LESSON 3.1

From requirements to classes

Read a requirement, underline the nouns and verbs, then ask which noun owns the invariant. That process produces your first class list.

7 min read

A common mistake is to open a code editor before asking what classes exist and what rules they enforce. The result is a tangle of free functions, mutable globals and objects that can be in invalid states. Class discovery is the cheaper step.

The noun-verb pass

Start with the requirement written in plain language. Underline every noun and every verb. Nouns are candidate classes, fields or values. Verbs are candidate methods.

Requirement: A library lends books to members. A member may have at most three books on loan at a time. A book cannot be loaned if it is already on loan.

Nouns: Library, Book, Member, Loan. Verbs: lends, has on loan, is on loan.

Now ask for each noun: does it enforce a rule? If yes, it is a class. If no, it might be a plain record or a field of another class.

Library enforces the "at most three" rule → class.
Book knows whether it is on loan → class.
Member tracks its loans → class.
Loan records a borrow event → value or simple record.

Invariants first

An invariant is a condition that must always hold. State it in one sentence: A Member can never have more than three active loans. Write the constraint into the class that owns the state that determines it.

If the invariant spans two classes, pick the one that knows the most. The Library knows the member's loan count because it manages both members and books. Putting the check inside Library.lend() is correct.

Laying out the diagram…

Library enforces the three-loan rule. Book owns the on-loan flag. Loan records the event.

In code

What to do with leftover nouns

After the invariant pass, some nouns have no rules and no state. They are data, not objects. Loan above is just a borrow event — it can be a simple object or a record with a timestamp and references to the member and book. Making it a full class with validation would be overengineering until an invariant appears.

The test: What goes wrong if this noun is in an invalid state? If the answer is "nothing, because it has no invariants", it is probably a value, not an entity.

Where you have already seen it

Domain-Driven Design formalizes this distinction as entities (have identity and invariants) versus value objects (immutable, compared by value). JPA @Entity annotations reflect the same idea. React's distinction between controlled and uncontrolled inputs is also an invariant question: who owns and enforces the current value?

Quick check

'A book cannot be loaned if it is already on loan.' Where does this invariant live?

Try it

Parking Lot LLD

List the nouns, state the invariants, then draw the class boundary before writing any methods.

Check your understanding

1.A parking lot requirement says 'a ticket records entry time and must not allow a negative duration'. What does 'must not allow' tell you?
2.Which of the following is a verb that becomes a method, not a noun that becomes a class?
3.You find three nouns in a requirement: Customer, Order and OrderLine. OrderLine has no invariant of its own and always belongs to exactly one Order. What does that suggest?

Practice it

Parking Lot LLDModel spots, tickets, and pricing with clear responsibilities and safe allocation.Library Management LLDModel titles, physical copies, loans and reservations without double lending.

Your notes

1 class Book { 2 #title; #onLoan = false; 3 constructor(id, title) { this.id = id; this.#title = title; } 4 get title() { return this.#title; } 5 get onLoan() { return this.#onLoan; } 6 checkOut() { 7 if (this.#onLoan) throw new Error(`${this.#title} is already on loan`); 8 this.#onLoan = true; 9 } 10 returnBook() { this.#onLoan = false; } 11 } 12 class Member { 13 #name; #loans = []; 14 constructor(id, name) { this.id = id; this.#name = name; } 15 get name() { return this.#name; } 16 get loanCount() { return this.#loans.length; } 17 addLoan(book) { this.#loans.push(book); } 18 removeLoan(bookId) { this.#loans = this.#loans.filter(b => b.id !== bookId); } 19 } 20 class Library { 21 #members = new Map(); #books = new Map(); 22 addMember(member) { this.#members.set(member.id, member); } 23 addBook(book) { this.#books.set(book.id, book); } 24 lend(memberId, bookId) { 25 const member = this.#members.get(memberId); 26 const book = this.#books.get(bookId); 27 if (!member || !book) throw new Error("Not found"); 28 if (member.loanCount >= 3) throw new Error("Loan limit reached"); 29 book.checkOut(); 30 member.addLoan(book); 31 return { member: member.name, book: book.title }; 32 } 33 } 34 const lib = new Library(); 35 lib.addMember(new Member("M1", "Alice")); 36 lib.addBook(new Book("B1", "Clean Code")); 37 const loan = lib.lend("M1", "B1"); 38 console.assert(loan.book === "Clean Code"); 39 console.log(loan);

1 public class Example { 2 static final class Book { 3 final String id, title; 4 private boolean onLoan; 5 Book(String id, String title) { this.id = id; this.title = title; } 6 boolean isOnLoan() { return onLoan; } 7 void checkOut() { 8 if (onLoan) throw new IllegalStateException(title + " already on loan"); 9 onLoan = true; 10 } 11 void returnBook() { onLoan = false; } 12 } 13 static final class Member { 14 final String id, name; 15 private final java.util.List<Book> loans = new java.util.ArrayList<>(); 16 Member(String id, String name) { this.id = id; this.name = name; } 17 int loanCount() { return loans.size(); } 18 void addLoan(Book b) { loans.add(b); } 19 } 20 static final class Library { 21 private final java.util.Map<String,Member> members = new java.util.HashMap<>(); 22 private final java.util.Map<String,Book> books = new java.util.HashMap<>(); 23 void addMember(Member m) { members.put(m.id, m); } 24 void addBook(Book b) { books.put(b.id, b); } 25 String lend(String memberId, String bookId) { 26 var m = members.get(memberId); var b = books.get(bookId); 27 if (m == null || b == null) throw new IllegalArgumentException("Not found"); 28 if (m.loanCount() >= 3) throw new IllegalStateException("Loan limit reached"); 29 b.checkOut(); m.addLoan(b); 30 return m.name + " borrowed " + b.title; 31 } 32 } 33 public static void main(String[] args) { 34 var lib = new Library(); 35 lib.addMember(new Member("M1", "Alice")); 36 lib.addBook(new Book("B1", "Clean Code")); 37 System.out.println(lib.lend("M1", "B1")); 38 } 39 }