Substitutivity of Implementations

We want the freedom to experiment with different implementations (e.g. to find the best).
What is the cost of changing from one implementation to another?
Answer: We must find and change every line of code that depends upon the specific details of the implementation (e.g. available operations, naming conventions, details of syntax).

Abstract Data Types

Solution: minimize the reliance on details of implementation.

An abstract data type specifies the values and operations of a data type, without referring to how it will be implemented.

Applications coded only in terms of abstract data types can switch between implementations at no maintenance cost.