📅Day 4 - Understanding UML Diagrams: Real Examples of Class and Sequence Diagrams

NOTE: - I started my 8-week system design journey with Coder Army. I will be journaling every day, recording what I learn, reflecting on it, and sharing it with my network to help newcomers to system design.

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1.) What is UML and why it’s used in Software Design?

💠 What is UML (Unified Modeling Language)?

UML stands for Unified Modeling Language – a standard way to visually represent the design of a system.

(Hindi: UML ka matlab hai ek aisa standardized diagram jisse hum software system ke design ko visual form mein dikhate hain.)

• Imagine UML as the blueprint of a house, but for software systems.

📍 Real-life analogy:

Before building a house, an architect creates floor plans and designs. Similarly, in software, developers create UML diagrams before writing code – so that the team understands what they’re building.

💠 Why is UML important in Software Design?

• Helps visualize the structure and flow of a system

• Makes team collaboration easier

• Simplifies problem-solving and planning

• Used in Low-Level Design (LLD) to build scalable, clean systems

📍Real example:

Suppose you’re building a food delivery app like Zomato. UML will help you design how:

• Users place orders 🍔

• Restaurants receive them 🍽️

• Delivery partners pick them up 🛵

Payments are handled 💳

💠 UML Diagram Parts: Structural & Behavioral

UML diagrams are mainly divided into two parts:

1) Structural (Static) Diagrams

These diagrams represent the structure of a system — what things exist and how they’re related.

(Hindi: Structural ya static diagrams system ke parts aur unke relationships ko dikhate hain — jaise ek skeleton ya blueprint.)

They focus on what the system is.

2) Behavioral (Dynamic) Diagrams

These diagrams represent interactions and how things behave over time.

(Hindi: Behavioral ya dynamic diagrams system ke components ke beech hone wali activity ko dikhate hain.)

They focus on what the system does.

📍Why These Are Useful?

• Better understanding of system structure & flow

• Team collaboration made easy

• Easy debugging and system maintenance

• Foundation for real-world coding

  💠7 Types of Structural (Static) Diagrams

• Class Diagram – Most commonly used

• Object Diagram

• Component Diagram

• Deployment Diagram

• Composite Structure Diagram

• Package Diagram

• Profile Diagram

  🎯 Most Used: Class Diagram

📍 Real-life Example:
In an e-commerce app like Amazon:

• You’ll have a User class, Product class, Order class

• These classes connect to show how users place orders, view products, and make payments.

  💠7 Types of Behavioral (Dynamic) Diagrams

• Sequence Diagram – Most commonly used

• Use Case Diagram

• Activity Diagram

• Communication Diagram

• State Machine Diagram

• Timing Diagram

Interaction Overview Diagram

🎯 Most Used: Sequence Diagram

📍 Real-life Example:
In a food delivery app:

• A user places an order

• The restaurant gets the order

• The delivery partner picks it up

• Payment confirmation is sent

The Sequence Diagram shows how this flow happens step by step over time.

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2.) How to draw and understand Class Diagrams (with real-world examples)

💠What is a Class Structure?

Class diagrams show the main entities (classes) of the system, their attributes, and the connections between them.

(Hindi : Class Diagram system ke main entities (classes), unke attributes aur unke beech ke connections ko show karta hai.)

📍 Real-world example:
Class: Car
Attributes: brand, model, engine
Methods: startEngine(), brake()

class Car {
  String brand;
  String model;
  void startEngine() { ... }
}

Visibility Notation:

• + for public (visible to all classes)

• - for private (visible only within the class)

• # for protected (visible to subclasses)

• ~ for package or default visibility (visible to classes in the same package)

💠What is an Association / Connection?

An association represents a bi-directional relationship between two classes. It means instances of one class are connected or linked to instances of another class.

(Hindi : Association ek bi-directional relationship hoti hai do classes ke beech. Iska matlab hai ki ek class ke instances doosri class ke instances se connected ya linked hote hain.)

Association types :

1. Class Association : a) Inheritance

2. Object Association : a) Simple Association , b) Aggregation, c) Composition

• Depicted as: A solid line connecting two class boxes in UML.

• Optional arrows can indicate direction (one-way or two-way).

  📍 Real-world example:

  Class 1: Library

  Class 2: Book

  The Library class has a collection of Book objects.

  • Library is the source class (has a reference to books).

