Create a class called Invoice that a hardware store might use to represent an invoice for an item sold at the store. An Invoice should include four pieces of information as instance variables - a part number (type String, the part number should start by SN and has a length of 6 characters), a part description (type String), a quantity of the item being purchased (type int) and a price per item (double). Also, it has a shared attribute called counter (int) to count the number of objects created by this class. 1) Your class should have a constructor that initializes the four instance variables. 2) Provide a set and a get method for each instance variable. In addition, provide a method named getInvoiceAmount() that calculates the invoice amount (i.e., multiplies the quantity by the price per item), then returns the amount as a double value. If the quantity is not positive, it should be set to 0 . If the price per item is not positive, it should be set to 0.0. Write a test app named InvoiceTest that demonstrates class Invoice's capabilities. In addition, print the number of objects created by the Invoice class