Defining Relationships - Learning Module

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Relationship Types Review

The Invisible Architecture of Software

In object-oriented design, classes are the nouns—but relationships are the verbs that give your system meaning. A perfectly designed class in isolation is useless; it's the connections between classes that enable behavior, enforce constraints, and ultimately determine whether your system is robust or fragile.

Consider a banking system: a Customer class means nothing without its relationship to Account. An Account without connection to Transaction cannot track history. The relationships define not just structure, but the very semantics of what operations are possible and how data flows through your system.

What You Will Learn

By the end of this page, you will have a comprehensive understanding of all major relationship types in OOP—association, aggregation, composition, dependency, inheritance, and realization. You'll know when to use each, how they differ semantically, and how to represent them in code and UML.

Why Relationships Matter in LLD

Relationships in software design serve multiple critical purposes that directly impact system quality:

1. Semantic Precision Relationships encode meaning. The difference between "a Car has an Engine" (composition) versus "a Car uses a GasStation" (dependency) affects how we reason about the system, what invariants we can guarantee, and how the system evolves.

2. Lifecycle Management Relationships determine object lifecycles. In composition, destroying the whole destroys the parts. In aggregation, parts outlive the container. Getting this wrong leads to memory leaks, dangling references, or inconsistent state.

3. Coupling Control Different relationship types create different levels of coupling. Dependency is weaker than association, which is weaker than inheritance. Choosing the right relationship type directly affects maintainability and testability.

4. API Design Relationships dictate your public interfaces. Should Order.getCustomer() return a full Customer object or just a customer ID? The relationship type guides this decision.

The Cost of Wrong Relationships

Misidentifying relationships is one of the most expensive design mistakes. Using inheritance when composition is appropriate leads to fragile base class problems. Using composition when aggregation is correct creates artificial lifecycle constraints. These mistakes compound as systems grow, becoming increasingly expensive to fix.

Association: The General Relationship

Association is the most general form of relationship between classes. It represents a structural connection where objects of one class are connected to objects of another class. Association implies that objects know about each other and can interact.

Key Characteristics:

Both objects can exist independently
Neither object owns the other
The relationship can be temporary or permanent
Navigation can be unidirectional or bidirectional

Real-World Examples:

A Student is associated with a Course (enrolled in)
A Doctor is associated with a Patient (treats)
An Employee is associated with a Project (works on)

association-example
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// Association: Student and Course exist independently
// A student can enroll in courses; courses can have students
 
class Course {
    private courseName: string;
    private enrolledStudents: Student[] = [];
    
    constructor(courseName: string) {
        this.courseName = courseName;
    }
    
    enrollStudent(student: Student): void {
        this.enrolledStudents.push(student);
        // Note: We don't create the student - they exist independently
    }
    
    getEnrolledStudents(): Student[] {
        return [...this.enrolledStudents];
    }
}
 
class Student {
    private studentId: string;
    private enrolledCourses: Course[] = [];
    
    constructor(studentId: string) {
        this.studentId = studentId;
    }
    
    enrollInCourse(course: Course): void {
        this.enrolledCourses.push(course);
        course.enrollStudent(this);
    }
    
    // Student continues to exist even if dropped from all courses
    dropAllCourses(): void {
        this.enrolledCourses = [];
    }
}
 
// Usage: Both objects have independent lifecycles
const student = new Student("S001");
const course = new Course("Data Structures");
student.enrollInCourse(course);
 
// Deleting the course doesn't delete the student
// Deleting the student doesn't delete the course

UML Representation: Association is represented by a solid line connecting two classes. Role names and multiplicities can appear at each end.

Aggregation: The 'Has-A' Relationship

Aggregation is a specialized form of association that represents a whole-part relationship where parts can exist independently of the whole. It's often called a "has-a" relationship with shared ownership.

