OOP Concepts Tutorial

Create a Temperature class with a static method fromFahrenheit(f) that returns a new Temperature in Celsius, and a static constant FREEZING = 0.

Create a QueryBuilder class with where(field), orderBy(field) and build() methods. where and orderBy store their values and return this, so you can write new QueryBuilder().where("age").orderBy("name").build().

Create a Temperature class with a private #celsius field. Add a celsius getter, a celsius setter that rejects values below -273.15, and a fahrenheit getter.

Write a TypeScript interface Shape with area(): number, and two classes Circle and Rectangle that implement it. Write a function totalArea(shapes: Shape[]).

Create a Shape base class with a name and an area() method returning 0. Create Circle and Square subclasses that override area(). Log the area of each.

Create Notification classes — Email, Sms and Push — each with a send(message) method that returns a different string. Send the same message through all three in a loop.

Write a JavaScript function area(a, b) that returns the area of a square when called with one number and of a rectangle when called with two.

Create a Shape class with describe() returning "A shape", and a Circle subclass whose describe() returns the parent text plus " that is round". Use super.

Create a base class Animal with a "virtual" describe() that returns "<name> is an animal" and an "abstract" sound() that throws. Create Dog with sound() returning "Woof", then log both methods.

Create an abstract Account class with a balance and an abstract interestRate(). Create SavingsAccount (4%) and FixedDeposit (7%), and a shared method yearlyInterest() = balance × rate / 100.

Define a Logger “interface” with one method log(message). Implement ConsoleLogger (returns "console: msg") and MemoryLogger (stores messages in an array). Write a function process(logger) that logs "started" and works with both.

Model a Bird (abstract class with name and a shared describe()) and a Flyer capability (fly()). Create Sparrow (bird + flyer), Penguin (bird only) and Drone (flyer only, not a bird). Log which of the three can fly.

Create a Playlist that aggregates Song objects (songs exist independently) and a Song that is composed of a private Lyrics object it creates itself. Log the playlist length and the first line of a song’s lyrics.

Build a Character that has a weapon part. Create Sword (attack returns "slashes for 10") and Bow (attack returns "shoots for 7"). Make a character, attack, switch weapon, attack again.

Write a Greeter class that receives a clock object with a getHour() method. greet() returns "Good morning" before 12 and "Good afternoon" otherwise. Test it with two fake clocks.

Identify which SOLID principle each smell violates: (1) a PaymentProcessor with a switch on payment type that grows every month; (2) an IWorker interface with Work() and Eat() that robots must implement; (3) a ReportService that creates new SqlConnection() inside its methods.

This class does two jobs: class Report { constructor(data) { this.data = data; } average() { ... } toCsv() { ... } }. Split it into Report (average) and ReportCsvFormatter (toCsv), then use both.

Make shipping cost extensible: create StandardShipping (₹50) and ExpressShipping (₹150), each with cost(). Write a function orderTotal(amount, shipping) that never needs editing when a new shipping type appears. Then add FreeShipping.

Bird has fly(). Penguin extends Bird and throws in fly() — an LSP violation. Redesign with a Bird base (eat()) and a separate FlyingBird subclass (fly()), so Penguin never promises to fly.

A Worker contract has work() and eat(). A Robot can work but not eat. Split it into Workable and Eatable roles, implement Human (both) and Robot (work only), and run a shift that calls work() on everyone who can work.

Write a PriceService that depends on a rateProvider with getRate(currency). It converts an INR amount into another currency. Use a FixedRateProvider ({ USD: 0.012 }) for a quick test.

Write a tiny Decorator (structural pattern): a function withTimestamp(logger) that returns a new logger whose log(msg) adds "2026-10-28 " in front of the original logger’s output.

Create a Logger singleton with a static getInstance() method and a log(msg) method that stores messages in an array. Log from two “different places” and show both messages are in the same logger.

Create a ShapeFactory.create(type, size) that returns a Circle or Square object with an area() method. Create one of each and log their areas (round the circle to 2 decimals).

Create a TextFormatter that uses a strategy to format a string: UpperCaseStrategy, LowerCaseStrategy and TitleCaseStrategy. Format "hello oop world" with each.

Create a NewsChannel subject with subscribe(fn) and publish(headline). Subscribe two readers that log "Asha read: …" and "Ravi read: …", publish one headline, unsubscribe Ravi, and publish another.

Write a UserRepository (in memory) with add(user), findByEmail(email) and count(). Add two users and look one up by email, ignoring case.

Write a Serializable mixin that adds toJSONString() (returns JSON.stringify(this)) to any class. Apply it to a Product class with name and price and log the result.

Add a FixedDepositAccount (sealed) that never allows withdrawals and pays 7% interest via IInterestBearing. Create one with 10000, try to withdraw 100 and print the interest.

Write a Mermaid class diagram for: a Library has many Books (books can exist without the library); a Member borrows 0..5 Books; StudentMember and StaffMember inherit from Member.

Extend the system without editing any existing class: add a PremiumMember (limit 10) and a NoFine strategy (always 0). Borrow three books with a premium member, return one 10 days late with NoFine, and log the fine.