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 best appreciated in pig latin. Objective-C is, at best, polarizing.

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.