BillingItem class

o Must have $3$ fields:

$1.$ An ArrayList which holds BillingSubItems, called $$subitems$$

$2.$ A double called $$amount$$

$3.$ A string called $$description$$

o It must have $3$ constructors:

$1.$ The default constructor, which initializes all fields with $0$ or an empty string,

as appropriate.

$2.$ An overloaded constructor, which takes in a double to initialize amount, and

initializes description with an empty string.

$3.$ An overloaded constructor, which takes in a double to initialize amount and

a string to initialize description.

$4.$ All constructors should initialize subitems.It must have the following methods:

$1.$ getAmount$(),$ which takes in nothing and returns a double. If subitems is

empty, it returns amount. Otherwise, this method adds up the amount of all

of the BillingSubItems in subitems, returning that summation. If subitems

has elements in it$,$ do not add the BillingItem$$s amount to the

summation.

$2.$ setAmount$(),$ which takes in a double and returns nothing. It updates the

value in amount with the value in the parameter. Note that no guard for the

input is necessary, as we want to allow for negative values in case we want

to offer a discount on something.

$3.$ addSubItem$(),$ which takes in a BillingSubItem and returns nothing, adding

it to subitems.

$4.$ removeSubItem$(),$ which takes in a BillingSubItem and returns nothing,

removing it from subitems.

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

integer is in bounds of subitems, return the BillingSubItem at that position.

Otherwise, return null.

$6.$ seeSubItems$(),$ which takes in nothing and returns a string. This method

works exactly as seeItems$()$ in Bill, but it uses the BillingSubItems stored

inside subitems.

$7.$ A getter and a setter for description.

$8.$ An overload of toString$().$ If the BillingItem has no BillingSubItems, it will

simply return its description and its amount. If the BillingItem does have

BillingSubItems, it must return:$$ Its own description, in its own line

$$ Every BillingSubItem in the BillingItem $($which you can retrieve by

calling every BillingSubItem$$s toString$()),$ each in its own line

$$ Every BillingSubItem must be preceded by a tab character $(\backslash$t$).$