The code for the Invoice class

import java.text.NumberFormat;
import java.util.ArrayList;

public class Invoice
{
    // the instance variable
    private ArrayList lineItems;

    // the constructor
    public Invoice()
    {
        lineItems = new ArrayList();
    }

    // a method that adds a line item
    public void addItem(LineItem lineItem)
    {
        this.lineItems.add(lineItem);
    }

    // the get accessor for the line item collection
    public ArrayList getLineItems()
    {
        return lineItems;
    }

    // a method that gets the invoice total
    public double getInvoiceTotal()
    {
        double invoiceTotal = 0;
        for (LineItem lineItem : this.lineItems)
        {
            invoiceTotal += lineItem.getTotal();
        }
        return invoiceTotal;
    }

    // a method that returns the invoice total in currency format
    public String getFormattedTotal()
    {
        NumberFormat currency = NumberFormat.getCurrencyInstance();
        return currency.format(this.getInvoiceTotal());
    }
}