Vending Machine – Part 1

Goal: Quality control the Item and Machine classes.

Setting up

From Java Bootcamp Resources -> Module 2 -> 8\. Exception Handling -> Exception Handling Workbooks, open the folder vending-machine.

Unchecked exceptions

An unchecked exception crashes the app as a result of badly written code.

You should throw an:

• IllegalArgumentException when the caller passes faulty arguments into a method/constructor.

• IllegalStateException when an object calls its method at a "bad time".

Throwing an unchecked exception forces the caller to improve/fix their code.

Run the code

---

Some of the items have negative price values, negative quantities, and blank names.

Such values are illegal, and the constructor should forbid the caller from passing them.

Task 1 - Item Constructor

---

The first place to apply quality control is the Item constructor.

• name is null OR blank

  • throw new IllegalArgumentException("name cannot be null/blank.");

• price is less than zero.

  • throw new IllegalArgumentException("price cannot be less than zero.");

• quantity is less than zero.

  • throw new IllegalArgumentException("quantity cannot be less than zero.");

Now, the code will keep crashing until the caller passes the correct values.

Task 2 - Use the following values

---

Item[][] items = new Item[][] { 
{ new Item("Pepsi", 1.99, 3) , new Item("Fresca", 1.49, 3), new Item("Brisk", 2.49, 2) }, 
{ new Item("Fanta", 1.99, 2) , new Item("Barq's", 1.49, 2), new Item("A & W", 2.49, 3) }, 
{ new Item("Crush", 1.99, 2) , new Item("C-Cola", 1.49, 2), new Item("Berry", 2.49, 1) }
};

Task 3 - setName

---

Inside setName, throw an IllegalArgumentException if the caller passes a name that is blank or null.

Task 4 - setPrice

---

Inside setPrice, throw an IllegalArgumentException if the caller passes a price that is less than zero.

Task 5 - setQuantity

---

Inside setQuantity, throw an IllegalArgumentException if the caller passes a quantity that is less than zero.

Task 6 - Inspecting the Machine class

---

1. Should the Machine constructor check for a null? No.

   • The code would already throw a NullPointerException. So, throwing another exception would be redundant.

2. Should the setItem setter check a null? No.

   • The code would already throw a NullPointerException. So, throwing another exception would be redundant.

3. Should the dispense method check if the row and spot are valid? No.

   • The code would already throw an ArrayIndexOutOfBoundsException. So, throwing another exception would be redundant.

4. Should dispense throw an IllegalArgumentException? Yes.

   • The caller should not call dispense for an item with a quantity of zero.

5. Is there a need to throw an IllegalStateException anywhere? No.

   • There isn't a clear scenario when the machine object can call a method at a "bad time". In other words, the machine is always in a valid state to call any method.

That's all!

You added checkpoints to the Item and Machine classes. Each checkpoint forbids the caller from misusing the methods/constructors.