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Easy Learn Java: Programming Articles, Examples and Tips - Page 361


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Design By Contract

Go to all tips in 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

8 comments | Printer Friendly Page  Send to a Friend | Score: 0
Posted by jalex on Friday, May 20, 2005 (00:00:00) (2646 reads)

Development Techniques

Go to all tips in Java Notes by Fred Swartz
Beginning programming textbooks usually concentrate on two topics: the programming language and the general ideas the programming language constructs implement. This is good, but there are basic program development techniques that are often not taught, but which make program development much easier, and which are used by professional programmers for maximum effectiveness.

Basic techniques

Every programmer, beginner or more advanced, should use these techniques where possible.
  • Tools - Choose good tools and learn them well.
  • Indentation - Be scrupulous about indentation. See Java Coding Standards.
  • Start - Start with a working program.
  • Small increments - Work in small increments.
  • Pair programming - Work with someone else.

Intermediate techniques

All of the above plus
  • Save your programs -- you may be able to use them as the nucleus for another program.
  • Create you own library. When you write a good general class or method, add it to this library.
  • Refactor (rewrite) parts of a program if you realize there is a better way. A better written program will be less likely to have bugs and will be easier to extend for the next iteration.
  • Use an IDE (Integrated Development Environment), eg, NetBeans, Eclipse, or JBuilder (A Java IDE requires learning, and therefore may not be a good choice for beginning programmers. However, after you are comfortable developing simple Java programs with a text editor and the JDK, the advantages of an IDE for creating user interfaces and debugging can be significant. As with all tools, learn to use it well. Check the menus for options that might be useful.


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Posted by jalex on Thursday, May 19, 2005 (00:00:00) (2365 reads)

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