Abstraction — Define contracts, hide complexity

Callers talk to a small surface; complexity lives behind it. · Interfaces / abstract classes document the contract.

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In code

Shape abstractionTypeScript

interface Shape {
  area(): number;
}

class Circle implements Shape {
  constructor(private r: number) {}
  area() {
    return Math.PI * this.r * this.r;
  }
}

function totalArea(shapes: Shape[]) {
  return shapes.reduce((s, x) => s + x.area(), 0);
}

📘 Key ideas

What it is

Hiding implementation complexity behind a simple interface. Callers know WHAT an object does, not HOW it does it.

Abstract class vs Interface

Use an abstract class when subclasses share code. Use an interface when you only need a contract (no shared state or logic).

Example

A Shape interface declares area() and perimeter(). Circle, Square, and Triangle implement it differently — the caller doesn't care.

Benefit

You can swap implementations (e.g. MySQL → Postgres) without touching callers — they only depend on the abstraction.

🧠 Practice — Apply What You Learned

Abstraction: User – Account – Transaction

Design a banking model where User holds multiple Accounts (Savings, Current, Fixed Deposit…

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