Key Characteristics:

Whole contains or aggregates parts
Parts have their own lifecycle
Parts can belong to multiple wholes simultaneously
Destroying the whole does NOT destroy the parts

Real-World Examples:

A Department has Employees (employees can move between departments)
A Playlist has Songs (songs exist in multiple playlists)
A Team has Players (players can be traded)

aggregation-example
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// Aggregation: Playlist aggregates Songs
// Songs exist independently and can belong to multiple playlists
 
class Song {
    constructor(
        public readonly id: string,
        public readonly title: string,
        public readonly artist: string,
        public readonly durationSeconds: number
    ) {}
}
 
class Playlist {
    private name: string;
    private songs: Song[] = [];
    
    constructor(name: string) {
        this.name = name;
    }
    
    // Aggregation: we receive an existing song, we don't create it
    addSong(song: Song): void {
        if (!this.songs.find(s => s.id === song.id)) {
            this.songs.push(song);
        }
    }
    
    removeSong(songId: string): void {
        this.songs = this.songs.filter(s => s.id !== songId);
        // Note: Song still exists after removal from playlist
    }
    
    getTotalDuration(): number {
        return this.songs.reduce((sum, song) => sum + song.durationSeconds, 0);
    }
}
 
// Usage: Songs have independent lifecycles
const song1 = new Song("1", "Bohemian Rhapsody", "Queen", 354);
const song2 = new Song("2", "Stairway to Heaven", "Led Zeppelin", 482);
 
const rockPlaylist = new Playlist("Classic Rock");
const favoritesPlaylist = new Playlist("My Favorites");
 
// Same song in multiple playlists (shared ownership)
rockPlaylist.addSong(song1);
favoritesPlaylist.addSong(song1);
 
// Deleting playlist doesn't delete songs
// Songs persist across playlist deletions

UML Notation

Aggregation is represented by a line with an empty diamond on the whole side. The diamond points to the container/whole, indicating 'this class aggregates that class.'

Composition: Strong Ownership

Composition is a strong form of aggregation where parts cannot exist without the whole. It represents exclusive ownership with coupled lifecycles.

Key Characteristics:

Parts are created and destroyed with the whole
Parts cannot be shared between wholes
The whole is responsible for part lifecycle
Destroying the whole automatically destroys all parts

Real-World Examples:

A House is composed of Rooms (rooms don't exist outside a house)
An Order is composed of OrderItems (items belong to one order)
A Car is composed of an Engine (engine is part of exactly one car)

composition-example
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// Composition: Order owns OrderItems exclusively
// OrderItems cannot exist without their parent Order
 
class OrderItem {
    constructor(
        public readonly productId: string,
        public readonly productName: string,
        public readonly quantity: number,
        public readonly unitPrice: number
    ) {}
    
    getSubtotal(): number {
        return this.quantity * this.unitPrice;
    }
}
 
class Order {
    private readonly orderId: string;
    private readonly items: OrderItem[] = [];
    private readonly createdAt: Date;
    
    constructor(orderId: string) {
        this.orderId = orderId;
        this.createdAt = new Date();
    }
    
    // Composition: We CREATE the OrderItem here
    // It's not passed in from outside
    addItem(productId: string, name: string, qty: number, price: number): void {
        const item = new OrderItem(productId, name, qty, price);
        this.items.push(item);
    }
    
    getTotal(): number {
        return this.items.reduce((sum, item) => sum + item.getSubtotal(), 0);
    }
    
    getItems(): ReadonlyArray<OrderItem> {
        return this.items;
    }
    
    // When order is cancelled/deleted, all items are gone
    // There's no separate "delete item and it still exists" operation
}
 
// Usage demonstrates exclusive ownership
const order = new Order("ORD-001");
order.addItem("PROD-1", "Keyboard", 2, 49.99);
order.addItem("PROD-2", "Mouse", 1, 29.99);
 
// Items are PART OF the order
// If order is deleted, items are deleted
// Items cannot be "moved" to another order

UML Notation

Composition is represented by a line with a filled diamond on the whole side. The filled diamond indicates strong ownership—the whole controls the lifecycle of the parts.

Dependency: The 'Uses-A' Relationship

Dependency is the weakest form of relationship. It exists when one class uses another class without maintaining a persistent reference. The relationship is transient—it exists only during method execution.

