📘 OO Patterns (Object-Oriented Design Patterns)

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🔹 1. Definition

Object-Oriented (OO) Patterns are reusable solutions to common software design problems that occur in object-oriented systems.

👉 Simple idea: Instead of solving the same design problem again and again, we use a proven template (pattern).

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🔹 2. Why OO Patterns are Important ⭐

• Reusable solutions (save time)
• Improve code quality
• Reduce design errors
• Make system easier to maintain
• Encourage best practices

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🔹 3. Key Idea

A design pattern is NOT code. It is a:

"Blueprint for solving a recurring design problem"

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🔹 4. Types of OO Design Patterns

Design patterns are mainly divided into 3 categories:

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🔷 4.1 Creational Patterns (Object Creation)

👉 These patterns deal with how objects are created

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🔸 1. Singleton Pattern ⭐

Definition: Ensures only one instance of a class exists.

Example:

• Database connection
• Printer spooler

class Database {
  private static Database instance;
}

👉 Only one object is shared globally.

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🔸 2. Factory Pattern

Definition: Creates objects without exposing creation logic.

Example: Car factory → produces different cars

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🔸 3. Builder Pattern

Definition: Builds complex objects step by step.

Example: Burger customization (bun, cheese, sauce)

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🔷 4.2 Structural Patterns (Class/Object Structure)

👉 These patterns deal with how classes and objects are structured

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🔸 1. Adapter Pattern ⭐

Definition: Converts one interface into another compatible interface.

Example: Mobile charger adapter

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🔸 2. Decorator Pattern

Definition: Adds new behavior to an object without changing its structure.

Example: Adding toppings to pizza 🍕

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🔸 3. Facade Pattern

Definition: Provides a simple interface to a complex system.

Example: ATM hides banking system complexity

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🔷 4.3 Behavioral Patterns (Object Communication)

👉 These patterns deal with how objects interact and communicate

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🔸 1. Observer Pattern ⭐

Definition: One-to-many relationship where one object notifies others.

Example: YouTube channel subscription 🔔

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🔸 2. Strategy Pattern

Definition: Defines multiple algorithms and selects one at runtime.

Example: Payment methods (Card, UPI, Cash)

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🔸 3. Command Pattern

Definition: Encapsulates a request as an object.

Example: Remote control buttons 📺

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🔹 5. Real-Life Example of Patterns ⭐

🎯 ATM System

• Singleton → ATM machine instance
• Strategy → Withdrawal methods (cash/card)
• Facade → ATM hides banking system complexity

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🔹 6. Benefits of OO Patterns

• Reusability
• Flexibility
• Better communication among developers
• Faster development
• Standardized solutions

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🔹 7. Problems Without Patterns

• Duplicate code
• Poor structure
• Difficult maintenance
• Hard to scale system

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🔹 8. Important Rules / Guidelines

• Use patterns only when needed
• Do NOT overuse patterns
• Choose correct pattern for correct problem
• Keep design simple

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🔹 9. Pattern vs Algorithm

Feature	Design Pattern	Algorithm
Purpose	Design solution	Step-by-step logic
Level	High-level design	Low-level implementation
Focus	Structure & interaction	Computation

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🔹 10. Likely Exam Questions

1. Define OO design patterns.
2. Why are design patterns important?
3. Explain Singleton pattern.
4. What is Factory pattern?
5. Explain Adapter pattern with example.
6. What is Observer pattern?
7. Difference between design pattern and algorithm.
8. Types of design patterns.
9. Explain Strategy pattern.
10. Real-life applications of OO patterns.

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🔹 11. Quick Revision Summary 🧠

🔹 OO Patterns = Reusable design solutions

🔹 Types:

• Creational → Object creation
• Structural → Object structure
• Behavioral → Object interaction

👉 Trick to Remember: "CSB" → Creational, Structural, Behavioral

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