Object Relationships Deep Dive

Why Relationships Matter

Suppose I give you these classes:

class User {}
class Order {}
class Product {}
class Payment {}

Do we have a design?

No

We only have isolated objects.

The real design begins when we answer:

How are these objects connected?

Relationships are the backbone of every software system.

Without relationships:

Objects are islands

With relationships:

Objects become a system

Think Like a Real World System

Imagine:

Customer places Order
Order contains Products
Payment pays for Order

This immediately creates a network.

Customer
    |
    |
    v
 Order
   |
   |
   v
Product

Order
   |
   |
   v
Payment

This network is called an:

Object Graph

Types of Relationships

There are five major types of relationships every designer must know:

1. Association
2. Aggregation
3. Composition
4. Dependency
5. Inheritance

Think of them as different strengths of connection.

Relationship 1 – Association

The simplest relationship.

Meaning:

Object A knows Object B

Example:

Teacher teaches Student

Teacher exists independently.

Student exists independently.

Suppose:

There are two classes Student and Course.

A student enrols in a course.

Question: Does Student own Course?
No.

Question: Does Course own Student?
No.

They simply know about each other.
This is association.

Definition

Association is:

A relationship where one object knows about another object.

No ownership implied.

No lifecycle dependency implied.

Diagram:

Teacher ------ Student

Code:

class Student {}

class Teacher {
	students: Student[];
}

Teacher knows students.

Association.

class Course
{
};

class Student
{
private:
    Course* course;
};

Student knows about Course. Nothing more.

Characteristics:

Objects can exist independently.

Association implies:

Awareness
Communication
Collaboration

But NOT:

Ownership
Lifecycle Control

Example:

Teacher resigns

Students still exist.

Example:

Student leaves school

Teacher still exists.

Therefore:

Weak relationship

Real Examples

WhatsApp

User <--> Group

Food Delivery

Customer <--> Restaurant

Cardinality

Association usually has cardinality.

One-to-One

Person <--> Passport

One passport.

One person.

One-to-Many

Customer <--> Orders

One customer.

Many orders.

Many-to-Many

Student <--> Course

Many students.

Many courses.

Relationship 2 – Aggregation

Aggregation means:

Whole-Part Relationship

but

Part can exist independently.

Example:

Team --> Players

A team contains players.

But players can exist without team.

Definition

Aggregation is:

A whole-part relationship where parts can exist independently of the whole.

Diagram:

Team ◇──── Player

Empty White diamond

Code:

class Player {}

class Team {
   players: Player[];
}

If team is deleted:

Players survive.

Real World Example

Department

Department
     |
     +---- Employees

Delete department.

Employees still exist.

Lifecycle

Department destroyed:

Department dies

Employees:

Still alive

Aggregation Rule

Ask:

If parent dies,
does child survive?

If yes:

Aggregation

Relationship 3 – Composition

Composition is stronger.

Meaning:

Part cannot exist without whole.

Example:

House ---> Room

Room belongs to house.

Destroy house.

Room also disappears.

Definition

Composition is:

A strong ownership relationship where parts cannot exist independently.

Diagram:

House ◆──── Room

Filled Black diamond

Code:

class Room {}

class House {
    rooms: Room[];
}

Difference:

Aggregation:
			Parent removed
			Child survives
			
Composition:
			Parent removed
			Child removed

Real Example

Order

Order
   |
   +---- OrderItems

OrderItem has no meaning outside Order.

Composition.

Example:

class Order {
   items: OrderItem[];
}

Delete Order.

OrderItems disappear.

Aggregation vs Composition

Aggregation:

Department → Employee

Team → Player

Library → Book

Child survives.

Composition:

Order → OrderItem

House → Room

Invoice → InvoiceLine

Child dies with parent.

Comparison Table

Easy Memory Trick

Ask:

Can child live alone?

YES

Aggregation

NO

Composition

Relationship 4 – Dependency

Weakest relationship.

Meaning:

Object temporarily uses another object.

Example:

class PaymentService {
	process(order: Order) {}
}

PaymentService uses Order.

But doesn't own it.

Dependency.

Definition

Dependency is:

A relationship where one object temporarily relies on another.

Diagram:

PaymentService -----> Order

Characteristics:

Short-lived
Temporary
No ownership

Dependency means:

Uses
Needs
Calls

but not:

Contains
Owns

Real Example

class EmailService
{
public:
    void sendEmail()
    {
    }
};

class PaymentService
{
public:

    void processPayment(
        EmailService& email)
    {
        email.sendEmail();
    }
};

Temporary usage. No ownership.

Why Dependency Matters

Because dependency creates coupling.

Bad:

class UserService {
    private paymentService =
        new RazorpayService();
}

Strong dependency.

Good:

class UserService {
   constructor(
      paymentService:
      PaymentService
   ) {}
}

Dependency Injection.

Relationship 5 – Inheritance

Most abused relationship.

Meaning:

IS-A

Example:

Dog IS-A Animal

Code:

class Animal {}

class Dog extends Animal {}

Inheritance.

Diagram:

        Animal
           ▲
           |
      +----+----+
      |         |
     Dog       Cat

Valid Examples

Car IS-A Vehicle

Dog IS-A Animal

Admin IS-A User

Invalid Examples

Engine IS-A Card

No

Engine is part of car.

Composition.

Inheritance Test

Ask:

Can I replace “is a”?

Example:

Dog is an Animal

Valid.

Example:

Engine is a Car

Invalid.

No inheritance.

The Inheritance Trap

Beginners overuse inheritance.

Bad:

class Vehicle {}

class Car extends Vehicle {}

class ElectricCar extends Car {}

class Tesla extends ElectricCar {}

class ModelS extends Tesla {}

Deep inheritance tree.

Nightmare.

Modern design prefers:

Composition over Inheritance

Real Example:

Bad:

class Bird {
   fly()
}

Then:

class Penguin extends Bird {}

Problem:

Penguins cannot fly.

Inheritance broken.

This violates:

Liskov Substitution Principle

Relationship identification Framework

When modeling systems ask these questions:

Question 1:

Is-A?

If yes:

Inheritance

Question 2:

Whole-Part?

If yes:

Continue.

Question 3:

Can part exist independently?

If yes:

Aggregation

If no:

Composition

Question 4:

Does object simply know another?

If yes:

Association

Question 5

Does object merely use another?

If yes:

Dependency

Relationship Selection Framework

When modeling objects ask:

Question 1

Is it IS-A?

Yes

Inheritance

Question 2

Is it Part-Of?

Yes, then either Aggregation or Composition.

Ask:

Can child survive?

Yes

Aggregation

No

Composition

Question 3

Does object merely know another object?

Association

Question 4

Temporarily uses?

Dependency

Golden Rule of Professional LLD

When in doubt:

Prefer Composition

Over:

Inheritance

This single rule prevents many design disasters.

Relationship Strength Spectrum

Weakest
   |
   v

Dependency

Association

Aggregation

Composition

Inheritance*

   ^
   |
Strongest

(*Inheritance is conceptually different but often forms strong coupling.)