Many subroutines are constructed in such a way that the details of their operation are made unavailable to other parts of the program through the use of locally defined variables, private subroutines, and so on. Other parts of a large program have access to only the output, not the underlying operations. This is not an accident. Encapsulating a subroutine's procedures—putting them behind what is called an “abstraction barrier”—carries important advantages, including the ability to modify the code without having to worry what other parts of the program will be affected.
