Lesson 1

LLD Essentials: Low-Level Design

Learn how to break a problem into classes, relationships, and responsibilities that are easier to build, test, and change.

What’s in this lesson: LLD basics, coupling vs cohesion, SOLID, UML thinking, and a short scored check.
Why this matters: Strong LLD helps you design code that is simpler to extend, debug, and explain in interviews.
Attention Activity

What changes when one module changes?

Imagine an order system where payment rules change. Tap through the design and notice which pieces should stay isolated.

Change one thingWatch impactSpot boundaries
LLD attention activity illustration showing modules, class diagram, and change impact
Hook: one small change can ripple through a design.
Core idea

LLD turns requirements into classes and responsibilities

Start by identifying nouns, actions, and rules. Then group related behavior into focused classes.

  • Class: what data and behavior belong together?
  • Responsibility: who owns each rule?
  • Relationship: who uses, contains, or extends whom?
Illustration of high cohesion and low coupling
Good LLD keeps pieces grouped by purpose and loosely connected.
Design rule

Prefer high cohesion and low coupling

High cohesion means a class does one related job well.

Low coupling means classes depend on each other as little as possible.

If a change forces many files to change, your design is probably too tightly coupled.
Visual metaphor for design principles and structure
Design choices work best when they support clear ownership and fewer dependencies.
Modeling

Use UML sketches to communicate design

Class diagrams help you show classes, attributes, operations, and relationships before coding.

  1. List major classes.
  2. Draw relationships.
  3. Check for missing responsibilities.
UML class diagram activity
A quick sketch often exposes design gaps before implementation starts.
Rule set

SOLID gives you practical guardrails

  • S: one reason to change
  • O: open for extension, closed for modification
  • L: substitute safely
  • I: small focused interfaces
  • D: depend on abstractions
Cohesion and coupling reinforcement visual
SOLID supports maintainable structure when your system grows.
Summary

Key Takeaways

  • Review the core ideas.
  • Connect concepts to practice.
  • Prepare for assessment.
Assessment

Assessment Starting

You are about to begin the assessment. Select the best answer for each question.

Assessment Q1

Which statement best describes high cohesion?

Assessment Q2

Why is low coupling valuable in LLD?

Assessment Q3

What is the best first move when creating a class diagram?

Results

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