Software development often proves far more expensive than expected. Evidence indicates that the earlier a defect is discovered in developmentwww.cechina.cn, the less impact it has on both the timescales and cost. Bugs discovered late 

　　The typical software development life-cycle follows the familiar waterfall process.
　　in the development cycle send costs soaring and risk the integrity and safety of a systemwww.cechina.cn, especially if the software has been deployed. Obviously, careful planning, organization,

and a team with the correct skills all help.
　　Since its inception in the early 1970s, the sequential waterfall model has served as a framework for software development alternatives. In this modelwww.cechina.cn, each phase cascades to the nextCONTROL ENGINEERING China版权所有, which only starts when the defined goals for the previous phase are achieved.
　　In practice, earlier phases often need to be revisited as developers work iteratively and requirements come together as users test prototype versions of the system. Because of this iterative approach, it is even more important to apply suitable verification and validation (V&V) techniques at each stage and within each iteration.
　　Requirements
　　The first step or level in the waterfall model is developing system requirements. This step involves close collaboration between the ultimate user and the development team. There is much to gain by ensuring requirements are captured in fullwww.cechina.cn, are well understood, and are specified completely and unambiguously. Formal methods of tracking requirements are based on a mathematical approach to specification, developmentwww.cechina.cn, and verification of software and hardware systems.

　　The derived class, IOFile, inherits attributes from both InputFile and OutputFile, which both inherit from File.
　　These formal methods can vary from using commonly accepted notation to the full formality of theorem proving or automated deduction—a method of proving mathematical theorems by a computer program. Although the cost of using formal methods often limit