From requirements to classes — nouns, verbs and responsibilities — Low-Level Design (LLD) — Object Design, UML and Classic Problems in C#

1. From requirements to classes — nouns, verbs and responsibilities

Basic★ Must knowLesson 1 of 8

Technique: write the use cases as sentences, underline nouns (candidate classes) and verbs (methods), then assign each responsibility to the class that owns the data it needs (Information Expert).

Example — "A member borrows a book copy from the library; the system records the due date and charges a fine if it is returned late."

Nouns → classes	Verbs → methods	Owner
Member, Book, BookCopy, Library, Loan, Fine	borrow, return, record due date, charge fine	`Library.Borrow(member, copy)` creates a `Loan`; `Loan.Return(date)` computes overdue days; `IFinePolicy.Calculate(loan)` gives the amount

Guidelines:

Each class has one reason to change (SRP).
Prefer composition ("has-a") over inheritance; inherit only for true "is-a" with shared behaviour.
Depend on interfaces for things that vary (payment methods, pricing rules, notification channels).
Use enums for fixed sets of states/types; use State/Strategy when behaviour differs per state/type.
Make invalid states impossible (private setters, constructor validation, value objects like Money).