Classes, Interfaces & Cohesion

Cohesion, coupling, and public surface

A well-designed class does one job well (high cohesion) and exposes only what callers need (low coupling). In LLD, your first sketch should list public methods before fields — the API is the contract; implementation is negotiable.

Concept	Good sign	Red flag
Cohesion	Methods share a single noun's lifecycle	UserManager + EmailSender + CsvExporter
Coupling	Depends on interfaces, not concrete types	new SmtpClient() inside domain logic
Encapsulation	Invariants enforced inside the class	Callers set balance directly
API size	3–7 meaningful public methods	20 getters/setters on a data bag

Abstract class vs interface

Use an interface when multiple unrelated classes share a role (Payable, Cacheable) — especially when you need multiple inheritance of type. Use an abstract class when subtypes share substantial code and the is-a relationship is stable (AbstractList in the JDK). In interviews, default to interfaces for behavior contracts; reach for abstract classes only when you can name shared implementation.

Programming to interfaces

High-level code should read: void process(PaymentMethod method), not void process(CreditCard card). Factory or DI constructs the concrete type at the edge. This is DIP in code form and makes unit tests trivial (inject a fake).

Law of Demeter (practical form)

A method should only call: itself, its parameters, objects it creates, its direct fields. Avoid order.getCustomer().getAddress().getZip() — expose order.getShippingZip() instead. Reduces ripple effects when Customer internals change.

Senior-level signal

Proactively shrink APIs: "Callers don't need getInternalQueue() — they need submit(Job) and cancel(id)." Mention package-private helpers or private inner classes for implementation details. If the interviewer asks about immutability, note that value objects (Money, ISBN) should be immutable with validation in the constructor.

Where this goes next

Creational Patterns covers how objects enter the system — Factory, Builder, and why Singleton rarely belongs in a clean design.