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CHAPTER 1

CHAPTER 1. INTRODUCTION TO SOFTWARE ENGINEERING. Outline. Historical aspects Economic aspects Maintenance aspects Specification and design aspects Team programming aspects The object-oriented paradigm Terminology . Scope of Software Engineering. Historical Aspects

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CHAPTER 1

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  1. CHAPTER 1 INTRODUCTION TO SOFTWARE ENGINEERING

  2. Outline • Historical aspects • Economic aspects • Maintenance aspects • Specification and design aspects • Team programming aspects • The object-oriented paradigm • Terminology

  3. Scope of Software Engineering • Historical Aspects • A NATO study group in 1967 coined the term software engineering – “building software is similar to other engineering tasks” • Endorsed by the 1968 NATO Conference in Garmisch, Germany • Aim: to solve the “Software Crisis” • Software is delivered • Late • Over budget • Low quality with lots of faults • Software crisis is still present over 35 years later!

  4. Scope of Software Engineering (contd) • Why cannot bridge-building techniques be used to build operating systems? • Attitude to collapse • Bridge collapse - redesigned and rebuilt from scratch • Operating system crash – reboot! • Imperfect engineering • Bridges are assumed to be perfectly engineered • OSs are assumed to be imperfectly engineered • Complexity is growing faster than we can master • Maintenance • Bridge maintenance – painting, repairing cracks, resurfacing the road • Software maintenance – porting to new hardware, adding more functions, re-write some parts, etc.

  5. Conclusion • Software engineering is a discipline whose aim is the production of fault-free software that satisfies the user’s needs and is delivered on time within budget • Software Engineering is not “Engineering” • Not as the same way viewed in other engineering disciplines

  6. Economic Aspects • Computer scientists investigate a variety of ways to produce software but software engineers are interested in economically viable techniques • Coding method CMnew is 10% faster than currently used method CMold. Should it be used? • Common sense answer • Of course! • Software Engineering answer must consider • the cost of introducing CMnew into an organization • the effect of CMnew on maintenance

  7. Maintenance Aspects • Software Life Cycle • The way we produce software, including • The life-cycle model • The individuals • CASE tools • Until the end of 1970s, most organizations were producing software using the Waterfall Model

  8. Life-cycle Model 1. Requirements phase - Client’s requirements are elicited 2. Specification phase - what is the software supposed to do? 3. Design phase - how the software does it 4. Implementation phase – coding and testing the components 5. Integration phase - combined and tested 6. Maintenance phase • Corrective (repair) • Enhancement (update) 7. Retirement • Removed from service

  9. Approximate Relative Cost of Each Phase • 1976–1981 data • Maintenance constitutes 67% of total cost

  10. Comparative Relative Cost of Each Phase

  11. Good and Bad Software • Good software is maintained—bad software is discarded • Different types of maintenance • Corrective maintenance [about 20%] • Enhancement • Corrective (Perfective) maintenance [about 60%] • Enhancement (Adaptive) maintenance [about 20%] • Effect of CMnew on maintenance

  12. Specification and Design Aspects • 60 to 70 percent of faults are specification and design faults [Boehm, 1979] • Data of Kelly, Sherif, and Hops [1992] • 1.9 faults per page of specification • 0.9 faults per page of design • 0.3 faults per page of code

  13. Cost to Detect and Correct a Fault

  14. Specification and Design Aspects (contd) • Data of Bhandari et al. [1994] • Faults at end of the design phase of the new version of the product • 13% of faults from previous version of product • 16% of faults in new specifications • 71% of faults in new design • Faults must be found and corrected early in the development stage, otherwise it will cost a lot!

  15. Team Programming Aspects • Hardware is cheap • performance-price factor = time to perform 1 million additions x cost of CPU and main memory • We can build products that are too large to be written by one person in the available time • Large product must be developed by a team • But team programming leads to • Interface problems • Communication problems • Good team organization and management is essential

  16. The Object-Oriented Paradigm • Structured paradigm had great successes initially • Structured systems analysis, data flow analysis, structured programming, structured testing • It started to fail with larger products (> 50,000 LOC) • Maintenance problems (today, up to 80% of effort) • Reason: structured methods are • Action oriented (finite state machines, data flow diagrams); or • Data oriented (entity-relationship diagrams, Jackson’s method); • But not both

  17. The Object-Oriented Paradigm (contd) • Both data and actions are of equal importance • Object: • Software component that incorporates both data and the actions that are performed on that data • Example: • Bank account • Data: account balance • Actions: deposit, withdraw, determine balance

  18. Structured versus Object-Oriented Paradigm

  19. Key Aspects of Object-Oriented Solution • Conceptual independence • Encapsulation • Physical independence • Information hiding • Impact on development • Physical counterpart • Impact on maintenance • Independence effects: only changes that need be made are within the object itself • Increased reusabilty • Can be utilized in the future products

  20. Responsibility-Driven Design • Also called “Design by Contract” • Send flowers to your aunt in Iowa City • Call 1-800-FLOWERS • Where is 1-800-FLOWERS? • Which Iowa City florist does the delivery? • Information hiding • Object-oriented paradigm • “Send a message to a method [action] of an object“

  21. Transition From Analysis to Design • Structured paradigm: • A sharp transition between analysis (what) and design (how) • Object-oriented paradigm: • Objects enter from very beginning

  22. Analysis/Design “Hump” • Systems analysis • Determine what has to be done • Design • Determine how to do it • Architectural design—determine modules • Detailed design—design each module

  23. Removing the “Hump” • Object-oriented analysis • Determine what has to be done • Determine the objects • Object-oriented design • Determine how to do it • Design the objects

  24. In More Detail • Objects enter here

  25. Warning • Do not use the object-paradigm to enhance a product developed using the structured paradigm • Water and oil do not mix • Exception: if the new part is totally disjoint • Example: adding a GUI (graphical user interface)

  26. Terminology • Program, system • Software: program + documentation • Product • a nontrivial piece of software • The end result of software production • Software production: development + maintenance • Methodology, paradigm: a collection of techniques for carrying out the software production • Bug  • “A bug  crept into the code” instead of • “I made an error”

  27. Object-Oriented Terminology • Data component of an object • State variable • Instance variable (Java) • Field (C++) • Attribute (generic) • Action component of an object • Member function (C++) • Method (generic)

  28. Object-Oriented Terminology (contd) • C++: A member is either an • Attribute (“field”), or a • Method (“member function”) • Java: A field is either an • Attribute (“instance variable”), or a • Method • Read Chapter 1 • Do Problems 1.1-1.11

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