Key Characteristics:

No persistent reference between classes
Used class appears as method parameter, local variable, or return type
Changes to the used class may affect the using class
Represents a "uses" semantic

Real-World Examples:

A ReportGenerator depends on DateFormatter (uses it to format dates)
An OrderService depends on EmailSender (uses it to send confirmations)
A PaymentProcessor depends on CurrencyConverter (uses it for conversion)

dependency-example
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// Dependency: ReportGenerator uses DateFormatter
// No persistent reference - just uses it when needed
 
interface DateFormatter {
    format(date: Date): string;
}
 
class ISODateFormatter implements DateFormatter {
    format(date: Date): string {
        return date.toISOString();
    }
}
 
class HumanReadableDateFormatter implements DateFormatter {
    format(date: Date): string {
        return date.toLocaleDateString('en-US', {
            weekday: 'long',
            year: 'numeric',
            month: 'long',
            day: 'numeric'
        });
    }
}
 
class ReportGenerator {
    // No field storing DateFormatter - that would be association
    
    // Dependency: DateFormatter is received as parameter
    generateReport(data: ReportData, formatter: DateFormatter): string {
        const formattedDate = formatter.format(new Date());
        return `Report generated on: ${formattedDate}\n${data.content}`;
    }
    
    // Another form of dependency: using a static method
    generateQuickReport(data: ReportData): string {
        // Depends on Date class
        const timestamp = new Date().getTime();
        return `Quick Report [${timestamp}]: ${data.summary}`;
    }
}
 
// Dependency allows flexibility - different formatters can be used
const generator = new ReportGenerator();
const data = { content: "Sales figures...", summary: "Q4 Report" };
 
const isoReport = generator.generateReport(data, new ISODateFormatter());
const readableReport = generator.generateReport(data, new HumanReadableDateFormatter());

UML Notation

Dependency is represented by a dashed arrow pointing from the dependent class to the class it depends on. The arrow indicates 'this class depends on that class.'

Inheritance: The 'Is-A' Relationship

Inheritance represents a generalization/specialization relationship. A subclass "is-a" type of its superclass, inheriting its structure and behavior while adding or overriding capabilities.

Key Characteristics:

Represents an "is-a" semantic (a Dog is-a Animal)
Subclass inherits fields and methods from superclass
Subclass can override behavior
Creates the strongest coupling between classes
Enables polymorphism

When to Use Inheritance:

True is-a relationship (not just code reuse)
Subclass is a proper subtype (Liskov Substitution Principle)
Shared behavior is fundamental, not incidental

inheritance-example
TypeScript
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// Inheritance: Different payment methods inherit from base
abstract class PaymentMethod {
    protected readonly id: string;
    protected readonly createdAt: Date;
    
    constructor(id: string) {
        this.id = id;
        this.createdAt = new Date();
    }
    
    abstract processPayment(amount: number): Promise<PaymentResult>;
    abstract validate(): boolean;
    
    // Shared behavior
    getPaymentMethodId(): string {
        return this.id;
    }
}
 
class CreditCard extends PaymentMethod {
    constructor(
        id: string,
        private cardNumber: string,
        private expiryMonth: number,
        private expiryYear: number,
        private cvv: string
    ) {
        super(id);
    }
    
    validate(): boolean {
        const now = new Date();
        const expiry = new Date(this.expiryYear, this.expiryMonth - 1);
        return expiry > now && this.cardNumber.length === 16;
    }
    
    async processPayment(amount: number): Promise<PaymentResult> {
        // Credit card specific processing
        return { success: true, transactionId: `CC-${Date.now()}` };
    }
    
    // Credit card specific method
    getMaskedNumber(): string {
        return `****-****-****-${this.cardNumber.slice(-4)}`;
    }
}
 
class BankTransfer extends PaymentMethod {
    constructor(
        id: string,
        private routingNumber: string,
        private accountNumber: string
    ) {
        super(id);
    }
    
    validate(): boolean {
        return this.routingNumber.length === 9;
    }
    
    async processPayment(amount: number): Promise<PaymentResult> {
        // Bank transfer specific processing
        return { success: true, transactionId: `BT-${Date.now()}` };
    }
}

Inheritance Pitfalls

Inheritance creates the tightest coupling. Favor composition over inheritance when possible. Use inheritance only for true is-a relationships where substitutability is guaranteed. Overusing inheritance leads to fragile base class problems and rigid hierarchies.

