Object-oriented programming allows procedures about objects to be created whose exact type is not known until runtime. For example, a screen cursor may change its shape from an arrow to a line ...

To argue that Objective-C resembles a metaphysically divine language, or even a good language, is like saying Shakespeare is ...

viewing and totalling are examples of its behavior. Objects (individual invoices) inherit this behavior and combine it with their own data elements. See CRC cards and object-oriented programming.