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Introduction

Introduction. Administrative. 70% Exam, 30% Homeworks Must pass exam 4-5 Homeworks Homework grades: 80 is easy, 100 is hard 1-2 Class exercises Lecture in tutorial slot Office hours: by e-mail appointment imaman@cs. An academic course is limited …. We want to emulate Changing requests

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Introduction

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  1. Introduction

  2. Administrative • 70% Exam, 30% Homeworks • Must pass exam • 4-5 Homeworks • Homework grades: 80 is easy, 100 is hard • 1-2 Class exercises • Lecture in tutorial slot • Office hours: by e-mail appointment • imaman@cs

  3. An academic course is limited … • We want to emulate • Changing requests • Partial/Underspecified requests • Project is never closed – no end point • Large teams • Large code base • Time factor – human memory is limited • Multiple users • Multiple versions • Wide area – many non-overlapping topics • Interfacing with external “things” • Lecture time is limited - cannot present complex code • Too much code vs. too much explanations

  4. Learning this Course • Close to social science/literature • No formulas, no proofs, no right answer • You need to forget a few things • Wide but not deep, no linear order • Extrapolation: from the simple examples to complex scenarios • Requires a different state of mind

  5. Our Context • OOP • Mostly static typing • Performance is not issue • Things everyone can use • You will write slower!

  6. Motivation • We need to deliver good programs • Bugs: results may be catastrophic • Project is 95% done, 80% of the time • Expensive to change: software is not “soft” (!) • In short: programs are complicated • Two issues • Definition of quality • Techniques for achieving quality

  7. A One-Line Summary • Write test!

  8. A Two-Line Summary • Rule #1:Write test! • Rule #2:Programming is a process of trial-and-error process

  9. A Slightly Deeper Summary • Rule #1:Write test! • Rule #2:Programming is a process of trial-and-error process • Rule #3, #4, #5: Turing Church • Rule #6: There is no rule #7

  10. The Turing & Church Rules • A program may contain other programs • You cannot know if a program is doing what you expect it to do • If it can be done at all, it can be done in many ways

  11. Quality in Software ? • #bugs in the present • #bugs in the future • Time to develop • Time to add a feature • Time to learn the code • LOC ? • #Users • Revenues • Conclusions • Many contradicting factors • Measuring is hard

  12. Statistical Correlation • Conduct an experiment • Measure quality once over a large enough sample • Correlate with other properties (that are easier to measure) • Similar to clinical trials • Candidates for “other properties” • Source code metrics, development methodologies, language/technology/tools used • Does not work: Expensive • Sample size is too small compared to variance • Participants must be isolated of new techniques for the duration of the experiment • Hidden bugs • Conclusion: software metrics are very limited • Not surprising: measuring quality seems to be at odds with Turing

  13. Burn Charts • Time is important. let’s describe our progress vs. time • Vertical axis: tasks completed • Horizontal axis: time line

  14. Burn Up

  15. Burn Up Example

  16. Quality in Software • High-quality: A software whose burn curve is linear • Similar to Big-O notation of algorithms • Does not distinguish between two linear curves • Differences in domain, languages, … • States that a flattening is the #1 risk • Can be experienced even in student assignments • Result oriented • Unlike code metrics whose correlation with the net result is indirect (at best) • Requires constant feedback, short iterations • Past is measurable • Provides some confidence for the future

  17. Achieving Linear Burn Up • The reason why people are obsessed with design • A good design => High quality • Two questions • What is design? • How can one achieve good design?

  18. (Intelligent) Code Review • Context • Schedule considerations • Criticality • Legacy • Not all parts of a program should be of the same quality • Readability • “Code is for communication” • Code is read more than written

  19. What is Design (I) • Sufficient information for a craftsman oftypical skill to be able the build the artifact without excessive creativity. • (Guy Steele)

  20. public class Calculator { int x; public A(int n) { x = n; } public int add(int y) { return x + y; } public int sub(int y) { return x – y; }}

  21. Using a Calculator Object • Compute 5 + 9, put result in x • int x = 5 + 9; • int x = new Calculator(5).add(9); • Protocols induce a new programming language

  22. Class IndentingPrinter • public class IndentingPrinter { • private String indentation = ""; • private final StringBuilder sb = new StringBuilder(); • public void inside() { indentation += " "; } • public void outside() { indentation = indentation.substring(2); } • public void print(String s) { • sb.append(indentation).append(s).append('\n'); • } • @Override • public String toString() { return sb.toString(); } • }

  23. Using IndentingPrinter’s Language • public class Book { • private String name; • public Book(String name) { this.name = name; } • public void print(IndentingPrinter p) { p.print("Book: " + name); } • } • public class Library { • private String name; • private List<Book> books = new ArrayList<Book>(); • public Library(String name) { this.name = name; } • public void add(Book b) { books.add(b); } • public void print(IndentingPrinter p) { • p.print("Library: " + name); • for (Book b : books) • b.print(p); • } • }

  24. Class IndentingPrinter (II) • public static class IndentingPrinter { • private final String indentation; • private StringBuilder sb = new StringBuilder(); • public IndentingPrinter() { this("", new StringBuilder()); } • private IndentingPrinter(String indentation, StringBuilder sb) { • this.indentation = indentation; • this.sb = sb; • } • public IndentingPrinter inside() { • return new IndentingPrinter(indentation + " ", sb); • } • public void print(String s) { • sb.append(indentation).append(s).append('\n'); • } • @Override • public String toString() { return sb.toString(); } • }

  25. New Protocol => New Language • // Book remains the same ... • public static class Library { • private String name; • private List<Book> books = new ArrayList<Book>(); • public Library(String name) { this.name = name; } • public void add(Book b) { books.add(b); } • public void print(IndentingPrinter p) { • p.print("Library: " + name); • for (Book b : books) • b.print(p.inside()); • } • }

  26. Which Version do you like better?

  27. What is Design (II) • Design is the language induced by the protocols of the objects that are available at a certain context

  28. “Design, by nature, is a series of trade-offs. Every choice has a good and bad side, and you make your choice in the context of overall criteria defined by necessity. Good and bad are not absolutes, however. A good decision in one context might be bad in another. If you don't understand both sides of an issue, you cannot make an intelligent choice; in fact, if you don't understand all the ramifications of your actions, you're not designing at all. You're stumbling in the dark” • Allen Holub, “Why getter and setter methods are evil”, • http://www.javaworld.com

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