Realization: Implementing Interfaces

Realization (or Implementation) represents the relationship between an interface and a class that implements it. The class agrees to fulfill the contract defined by the interface.

Key Characteristics:

Interface defines the contract (what)
Implementing class provides the behavior (how)
Enables loose coupling through abstraction
Supports multiple interface implementation
Foundation for dependency inversion

realization-example
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// Realization: Classes implement the Repository interface
 
interface Repository<T, ID> {
    findById(id: ID): Promise<T | null>;
    findAll(): Promise<T[]>;
    save(entity: T): Promise<T>;
    delete(id: ID): Promise<boolean>;
}
 
// PostgreSQL implementation
class PostgresUserRepository implements Repository<User, string> {
    constructor(private pool: Pool) {}
    
    async findById(id: string): Promise<User | null> {
        const result = await this.pool.query(
            'SELECT * FROM users WHERE id = $1',
            [id]
        );
        return result.rows[0] ?? null;
    }
    
    async findAll(): Promise<User[]> {
        const result = await this.pool.query('SELECT * FROM users');
        return result.rows;
    }
    
    async save(user: User): Promise<User> {
        // PostgreSQL-specific save logic
        return user;
    }
    
    async delete(id: string): Promise<boolean> {
        const result = await this.pool.query(
            'DELETE FROM users WHERE id = $1',
            [id]
        );
        return result.rowCount > 0;
    }
}
 
// In-memory implementation for testing
class InMemoryUserRepository implements Repository<User, string> {
    private users: Map<string, User> = new Map();
    
    async findById(id: string): Promise<User | null> {
        return this.users.get(id) ?? null;
    }
    
    async findAll(): Promise<User[]> {
        return Array.from(this.users.values());
    }
    
    async save(user: User): Promise<User> {
        this.users.set(user.id, user);
        return user;
    }
    
    async delete(id: string): Promise<boolean> {
        return this.users.delete(id);
    }
}

Summary: Relationship Types at a Glance

Comparison of Relationship Types
Relationship	Coupling	Lifecycle	UML Symbol	Example
Association	Medium	Independent	Solid line	Student ↔ Course
Aggregation	Medium	Independent	Empty diamond	Playlist ◇→ Song
Composition	Strong	Coupled	Filled diamond	Order ◆→ OrderItem
Dependency	Weak	Transient	Dashed arrow	Service ⇢ Helper
Inheritance	Strongest	Coupled	Hollow arrow	CreditCard ▷ PaymentMethod
Realization	Medium	Independent	Dashed hollow arrow	Class ⇢▷ Interface

Page Complete

You now have a comprehensive understanding of all major relationship types in OOP. In the next page, we'll dive deep into cardinality and multiplicity—how to specify how many objects participate in each relationship.

Relationship Types Review

The Invisible Architecture of Software

What You Will Learn

Why Relationships Matter in LLD

Relationships in software design serve multiple critical purposes that directly impact system quality:

4. API Design Relationships dictate your public interfaces. Should Order.getCustomer() return a full Customer object or just a customer ID? The relationship type guides this decision.

