LLD, these are the most important UML diagrams:

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1. Class Diagram (Most Important)

📌 Purpose: Represents the static structure of a system, showing classes, attributes, methods, and relationships.
📌 Key Components:

• Classes (with attributes & methods)
• Relationships (Association, Aggregation, Composition, Inheritance)
• Access Modifiers (+ public, – private, # protected)

✔ Why Important? Used in LLD to define the object-oriented structure of a system.

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2. Sequence Diagram

📌 Purpose: Shows the interaction between objects over time (i.e., the order of method calls).
📌 Key Components:

• Actors & Objects (Who initiates the interaction)
• Lifelines (Object existence)
• Messages/Method Calls (Solid arrow for synchronous, dashed for asynchronous)

✔ Why Important? Defines the flow of control and communication between objects for a use case.

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3. Use Case Diagram

📌 Purpose: Represents how external users interact with the system.
📌 Key Components:

• Actors (Users or external systems)
• Use Cases (Functionalities)
• Relationships (Extend, Include, Generalization)

✔ Why Important? Helps in understanding functional requirements and user interactions.

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4. Activity Diagram

📌 Purpose: Represents workflow and business logic.
📌 Key Components:

• Start & End nodes
• Actions (Processes)
• Decision points (if/else conditions)
• Swimlanes (For role-based responsibilities)

✔ Why Important? Helps in defining process flows and decision logic.

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5. Component Diagram

📌 Purpose: Represents high-level system architecture, showing dependencies between components.
📌 Key Components:

• Components (Modules, Services)
• Interfaces (APIs)
• Dependency Relationships

✔ Why Important? Helps in understanding module interactions and dependencies.

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6. Deployment Diagram

📌 Purpose: Represents physical deployment of software components on hardware nodes.
📌 Key Components:

• Nodes (Servers, Databases, Cloud, etc.)
• Artifacts (Deployed software components)
• Connections (Communication links between nodes)

✔ Why Important? Used in system architecture discussions for scalability and deployment planning.

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Which UML Diagrams to Use in LLD?

UML Diagram	Used for
Class Diagram	Defining system structure (MUST HAVE)
Sequence Diagram	Explaining object interactions (MUST HAVE)
Use Case Diagram	Understanding user-system interaction
Activity Diagram	Modeling workflows and logic
Component Diagram	Showing module-level architecture
Deployment Diagram	Understanding infrastructure and deployment

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How to Use UML in an LLD Interview?

1️⃣ Start with a Use Case Diagram – Identify functionalities.
2️⃣ Create a Class Diagram – Define classes, attributes, and relationships.
3️⃣ Draw a Sequence Diagram – Show object interactions for key flows.
4️⃣ Use an Activity Diagram – If business logic is complex.
5️⃣ Component & Deployment Diagrams – For large-scale system designs.

 

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