Shopping Cart – Part 4

Test-Driven Development: write tests before writing code.

Unit testing results in modular code. Modular code is:

• easy to test.
• immune to bugs.
• easy to understand.
• scalable.

Task 1

1. Create a test that fails

---

Write a unit test named subtotalIsValid. subtotalIsValid asserts that the subtotal is calculated correctly.

Inside Cart.java, write code to make the test fail.

 /**
  * Function name: getSubtotal
  * @return double
  * 
  * Inside the function:
  *   1. return -2
  */

2. Make the test pass

---

Write code inside Cart.java to make the test pass.

 /**
  * Function name: getSubtotal
  * @return double
  * 
  * Inside the function:
  *   1. loop runs through the size of the ArrayList.
  *   2. adds the price field of each Item object. 
  *   3. returns the sum
  */

3. Refactor

---

Can getSubtotal be simpler?

• Yes. Run the ArrayList through a pipeline of functions:

Intermediate:

1. mapToDouble: maps (updates) every element to a Double. The lambda expression returns a Double value for each element.

Terminal:

2. sum(): returns the sum of every element in the sequence.

Important: As you refactor, run the unit test to make sure there are no bugs.

Task 2

1. Create a test that fails

---

Write a unit test named taxIsValid. taxIsValid asserts the tax is calculated correctly.

Inside Cart.java, write code to make the test fail.

 /**
  * Function name: getTax
  * @param double (subtotal)
  * @return double
  * 
  * Inside the function:
  *   1. return -2
  */

Task 2. Make the test pass

---

Write code inside Cart.java to make the test pass.

 /**
  * Function name: getTax
  * @param double (subtotal)
  * @return double
  * 
  * Inside the function:
  *   1. calculates tax
  *   2. rounds to two decimal places
  *   3. returns the result
  */

3. Refactor

---

Can getTax be simpler? Likely not.

Task 3

1. Create a test that fails

---

Write a unit test named totalIsValid. totalIsValid asserts the total is correct.

Inside Cart.java, write code to make the test fail.

 /**
  * Function name: getTotal
  * @param double (subtotal)
  * @param double (tax)
  * @return (double)
  * 
  * Inside the function:
  *   1. returns -2
  */

2. Make the test pass

---

 /**
  * Function name: getTotal
  * @param double (subtotal)
  * @param double (tax)
  * @return (double)
  * 
  * Inside the function:
  *   1. returns subtotal + tax
  */

3. Refactor

---

Can getTotal be simpler? Likely not.

Task 4

Define a checkout method that returns a receipt:

return "\tRECEIPT\n\n" +
  "\tSubtotal: $" + getSubtotal() + "\n" +
  "\tTax: $" + getTax(getSubtotal()) + "\n" +
  "\tTotal: $" + getTotal(getSubtotal(), getTax(getSubtotal())) + "\n";

Final Remarks

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Compare your old code to the new code.

Old code: a giant method that performs too many tasks. The method is impossible to unit test and is vulnerable to bugs.

New code: more polished. Each method performs one task (modular) and is easy to test.

Therein lies the beauty of Test Driven Development!