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Pavan Barnana
Pavan Barnana

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Low-Level System Design (LLD) in Java: A Beginner's Guide

#beginners #java #systemdesign #tutorial

Introduction

When preparing for software engineering interviews, many developers focus heavily on Data Structures and Algorithms. However, as you move toward mid-level and senior roles, Low-Level Design (LLD) becomes equally important.

Low-Level Design focuses on designing classes, objects, relationships, and interactions within a system. It helps developers create maintainable, scalable, and extensible applications.

In this blog, we'll understand the fundamentals of LLD and see how Java can be used to implement clean object-oriented designs.

What is Low-Level Design?

Low-Level Design is the process of converting business requirements into class-level designs.

It involves:

  • Identifying classes and objects
  • Defining relationships between classes
  • Applying Object-Oriented Programming principles
  • Using Design Patterns where necessary
  • Writing maintainable and reusable code

Example

Suppose we are building a Parking Lot System.

Instead of directly writing code, we first identify:

  • Vehicle
  • ParkingSpot
  • ParkingFloor
  • Ticket
  • Payment
  • ParkingLot

Each of these becomes a separate class in our design.

Core Object-Oriented Principles

1. Encapsulation

Encapsulation means hiding internal implementation details and exposing only necessary functionality.

public class BankAccount {

    private double balance;

    public void deposit(double amount) {
        balance += amount;
    }

    public double getBalance() {
        return balance;
    }
}
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Benefits:

  • Better security
  • Easier maintenance
  • Controlled access

2. Abstraction

Abstraction hides complexity from users.

public interface Payment {
    void pay(double amount);
}
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Users only know how to call pay(), not how the payment is processed.

3. Inheritance

Inheritance promotes code reuse.

public class Vehicle {
    protected String vehicleNumber;
}

public class Car extends Vehicle {
}
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4. Polymorphism

Different implementations through a common interface.

Payment payment = new CreditCardPayment();
payment.pay(1000);
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The same method behaves differently depending on the implementation.

SOLID Principles

A good LLD follows SOLID principles.

S - Single Responsibility Principle

A class should have only one reason to change.

Bad:

class User {
    void saveUser() {}
    void sendEmail() {}
}
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Good:

class UserService {
    void saveUser() {}
}

class EmailService {
    void sendEmail() {}
}
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O - Open Closed Principle

Classes should be open for extension but closed for modification.

Example:

interface Notification {
    void send();
}
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Now we can add EmailNotification, SMSNotification, PushNotification without changing existing code.

L - Liskov Substitution Principle

Child classes should be replaceable with parent classes.

I - Interface Segregation Principle

Do not force classes to implement methods they don't need.

D - Dependency Inversion Principle

Depend on abstractions, not concrete implementations.

class OrderService {

    private Payment payment;

    public OrderService(Payment payment) {
        this.payment = payment;
    }
}
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How to Approach Any LLD Problem

Whenever an interviewer gives an LLD problem:

Step 1: Understand Requirements

Example:

Design a Parking Lot System.

Questions:

  • Multiple floors?
  • Different vehicle types?
  • Ticket generation?
  • Payment support?

Step 2: Identify Entities

For Parking Lot:

  • Vehicle
  • ParkingSpot
  • Ticket
  • Floor
  • Payment

Step 3: Define Relationships

Vehicle -> Ticket

Ticket -> Parking Spot

Parking Lot -> Floors

Floor -> Parking Spots

Step 4: Create Class Diagram

Draw classes before coding.

This avoids unnecessary refactoring later.

Step 5: Apply Design Patterns

Common patterns used in LLD:

  • Singleton
  • Factory
  • Strategy
  • Observer
  • Builder

Why LLD Matters

LLD helps developers:

  • Write clean code
  • Build scalable applications
  • Improve maintainability
  • Perform better in interviews
  • Design enterprise-level systems

Companies often evaluate LLD skills for Software Engineer II, Senior Engineer, and Architect roles.

Conclusion

Low-Level Design is not about memorizing design patterns. It is about creating software that is easy to understand, extend, and maintain.

To master LLD:

  1. Learn OOP thoroughly.
  2. Understand SOLID principles.
  3. Practice design patterns.
  4. Solve real-world design problems.
  5. Implement systems in Java.

In upcoming blogs, we will design complete systems such as:

  • Parking Lot System
  • Library Management System
  • ATM System
  • BookMyShow
  • Splitwise
  • Elevator System

Stay tuned for more Low-Level Design examples using Java.

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