Project: Test the Checkout

A test suite is a set of tests that run together, one for each thing that must keep working. The shop's checkout takes real money, so it deserves one. It runs for you, hidden: POST /checkout takes a cart like {"items": [{"product_id": 1, "quantity": 2}]}, adds up the prices from the database, charges the card through the payment provider and answers 201 with {"status": "paid", "total": 24}.

A good suite tests the normal case and the edge cases, where bugs like to hide. Each test sends one request and checks the status code and the whole body. Here is one test of that shape, for a shop's product page:

void testMissingProduct() {
    ResponseEntity<Map> response = client.getForEntity("/products/99", Map.class);
    assertEquals(404, response.getStatusCode().value());
    assertEquals(Map.of("error", "Product not found"), response.getBody());
}

Your suite sends three carts. A Mug (product 1) costs 12, a Pen (product 4) costs 2 and a Gift card (product 8) costs 50. The payment provider's test mode declines every payment over 1000. Work the expected answers out by hand, never with the code you are testing: a test that repeats the code's calculation repeats its bugs too.

POST /checkout  2 Mugs + 5 Pens   →  201  {"status": "paid", "total": 34}
POST /checkout  {"items": []}     →  400  {"error": "The cart is empty"}
POST /checkout  25 Gift cards     →  402  {"error": "Payment declined"}

When you run the code, the check runs your suite on the checkout, where all three tests must pass (3 tests: OK). Then it runs it on three broken copies of the checkout, and each copy must fail at least one test.

Your task: in CheckoutTests, setUp already stores a new TestRestTemplate in client, and testCheckoutSucceeds already builds its cart. Finish that test, then add testEmptyCart and testDeclinedCard. Each one posts its cart to /checkout with client.postForEntity and checks the status code and the body with assertEquals.

Assignment
Follow the Coding Tutorial to write a test suite for the checkout: a paid order, an empty cart and a declined card.

Hint
Every test has the same shape: post the cart with client.postForEntity("/checkout", cart, Map.class), then one assertEquals for response.getStatusCode().value() and one for response.getBody(), with the value you expect first. Work out each expected total on paper first.