Finding Entities and Relationships

Nouns become classes, verbs become methods

Read the requirements and underline nouns (candidate classes or attributes) and verbs (candidate methods or services). Decide which nouns have behaviour and identity (classes) and which are just values (enums or value objects). Then name the relationships: is-a (inheritance or interface implementation), has-a (composition when the part cannot exist alone, aggregation when it can) and uses (a dependency passed in). Keep the first model small, cover the main use cases, and add classes only when a responsibility needs a home.

From requirements to classes

Good LLD answers find the right nouns and verbs, apply a few principles and use patterns only where they fit.

From a requirement to a model

A library example.

Requirement: "Members borrow book copies; each copy has a barcode.
 A member may hold at most 5 copies; late returns incur a fine."

Nouns  -> Member, Book, BookCopy, Loan, Fine
Verbs  -> borrow(), return(), calculateFine()

Relationships
  Book 1 --- * BookCopy        (composition: copies belong to a title)
  Member 1 --- * Loan          (a member has loans)
  Loan * --- 1 BookCopy        (each loan is for one copy)
  FinePolicy <- LoanService    (uses: injected dependency)

Prefer composition

Reach for inheritance only for a true is-a relationship with shared behaviour; otherwise compose objects and depend on interfaces.

Quick check: A Loan that holds a reference to a BookCopy is best described as:

• Inheritance from BookCopy
• An association between Loan and BookCopy
• A static utility method
• An enum value