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If you write your tests first, then you'll be forced to write code that is easy to test at the same

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time, code that is easy to test is immune to bugs, and it tends to be more polished.

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So the first step of test driven development is to identify meaningful test cases from the requirements

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when the cart adds an item to its collection.

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We need to check if the cart contains the item that was just added.

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So test if the cart contains the item.

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After it's added.

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We also got to make sure the cart doesn't add an item that it already contains.

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Test if the cart.

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Scheps duplicate item.

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And now there's another requirement about removing an item, so we're going to test if the item.

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Gets removed after being sold.

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And the code to throw an exception, if the user tries to remove from an empty cart, so test for an

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illegal state exception.

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We're trying to remove from an empty cart.

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And for this requirement, we got a test of the subtotal as calculated correctly.

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Test if the tax is calculated correctly.

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Test for the total.

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And we're also going to test for an illegal state acceptant for trying to check out an empty cart.

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If every test passes, we can rest assured that our code doesn't have any bugs.

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All right.

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That is all for part one.

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We've identified every meaningful test case and now we can start unit testing.

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See you soon.