System Design ( Day - 42 )

OOP & UML Revision: From Machine Code to Modeling

1. OOP Evolution: Why We Needed Objects

1. Machine Language (0s & 1s):

   • Tedious, error-prone, unscalable.

2. Assembly Language:

   • Mnemonics replace bits, but still low-level & brittle.

3. Procedural Programming:

   • Functions, loops, blocks—but no real-world modeling, limited reuse.

4. Object-Oriented Programming (OOP):

   • Models real-world entities as objects with state (variables) and behavior (methods).

   • Boosts data security, scalability, and reusability.

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2. Four Pillars of OOP

1. Abstraction 🕹️

   • Hide complexity: Use a TV remote without knowing its circuits.

   • In code: Expose only necessary methods; hide implementation details.

2. Encapsulation 🔐

   • Bundle data & behavior: A Class is a blueprint; access modifiers (public, private, protected) guard internal state.

   • Protect invariants: Prevent outside code from corrupting your object’s state.

3. Inheritance 🚗

   • Reuse common features: Car → ManualCar & AutomaticCar inherit start(), stop(), accelerate().

   • “is-a” relationship: Child classes extend parent capabilities without duplication.

4. Polymorphism 🦆🐯

   • Runtime (Overriding): run() behaves differently for Duck, Tiger, Human.

   • Compile-time (Overloading): run() vs run(speed) in Human—same method name, different signatures.

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3. UML Diagrams: Visualizing Structure & Behavior

UML is the lingua franca for system design. Two essentials:

A. Class Diagrams (Static Structure)

• Classes & Attributes

• Associations:

  • Inheritance (is-a): Parent —▷ Child

  • Simple (uses-a): A —> B

  • Aggregation (has-a, loose): A —<> B

  • Composition (has-a, tight): A —◇ B

B. Sequence Diagrams (Dynamic Behavior)

• Lifelines: Object lifespans

• Activation Bars: When an object is active

• Messages:

  • Synchronous (→) vs Asynchronous (–>)

• Special Messages: Create, Destroy, Lost, Found

Example: ATM Withdrawal

1. User inserts card → ATM

2. ATM verifies PIN → Transaction

3. Transaction checks Account → CashDispenser

4. CashDispenser dispenses cash → User