Internal Implementation is Hidden

Messages define a standard interface to objects. Objects may have different internal implementations as long as the message interface is maintained. If access is controlled via messages, OOP facilitates information hiding: application programs do not know and cannot depend on the internal implementation of the object.

Example: Vector (send v x) or v.getX()

• Vector type 1: x is stored.

  

• Vector type 2: r and theta are stored. x is computed as r * cos(theta)

The two kinds of vectors appear the same to the outside world.