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Design By Contract
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JavaFAQ Home » Java Notes by Fred Swartz
Bertrand Meyer orginated a programming methodology called Design by Contract,
which has become popular in some groups. In addition to specifying programming
code to carry out the operations of a function (method), the programmer also
specifies:
- Preconditions - assumptions the function makes. These are usually
expressed as statements that must be true about the parameters. This is the
part of the "contract" that the caller must agree to.
- Postconditions - conditions that are true when it finishes. These
conditions define the responsibilities of the function, and are the part of
the contract that the function agrees to.
- Invariants - conditions that should be true of a class in general
(class invariants. It's sometimes applied to loops (loop
invariants). These seem to be generally less useful than pre- and
postconditions.
Design benefits
The strength of this programming methodology is that it gets the programmer to
think clearly about what a function does, and it provides documentation for the
caller.
Programming language support for Design by Contract
A few programming languages, eg Eiffel and Sather
Sather home page,
implement pre- and postconditions in executable code so that they are checked at
run time. Most programming language don't have such support, so programmers who
want to use pre- and postconditions often write comments documenting the
conditions. This is not an ideal situation because the comments aren't verified
automatically, and they may not even be consistent with the actual code.
Assertions
Perhaps the closest one can come in pure Java to implementing pre- and
postconditions is to use the assert statement.
Trademarked?
It seems that Bertrand Meyer's company, Interactive Software Engineering, has
trademarked the phrase "Design by Contract".
Further reading
Building bug-free O-O software: An introduction to Design by Contract
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