Before we take that next step, however, let's introduce an idea that was
first describes during the period that modules were being developed.
This idea was described by a computer scientist named David Parnas, and
so it is named after him. Parnas's principles describe the proper use
The emphasis is placed on the second part, the part that limits the
amount of information that must be shared. Parnas's principles are
guidelines for information hiding. Because objects develop out of
this tradition of modules, the guidelines for the proper use of
modules eventually transform themsevles into guidelines for the
proper use of objects.
One must provide the intended user of a module with all the information
needed to use the module correctly, and with nothing more.
- One must provide the implementor of a module with all the information
needed to complete the module, and nothing more.