Code Challenge: Invoice Has-a Customer (Composition)

Objective

Practice composition by building a small business-domain model where an Invoice has-a Customer. Your code should safely handle a missing customer using a sentinel value.

Skills to Practice

• Creating classes with constructors and instance variables
• Using composition (has-a relationship)
• Using null safely
• Returning an AP-style sentinel value
• Designing simple methods that describe object state

Tasks

• Create a new project named CH-Composition-Invoice
• Create three files: Customer.java, Invoice.java, Program.java

Task 1: Customer Class

• Add private instance variables: name and email
• Create a constructor that initializes both values
• Add getters: getName(), getEmail()
• Add getContactCard() that returns something like: "Ada Lovelace <ada@company.com>"

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public class Customer
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{
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    // TODO: private instance variables (name, email)
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    // TODO: constructor(s)
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    // TODO: getters (getName, getEmail)
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    // TODO: getContactCard() -> returns a single formatted String
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}

Task 2: Invoice Class (Composition)

• Add private instance variables: invoiceNumber, amount, paid, and customer
• In the constructor, invoices begin UNPAID
• The customer field may start as null
• Add composition methods: setCustomer and clearCustomer
• Add an AP-style sentinel method getCustomerName():
  • Return the customer's name if customer is not null
  • Return "none" if customer is null
• Add markPaid() to set the invoice status to paid
• Add getSummary() that returns a single-line String summary exactly like the sample output format: Invoice #____ | Customer: ____ | Amount: $____ | Status: ____

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public class Invoice
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{
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    // TODO: private instance variables
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    // invoiceNumber (int), amount (double), paid (boolean), customer (Customer)
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    // TODO: constructor
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    // Hint: invoices begin unpaid, and customer may be null
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    // TODO: getters (getInvoiceNumber, getAmount, isPaid)
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    // TODO: composition methods
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    // setCustomer(Customer customer)
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    // clearCustomer()
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    // getCustomerName() -> returns "none" if customer is null (sentinel)
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    // TODO: behavior methods
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    // markPaid()
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    // getSummary() -> returns a single formatted String describing the invoice
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}

Task 3: Main Method

• Create an Invoice with invoice number 1001 and amount 199.99
• Print the invoice summary (should show Customer: none)
• Create a Customer named Ada Lovelace with any email you choose
• Assign the customer to the invoice and print the summary again
• Mark the invoice paid and print the summary one last time

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public class Program
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{
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    public static void main(String[] args)
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    {
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        // TODO: Create:
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        // 1) an Invoice with no Customer yet
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        // 2) a Customer
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        // 3) assign the Customer to the Invoice
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        // 4) mark the Invoice paid
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        // 5) print invoice summaries at each stage (see Sample Output)
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    }
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}

Sample Output

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-- Invoice Demo --
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Invoice #1001 | Customer: none | Amount: $199.99 | Status: UNPAID
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Assigning customer...
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Invoice #1001 | Customer: Ada Lovelace | Amount: $199.99 | Status: UNPAID
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Marking invoice paid...
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Invoice #1001 | Customer: Ada Lovelace | Amount: $199.99 | Status: PAID