POJOs for our simple invoice database

In the context of the book ZUGFeRD: The Future of Invoicing, we need a simple invoice database. This database has the following four tables:

We'll write a POJO for each of these four tables and we'll create a POJO factory to access the database.

invoice

##GITHUB:https://github.com/itext/i5js-sandbox/blob/master/src/main/java/zugferd/pojo/Invoice.java##

customer

##GITHUB:https://github.com/itext/i5js-sandbox/blob/master/src/main/java/zugferd/pojo/Customer.java##

item

##GITHUB:https://github.com/itext/i5js-sandbox/blob/master/src/main/java/zugferd/pojo/Item.java##

product

##GITHUB:https://github.com/itext/i5js-sandbox/blob/master/src/main/java/zugferd/pojo/Product.java##

pojofactory

##GITHUB:https://github.com/itext/i5js-sandbox/blob/master/src/main/java/zugferd/pojo/PojoFactory.java##