• Book is the target class (belongs to a specific library).

    public class Library {
        private List<Book> books;
    }
  
    public class Book {
        private String title;
    }
  

  💠Inheritance

• Represents: "is-a" relationship

• Arrow: Solid line with empty arrowhead pointing to the parent (--▷)

Example: Dog is an Animal

class A {
    void method1() {
        System.out.println("A");
    }
}

class B extends A {
    void method2() {
        System.out.println("B");
    }
}

public class Main {
    public static void main(String[] args) {
        B b = new B();
        b.method1(); // from A
        b.method2(); // from B
    }
}

💠Simple Association

• Represents: "uses-a" or "has-a" relationship (loose connection)

• Arrow: Solid line (→) or solid line with optional direction (no triangle for simple association)

• Example:
  Arjun → House
  (Arjun lives in a house)

💠Aggregation

• Represents: "has-a" but loosely bound

• Arrow: Empty diamond arrow (<>---)

• Example:
  Sofa <>--- Room, Chair <>--- Room
  (Room can exist even if sofa/chair is removed)

💠Composition

• Represents: Strong containment

• Arrow: Filled diamond arrow (<~>---)

Example:
Wheel <~> Chair, Arms <~> Chair
(Without chair, its parts don't exist)

class A {
    void method1() {
        System.out.println("From A");
    }
}

class B {
    A a;

    B() {
        a = new A(); // Strong containment (composition)
    }

    void method2() {
        System.out.println("From B");
    }
}

public class Main {
    public static void main(String[] args) {
        B b = new B();
        b.method2();       // From B
        b.a.method1();     // From A (through composition)
    }
}

3) How to create and analyze Sequence Diagrams to visualize system interactions

A sequence diagram is a behavioral UML diagram that shows the interactions between different objects in a time sequence.

(Hindi : Sequence Diagram ek Behavioral UML Diagram hai jo different objects ke beech mein hone wali interactions ko time ke order mein dikhata hai.)

📍Use When:

When you want to model how different components interact in a specific use case.

💠Main Components of a Sequence Diagram:

• Objects / Classes
  Example: A, B, C, D (entities involved)

• Lifeline
  When the object is active in the system

• Activation Bar
  The time period till which the object is active (send/receive messages)

• Messages

  • Synchronous: Sender waits for response

  • Asynchronous: Sender does not wait for response

💠Special messages:

• Create message: When an object is created

• Destroy message: When an object is destroyed

• Lost message: Message was sent but not received

• Found message: Message was received but not known who sent it

User -> App: Request Ride  
App -> Server: Check Availability  
Server -> Driver: Send Request  
Driver -> App: Accept Ride  
App -> User: Ride Confirmed

4. ) How UML helps in real-life Low-Level Design and interview discussions

UML (Unified Modeling Language) is like a visual blueprint for software design. It helps you explain and plan systems clearly, especially during coding and interviews. Here’s why it matters:

1. Clear Visuals
   UML diagrams (like class and sequence diagrams) break down complex systems into simple parts, making it easy for everyone to understand.

2. Shows Relationships
   It clearly represents key relationships like inheritance (is-a), aggregation (has-a), and composition (strong has-a), which helps build solid designs.

3. Makes Coding Easier
   With UML, you know what to build and how parts connect, reducing guesswork and speeding up development.

4. Impresses Interviewers
   Using UML in interviews shows you think systematically, understand OOP principles, and can communicate complex ideas clearly.

5. Bridges Teams
   UML helps developers and non-technical stakeholders (like product managers) stay on the same page.

User            ATM           Account        Transaction       CashDispenser
 |               |                |                |                  |
 |--insertCard()->|               |                |                  |
 |--enterPIN()--->|               |                |                  |
 |               |--validatePin->|                |                  |
 |               |<--valid/invalid--|             |                  |
 |--selectWithdraw(amount)------->|               |                  |
 |               |--createTxn()------------------->|                |
 |               |                  |--initiate()-->|                |
 |               |<----------------|<--complete()---|                |
 |               |--debit()------->|                |                |
 |               |<--success--------|               |                |
 |               |--dispenseCash()------------------------------->|  
 |               |<------------------cash dispensed---------------|
 |<--Take cash--------------------|                |                |

📍 Diagram

Day 4 Completed ✅ System Design

NOTE : - A big thanks to my mentors Rohit Negi Sir and Aditya Sir for launching this amazing 8-week course absolutely free on YouTube via CoderArmy9 :- https://www.youtube.com/@CoderArmy9🙌

👉 Share this blog with your connections! Let’s keep learning, growing, and supporting one another on this journey. 🚀

✍️ Payal Kumari 👩‍💻

Jai Hind 🇮🇳 | #CoderArmy #LearningInPublic #SystemDesign #TechForAll #MentorshipMatters #8weeksLLdChallenge #LowLevelDesign #Code #LLD #OOP #UMLDiagrams 👩‍💻