Lesson 1 · 15 min · 
School guide Classes, objects and state
- Use a wallet to see how an object stores data and controls changes to it. Its balance is state; pay() is behaviour.
- Use a constructor to set a valid starting balance.
- Keep the balance private. Change it through methods that reject invalid payments.
- Distinguish a field shared by the class from a field stored separately in each wallet.
classobjectconstructoraccess modifierstatic
Lesson 2 · 20 min · School guide The four pillars without the textbook
- Encapsulation: the wallet guards its own balance and exposes only the actions that keep it valid.
- Abstraction: callers see pay(amount, toVpa) and none of the bank calls behind it.
- Inheritance: an is-a relationship. Using it only to reuse code is a common reviewer flag.
- Polymorphism: one call, different behaviour. Overloading is decided at compile time, overriding at runtime.
- Explain each concept with a definition, an example and a limitation.
encapsulationabstractioninheritancepolymorphism
Lesson 3 · 20 min · School guide Interfaces, abstract classes and composition
- An interface is a contract: PaymentMethod with pay(). An abstract class is a contract plus shared code.
- Pick an interface when unrelated classes share a capability; pick an abstract class when siblings share real code.
- Composition over inheritance: an Order has a PricingPolicy, so the policy can change at runtime.
- UPI, card and BNPL as PaymentMethod implementations. Adding netbanking touches no existing class.
- Syntax in each language: Java interface and default methods, C++ pure virtual functions, Python abc.
interfaceabstract classcompositionhas-ais-a
Lesson 4 · 15 min · School guide Polymorphism under the hood
- C++: virtual functions, the vtable idea, object slicing when you pass by value, and why base classes need a virtual destructor.
- Java: every non-final, non-static method dispatches at runtime. final and static change that.
- Python: duck typing, so the interface is whatever methods you call.
- Work through five code snippets and explain their output step by step.
virtual functionvtableobject slicingdynamic dispatchduck typing
Lesson 5 · 20 min · School guide Class relationships and UML class diagrams
- Association, aggregation, composition and dependency, all shown on one parking lot.
- Multiplicity: one lot has many floors, one ticket belongs to one vehicle.
- Draw only what helps: class name, key fields, key methods, the arrows. Keep it to about five minutes.
- Reading your diagram out loud, top to bottom, is a big part of an LLD round. Practice saying it.
- Flipkart's 2026 SDE-2 review ended with a request to sketch the UML class diagram of the code just written. Be ready to draw it from code.
associationaggregationcompositiondependencymultiplicity
Lesson 6 · 15 min · School guide Enums, value objects and exceptions
- Enums for fixed sets like VehicleType and OrderStatus. Raw strings let typos compile.
- Money as a value object holding paise in a long. 0.1 + 0.2 is not 0.3 in floating point, so totals drift by paise.
- equals and hashCode (or __eq__ and __hash__) whenever objects become map keys.
- Custom exceptions like InsufficientBalanceException instead of returning -1. Flipkart's 2026 reviewers threw edge cases at the running code and expected no unexpected exception.
enumvalue objectimmutabilitycustom exception