The Cost of Wrong Relationships

Association: The General Relationship

Key Characteristics:

Both objects can exist independently
Neither object owns the other
The relationship can be temporary or permanent
Navigation can be unidirectional or bidirectional

Real-World Examples:

A Student is associated with a Course (enrolled in)
A Doctor is associated with a Patient (treats)
An Employee is associated with a Project (works on)

association-example
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// Association: Student and Course exist independently
// A student can enroll in courses; courses can have students
 
class Course {
    private courseName: string;
    private enrolledStudents: Student[] = [];
    
    constructor(courseName: string) {
        this.courseName = courseName;
    }
    
    enrollStudent(student: Student): void {
        this.enrolledStudents.push(student);
        // Note: We don't create the student - they exist independently
    }
    
    getEnrolledStudents(): Student[] {
        return [...this.enrolledStudents];
    }
}
 
class Student {
    private studentId: string;
    private enrolledCourses: Course[] = [];
    
    constructor(studentId: string) {
        this.studentId = studentId;
    }
    
    enrollInCourse(course: Course): void {
        this.enrolledCourses.push(course);
        course.enrollStudent(this);
    }
    
    // Student continues to exist even if dropped from all courses
    dropAllCourses(): void {
        this.enrolledCourses = [];
    }
}
 
// Usage: Both objects have independent lifecycles
const student = new Student("S001");
const course = new Course("Data Structures");
student.enrollInCourse(course);
 
// Deleting the course doesn't delete the student
// Deleting the student doesn't delete the course

UML Representation: Association is represented by a solid line connecting two classes. Role names and multiplicities can appear at each end.

Aggregation: The 'Has-A' Relationship

Key Characteristics:

Whole contains or aggregates parts
Parts have their own lifecycle
Parts can belong to multiple wholes simultaneously
Destroying the whole does NOT destroy the parts

Real-World Examples:

A Department has Employees (employees can move between departments)
A Playlist has Songs (songs exist in multiple playlists)
A Team has Players (players can be traded)

aggregation-example
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// Aggregation: Playlist aggregates Songs
// Songs exist independently and can belong to multiple playlists
 
class Song {
    constructor(
        public readonly id: string,
        public readonly title: string,
        public readonly artist: string,
        public readonly durationSeconds: number
    ) {}
}
 
class Playlist {
    private name: string;
    private songs: Song[] = [];
    
    constructor(name: string) {
        this.name = name;
    }
    
    // Aggregation: we receive an existing song, we don't create it
    addSong(song: Song): void {
        if (!this.songs.find(s => s.id === song.id)) {
            this.songs.push(song);
        }
    }
    
    removeSong(songId: string): void {
        this.songs = this.songs.filter(s => s.id !== songId);
        // Note: Song still exists after removal from playlist
    }
    
    getTotalDuration(): number {
        return this.songs.reduce((sum, song) => sum + song.durationSeconds, 0);
    }
}
 
// Usage: Songs have independent lifecycles
const song1 = new Song("1", "Bohemian Rhapsody", "Queen", 354);
const song2 = new Song("2", "Stairway to Heaven", "Led Zeppelin", 482);
 
const rockPlaylist = new Playlist("Classic Rock");
const favoritesPlaylist = new Playlist("My Favorites");
 
// Same song in multiple playlists (shared ownership)
rockPlaylist.addSong(song1);
favoritesPlaylist.addSong(song1);
 
// Deleting playlist doesn't delete songs
// Songs persist across playlist deletions

UML Notation

Aggregation is represented by a line with an empty diamond on the whole side. The diamond points to the container/whole, indicating 'this class aggregates that class.'

Composition: Strong Ownership

Composition is a strong form of aggregation where parts cannot exist without the whole. It represents exclusive ownership with coupled lifecycles.

Key Characteristics:

Parts are created and destroyed with the whole
Parts cannot be shared between wholes
The whole is responsible for part lifecycle
Destroying the whole automatically destroys all parts

Real-World Examples:

A House is composed of Rooms (rooms don't exist outside a house)
An Order is composed of OrderItems (items belong to one order)
A Car is composed of an Engine (engine is part of exactly one car)

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// Composition: Order owns OrderItems exclusively
// OrderItems cannot exist without their parent Order
 
class OrderItem {
    constructor(
        public readonly productId: string,
        public readonly productName: string,
        public readonly quantity: number,
        public readonly unitPrice: number
    ) {}
    
    getSubtotal(): number {
        return this.quantity * this.unitPrice;
    }
}
 
class Order {
    private readonly orderId: string;
    private readonly items: OrderItem[] = [];
    private readonly createdAt: Date;
    
    constructor(orderId: string) {
        this.orderId = orderId;
        this.createdAt = new Date();
    }
    
