Requirements

The features described below must be in your program.

$$ A total of four classes: the driver, Bill, BillingItem, and BillingSubItem.

$$ Bill class:

o Must have $4$ fields:

$1.$ An ArrayList which holds BillingItems, called $$items$$

$2.$ A static integer called $$nextId$,$ initialized at $0$

$3.$ An integer called $$id$$

$4.$ A static double called $$taxPercentage$,$ initialized at $0$

o It must only feature the default constructor, which initializes the arraylist, sets id

with the value of nextId, and then increments nextId by $1.$

o It must feature the following methods:

$1.$ setTax$(),$ which is static, takes in a double, and returns nothing. The double

in the parameter updates taxPercentage only if the parameter is greater

than or equal to $0.$ Otherwise, this method does nothing.

$2.$ A getter for taxPercentage, which is static.

$3.$ addItem$(),$ which takes in a BillingItem and returns nothing. It adds said

item to items.

$4.$ removeItem$(),$ which takes in a BillingItem and returns nothing. It removes

said item from items.

$5.$ getItem$(),$ which takes in an integer and returns a BillingItem. If the integer

in the parameter is within the bound of items, getItem$()$ returns the item in

that position. Otherwise, return null.

$6.$ seeItems$(),$ which takes in nothing and returns a string. If items is empty,

this method returns an empty string. Otherwise, it will return a list of all the

items in items in the following format:

POSITION. ITEM$-$DESCRIPTION: $ITEM$-$AMOUNT

e$.$g$.$: The bill below with $3$ items, when calling seeItems$(),$ would return the

following string:

$0.$ Item $1$: $$100\mathrm{.}00$

$1.$ Item $4$: $$30\mathrm{.}00$

$2.$ Item $5$: $$120\mathrm{.}00$

$7.$ calculateSubTotal$(),$ which takes in nothing and returns a double. This

method adds the amount of all BillingItems stored inside of items and then

returns this summation.

$8.$ calculateTotal$(),$ which takes in nothing and returns a double. This method

adds the amount of all BillingItems stored inside of items and then returns

this summation multiplied by the tax percentage.

$$ NOTE: The tax percentage is stored as a percentage $($e$.$g$.$: $10$ for

$10\%).$ This means you will have to convert it to the appropriate

decimal number before you can use it to calculate your total. In the

example above, if taxPercentage is $10,$ then you would multiply your

subtotal by $1\mathrm{.}1$ to get the total.

$9.$ An override of toString$().$ This override must contain the following

information:

$$ The Bill$$s id$,$ in its own line

$$ Every BillingItem in the Bill $($which you can retrieve by calling every

BillingItem$$s toString$()),$ each in its own line

$$ The subtotal, in its own line

$$ The total, in its own line