UML Design

What is UML?

• UML stands for Unified Modeling Language.

• It is used for Object-Oriented Analysis and Design (OOAD).

• Helps to visualize, specify, construct, and document software systems.

• UML consists of 13+ diagrams, categorized as:

  • Structural Diagrams (e.g., Class Diagram, Component Diagram)

  • Behavioral Diagrams (e.g., Use Case, Sequence Diagram)

Use Case

UML is heavily used in Low-Level Design (LLD) and System Design rounds to communicate architecture clearly.

What is a Class Diagram?

Definition

• A Class Diagram represents the static structure of a system.

• It models classes, their attributes, methods, and relationships between them.

Components

Element	Description
Class	Blueprint for objects
Attribute	Properties or data members
Method	Functions or operations of class
Relationships	Association, Aggregation, Composition, Inheritance

Purpose

• Helps in designing and understanding object-oriented systems.

• Directly maps to OOP code (Java, C++, etc.).

Real-World Analogy

Think of a “Car” class having attributes like engine, color, and methods like start(), accelerate().

Object Associations in UML

1. Composition (Strong Containment)

“Has-a” relationship where lifetime is tightly bound.

• One class owns another completely.

• Child object cannot exist independently.

• Represented using filled diamond.

Example

A House has-a Room. If the house is destroyed, the room doesn’t exist.

2. Aggregation (Weak Containment)

“Has-a” relationship but with independent lifecycle.

• One class contains another, but child can exist independently.

• Represented using hollow diamond.

Example

A Team aggregates Players. Players exist even without the team.

3. Association (Unidirectional)

One class knows about another.

• Interaction flows in one direction only.

• Represented using a simple arrow.

Example

A Customer places an Order, but Order doesn’t know about the Customer.

4. Association (Bidirectional)

Two classes are mutually aware.

• Both classes can interact with each other.

• Represented using a line without arrows.

Example

A Teacher and Student know each other.