Add input validation to a Java method with exact messages

About this exercise

QA found that createBooking accepts blank room codes, 0 nights and past dates. Add guard clauses at the top that throw IllegalArgumentException with exactly these messages: 'roomCode must not be blank', 'nights must be between 1 and 30', and 'checkIn must not be in the past'. The past-date check must use the class's clock field so tests can fix the date. Return only the updated method in exactly 1 code block of at most 20 lines, keeping the rest of it unchanged, with under 25 words of text.

Common mistakes when you ask AI for this

• Letting the AI write the messages. Tests assert on the exact message, and the AI writes its own wording. Try: Paste them: "Messages exactly: roomCode must not be blank / nights must be between 1 and 30 / checkIn must not be in the past."
• Ignoring the injected clock. LocalDate.now() reads the real date, so tests with a fixed clock can't pass reliably. Try: Add: "Use LocalDate.now(clock) with the existing clock field."
• A rewritten class. The AI reprints the class, adds a validator helper or switches to Bean Validation annotations. Try: Add: "Return only createBooking, guards at the top, the rest unchanged."
• An explanation of every check. The AI explains each guard and suggests extra ones. Try: Add: "One code block, no text."

What a strong prompt includes

• Gives the exact messages
• Mentions the clock
• Names the exception
• Asks for only the method
• Protects the existing code

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