    // Composition: We CREATE the OrderItem here
    // It's not passed in from outside
    addItem(productId: string, name: string, qty: number, price: number): void {
        const item = new OrderItem(productId, name, qty, price);
        this.items.push(item);
    }
    
    getTotal(): number {
        return this.items.reduce((sum, item) => sum + item.getSubtotal(), 0);
    }
    
    getItems(): ReadonlyArray<OrderItem> {
        return this.items;
    }
    
    // When order is cancelled/deleted, all items are gone
    // There's no separate "delete item and it still exists" operation
}
 
// Usage demonstrates exclusive ownership
const order = new Order("ORD-001");
order.addItem("PROD-1", "Keyboard", 2, 49.99);
order.addItem("PROD-2", "Mouse", 1, 29.99);
 
// Items are PART OF the order
// If order is deleted, items are deleted
// Items cannot be "moved" to another order

UML Notation

Composition is represented by a line with a filled diamond on the whole side. The filled diamond indicates strong ownership—the whole controls the lifecycle of the parts.

Dependency: The 'Uses-A' Relationship

Key Characteristics:

No persistent reference between classes
Used class appears as method parameter, local variable, or return type
Changes to the used class may affect the using class
Represents a "uses" semantic

Real-World Examples:

A ReportGenerator depends on DateFormatter (uses it to format dates)
An OrderService depends on EmailSender (uses it to send confirmations)
A PaymentProcessor depends on CurrencyConverter (uses it for conversion)

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// Dependency: ReportGenerator uses DateFormatter
// No persistent reference - just uses it when needed
 
interface DateFormatter {
    format(date: Date): string;
}
 
class ISODateFormatter implements DateFormatter {
    format(date: Date): string {
        return date.toISOString();
    }
}
 
class HumanReadableDateFormatter implements DateFormatter {
    format(date: Date): string {
        return date.toLocaleDateString('en-US', {
            weekday: 'long',
            year: 'numeric',
            month: 'long',
            day: 'numeric'
        });
    }
}
 
class ReportGenerator {
    // No field storing DateFormatter - that would be association
    
    // Dependency: DateFormatter is received as parameter
    generateReport(data: ReportData, formatter: DateFormatter): string {
        const formattedDate = formatter.format(new Date());
        return `Report generated on: ${formattedDate}\n${data.content}`;
    }
    
    // Another form of dependency: using a static method
    generateQuickReport(data: ReportData): string {
        // Depends on Date class
        const timestamp = new Date().getTime();
        return `Quick Report [${timestamp}]: ${data.summary}`;
    }
}
 
// Dependency allows flexibility - different formatters can be used
const generator = new ReportGenerator();
const data = { content: "Sales figures...", summary: "Q4 Report" };
 
const isoReport = generator.generateReport(data, new ISODateFormatter());
const readableReport = generator.generateReport(data, new HumanReadableDateFormatter());

UML Notation

Dependency is represented by a dashed arrow pointing from the dependent class to the class it depends on. The arrow indicates 'this class depends on that class.'

Inheritance: The 'Is-A' Relationship

Inheritance represents a generalization/specialization relationship. A subclass "is-a" type of its superclass, inheriting its structure and behavior while adding or overriding capabilities.

