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CSE 331: Software Design & Implementation

CSE 331: Procedural abstraction and specification

In this lecture, we will rebuild last lecture's Sum from methods and ask how to know that a program built that way works. Code that calls a method is a client of the method. A method specification has a precondition, which the client must make true when it calls the method, and a postcondition, which the method must make true when it returns or throws. The method may assume the precondition. A method meets its specification if every call that begins with the precondition true ends with the postcondition true.

Procedural abstraction is using a method through its specification alone. Procedural abstraction lets us argue that a large program works, one method at a time. We will also compare specifications. A specification S1 is stronger than a specification S2 if every implementation that meets S1 also meets S2. Choosing how strong to make a specification is part of designing a method. Finally, a client is correct if it works with every implementation that meets the specifications it relies on.