The Java & Design Patterns series explains design principles and patterns through runnable Java code: SOLID, the GoF patterns, dependency injection, Repository and Specification with JPA, DDD, Hexagonal Architecture, ...
Most order systems we run into share the same symptom: classes like Order, Customer and Product are just bags of getters and setters, while every business rule is scattered across services. Money is a...
Every order admin screen has a filter block: status, date range, customer city, product code, minimum total. Users can pick any combination, and on the backend the code that builds the query often tur...
Here is a very familiar snippet in Spring projects: a service class calls entityManager.createQuery("select o from Order o where ...") right in the middle of payment logic, elsewhere the same query is...
Open any Spring application and you will see an OrderService that receives a PaymentGateway and a Notifier through its constructor, with no new anywhere in sight. Who creates those objects, in what or...
The checkout screen of an online store usually has five components: a coupon code field, a shipping option, a payment method, a total label, and a "Pay" button. They constrain each other: switching to...
Every time you write for (Order o : orders), Java uses the Iterator pattern behind the scenes. The pattern lets you walk through the elements of a collection without exposing how the collection stores...
An e-commerce order moves through several states: new, paid, shipped, delivered, cancelled. Each action, such as paying, shipping or cancelling, is valid only in certain states, and the behavior diffe...
An order entering the system has to pass several checks: is the customer logged in, is the item in stock, does the payment exceed the limit. If you write all of that in one long method full of nested...
The shopping cart of an online store needs three things that direct method calls cannot give it: an "Undo" button after a customer accidentally deletes an item, a "Redo" button when they change their...
Three reports in a sales system: a CSV export, an HTML export, and a CSV export for orders of 1,000,000 VND or more. All three open a connection, filter, print a header, loop over rows, print a footer...
When an order moves to the "paid" state, the system has to decrement stock, email the invoice, write an audit log entry and notify the accounting system. If OrderService.pay() calls all four directly,...
An online store has to calculate shipping fees in several ways: standard delivery, express delivery, free shipping for orders of 500,000 VND or more, and next week marketing will come up with a fourth...
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