The Integrated Tracker

Assemble the WHOLE tracker in Solution (you may reuse your milestone code): - record Expense(String id, String category, int cents) with constructor validation (blank id/category, non-positive cents throw IllegalArgumentException with a specific message) - interface ExpenseStore { void save(Expense e); Optional<Expense> find(String id); List<Expense> all(); } + InMemoryExpenseStore - static class ExpenseService with constructor (ExpenseStore store): - void add(String id, String category, int cents) — constructs (letting validation throw), then saves - Map<String, Integer> totalsByCategory() — stream aggregation - Optional<String> topExpense() — highest cents, tie → smallest id The checkpoint test drives the three layers together, exactly as the milestones promised.