Why does money math need its own careful verification?

With money, even a tiny difference becomes an incident on receipts, payment checks, and settlement. That's why it matters to actually verify that the total and the discounted amount are exact to the won.

What cases should the tests feed in?

Build several different carts and check that the total and the discounted amount both match the values you expect.

How do I run this problem?

The sandbox has no network access and dependencies are pre-fetched into the image. Run the tests with gradle test --offline --console=plain.

Cart Discount Amount

Your task

Build cart amount calculation. The total is the sum of (unit price × quantity) for every item, and the discounted amount applies a 10% discount to the total, rounded to whole won (HALF_UP). Complete Money.

Ground rules

•Only one currency (KRW); unit prices are whole won integers.
•Only a single 10% discount rate is in scope.

Not required

•Other currencies and exchange rates.
•Coupon stacking and tiered discount policies.

▶About this challenge

Complete a plain-Java (no framework) Money value object that calculates cart amounts. The total is the sum of unit price × quantity, and the discounted amount applies a 10% discount to the total, rounded to whole won. You practice verifying that money math stays exact across every case.

Money calculation is an area where the result must not be off by a single won. So it matters to state clearly in code how numbers are held and computed, and when and by what rule rounding happens. This problem practices computing the total (the sum of unit price × quantity) and the 10% discount exact to the won, and verifying that the total and the discounted amount come out correct.

Why does money math need its own careful verification?: With money, even a tiny difference becomes an incident on receipts, payment checks, and settlement. That's why it matters to actually verify that the total and the discounted amount are exact to the won.
What cases should the tests feed in?: Build several different carts and check that the total and the discounted amount both match the values you expect.
How do I run this problem?: The sandbox has no network access and dependencies are pre-fetched into the image. Run the tests with gradle test --offline --console=plain.