CSC/ECE 517 Fall 2011/ch6 6a pc

6a (was 5c). Lecture 18, Programming by contract. My notes for Lecture 18 are largely taken from 20-year-old articles by Bertrand Meyer. The principles are timeless, but there are undoubtedly new embellishments that would provide better background to the topics discussed in the lecture. Write a narrative, generally following the lecture organization, that explains programming by contract in more detail.

Software Reliability

A major concern in software design is Reliability, which depends on two factors: 1) Correctness - system's ability to perform its job according to the specification and 2) Robustness - handling abnormal conditions. Reliability is especially important in Object oriented software because of the additional complexity imposed by Reusability. Some of the common problems encountered when writing software include:

• System failure - CPU crash, Disk/Memory access errors, Network errors
• Invalid input data - Out of range data, Bad file name,etc.
• Programming errors - Memory leaks, Buffer overruns, etc.

There are many programming language features that help tackle these issues. Static typing, for example, is a major help for catching inconsistencies before they have had time to become bugs. Similarly Garbage collection helps to remove the specter of devious memory management errors. Various software design methodologies also come handy when dealing with complex software. For instance, reusability itself can help eliminate lots of bugs, if we are reusing existing code that has already been thoroughly tested and deployed. Polymorphism also helps in handling issues related to maintainability by reducing the size of the code and making it more elegant. But in a large and complex software involving multiple developers, we need a more systematic approach of specifying and implementing object-oriented software elements and their interactions. This is exactly what "Programming by contract" promises to do.

Programming by contract

Basically programming by contract creates a contract between the software developer and software user, which are referred to as the supplier and the consumer. Every feature, or method, starts with a precondition that must be satisfied by the consumer of the routine. And each feature ends with postconditions which the supplier guarantees to be true after it is executed(if and only if the preconditions were met). Consider the following example of a static Intersect() method on a Set object (Quoted from Skrien 4.6):

public static Set intersect(Set s1, Set s2)
  Precondition: s1 and s2 are not null.
  Postcondition: 
    Returns a Set with the common elements of s1 and s2, if s1 and s2 are not null.
    If s1 or s2 is null, an IllegalArgumentException is thrown.

Note here that this documentation is not overly specific about the implementation nor is it very general and vague. This can be stated concisely as a guideline: "The external documentation for a method should be sufficiently specific to exclude implementations that are unacceptable but sufficiently general to allow all implementations that are acceptable".

But note here that, the above contract does not mention anything about the behavior of the method when preconditions are not satisfied. This is Meyer’s non-redundancy principle: "Under no circumstances shall the body of a routine ever test for the routine’s precondition".

In essence, this theory suggests associating a specification with every software element. These specifications (or contracts) govern the interaction of the element with the rest of the world. This is different from having a formal specification in that the specification language is embedded in the design and programming language (e.g. Eiffel). This makes it possible to use a single notation and a single set of concepts throughout the software life cycle, and since these are modeled based on the programming language itself, it provides a great deal of clarity about the requirements at each stage of software development.

Comparison with Defensive Programming

When not to use Programming by Contract

Asserts and "by contract" specifications catch programmer errors not run-time errors! (Add more details)