Key Characteristics:

Represents an "is-a" semantic (a Dog is-a Animal)
Subclass inherits fields and methods from superclass
Subclass can override behavior
Creates the strongest coupling between classes
Enables polymorphism

When to Use Inheritance:

True is-a relationship (not just code reuse)
Subclass is a proper subtype (Liskov Substitution Principle)
Shared behavior is fundamental, not incidental

inheritance-example
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// Inheritance: Different payment methods inherit from base
abstract class PaymentMethod {
    protected readonly id: string;
    protected readonly createdAt: Date;
    
    constructor(id: string) {
        this.id = id;
        this.createdAt = new Date();
    }
    
    abstract processPayment(amount: number): Promise<PaymentResult>;
    abstract validate(): boolean;
    
    // Shared behavior
    getPaymentMethodId(): string {
        return this.id;
    }
}
 
class CreditCard extends PaymentMethod {
    constructor(
        id: string,
        private cardNumber: string,
        private expiryMonth: number,
        private expiryYear: number,
        private cvv: string
    ) {
        super(id);
    }
    
    validate(): boolean {
        const now = new Date();
        const expiry = new Date(this.expiryYear, this.expiryMonth - 1);
        return expiry > now && this.cardNumber.length === 16;
    }
    
    async processPayment(amount: number): Promise<PaymentResult> {
        // Credit card specific processing
        return { success: true, transactionId: `CC-${Date.now()}` };
    }
    
    // Credit card specific method
    getMaskedNumber(): string {
        return `****-****-****-${this.cardNumber.slice(-4)}`;
    }
}
 
class BankTransfer extends PaymentMethod {
    constructor(
        id: string,
        private routingNumber: string,
        private accountNumber: string
    ) {
        super(id);
    }
    
    validate(): boolean {
        return this.routingNumber.length === 9;
    }
    
    async processPayment(amount: number): Promise<PaymentResult> {
        // Bank transfer specific processing
        return { success: true, transactionId: `BT-${Date.now()}` };
    }
}

Inheritance Pitfalls

Realization: Implementing Interfaces

Realization (or Implementation) represents the relationship between an interface and a class that implements it. The class agrees to fulfill the contract defined by the interface.

Key Characteristics:

Interface defines the contract (what)
Implementing class provides the behavior (how)
Enables loose coupling through abstraction
Supports multiple interface implementation
Foundation for dependency inversion

realization-example
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// Realization: Classes implement the Repository interface
 
interface Repository<T, ID> {
    findById(id: ID): Promise<T | null>;
    findAll(): Promise<T[]>;
    save(entity: T): Promise<T>;
    delete(id: ID): Promise<boolean>;
}
 
// PostgreSQL implementation
class PostgresUserRepository implements Repository<User, string> {
    constructor(private pool: Pool) {}
    
    async findById(id: string): Promise<User | null> {
        const result = await this.pool.query(
            'SELECT * FROM users WHERE id = $1',
            [id]
        );
        return result.rows[0] ?? null;
    }
    
    async findAll(): Promise<User[]> {
        const result = await this.pool.query('SELECT * FROM users');
        return result.rows;
    }
    
    async save(user: User): Promise<User> {
        // PostgreSQL-specific save logic
        return user;
    }
    
    async delete(id: string): Promise<boolean> {
        const result = await this.pool.query(
            'DELETE FROM users WHERE id = $1',
            [id]
        );
        return result.rowCount > 0;
    }
}
 
// In-memory implementation for testing
class InMemoryUserRepository implements Repository<User, string> {
    private users: Map<string, User> = new Map();
    
    async findById(id: string): Promise<User | null> {
        return this.users.get(id) ?? null;
    }
    
    async findAll(): Promise<User[]> {
        return Array.from(this.users.values());
    }
    
    async save(user: User): Promise<User> {
        this.users.set(user.id, user);
        return user;
    }
    
    async delete(id: string): Promise<boolean> {
        return this.users.delete(id);
    }
}

Summary: Relationship Types at a Glance

Comparison of Relationship Types
Relationship	Coupling	Lifecycle	UML Symbol	Example
Association	Medium	Independent	Solid line	Student ↔ Course
Aggregation	Medium	Independent	Empty diamond	Playlist ◇→ Song
Composition	Strong	Coupled	Filled diamond	Order ◆→ OrderItem
Dependency	Weak	Transient	Dashed arrow	Service ⇢ Helper
Inheritance	Strongest	Coupled	Hollow arrow	CreditCard ▷ PaymentMethod
Realization	Medium	Independent	Dashed hollow arrow	Class ⇢▷ Interface

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