Software Construction #1: Creating High-Quality Code

Transcription

1 Software Construction #1: Creating High-Quality Code Minsoo Ryu Hanyang University

2 Software construction Software construction is mostly coding and debugging But also involves detailed design, construction planning, unit testing, integration, integration testing, and other activities 2 2

3 Design in construction On small projects, a lot of (detailed) design is done while the programmer sits at the keyboard Design might be just writing a class interface in pseudocode before writing the details It might be drawing diagrams of a few class relationships before coding them It might be asking another programmer which design pattern seems like a better choice Design is a wicked problem Horst Rittel and Melvin Webber defined a wicked problem as one that could be clearly defined only by solving it, or by solving part of it (1973) This paradox implies, essentially, that you have to solve the problem once in order to clearly define it and then solve it again to create a solution that works 3 3

4 Outline Working Classes High-Quality Routines Defensive Programming The Pseudocode Programming Process 4 4

5 Working Classes

6 Abstractions Base classes are abstractions that allow you to focus on common attributes of a set of derived classes and ignore the details of the specific classes while you re working on the base class A good class interface is an abstraction that allows you to focus on the interface without needing to worry about the internal workings of the class From a complexity point of view, the principal benefit of abstraction is that it allows you to ignore irrelevant details 6 6

7 Encapsulation and information hiding Abstraction says, You re allowed to look at an object at a high level of detail Encapsulation says, Furthermore, you aren t allowed to look at an object at any other level of detail 7 7

8 Poor abstraction Poor interface It contains a collection of miscellaneous functions 8 8

9 Good interface Good abstraction Some of the original routines were moved to other, more appropriate classes Some were converted to private routines used by InitializeUserInterface() and the other routines 9 9

10 Present a consistent level of abstraction Mixed levels of abstraction Inheriting from ListContainer is convenient because it supports polymorphism, allowing an external search or sort function that takes a List Container object But, the class presents two ADTs Employee and ListContainer 10 10

11 Present a consistent level of abstraction Consistent levels of abstraction The class now presents one ADT: Employee The ListContainer is hidden as a private member 11 11

12 Beware of erosion of abstraction under modification There s no logical connection between employees and routines that check ZIP Codes, phone numbers, or job classifications The routines that expose SQL query details are at a much lower level of abstraction than the Employee class, and they break the Employee abstraction 12 12

13 Avoid putting private implementation details into a class s interface Including private declarations in the class header file might seem like a small transgression, but it encourages other programmers to examine the implementation details 13 13

14 Avoid putting private implementation details into a class s interface Scott Meyers describes a common way to address this issue in Item 34 of Effective C++, 2d ed. (Meyers 1998) You separate the class interface from the class implementation Within the class declaration, include a pointer to the class s implementation but don t include any other implementation details 14 14

15 Be wary of semantic violations of encapsulation Not calling Class A s InitializeOperations() routine because you know that Class A s PerformFirstOperation() routine calls it automatically Using a pointer or reference to ObjectB created by ObjectA even after ObjectA has gone out of scope, because you know that ObjectA keeps ObjectB in static storage and ObjectB will still be valid Using Class B s MAXIMUM_ELEMENTS constant instead of using ClassA.MAXIMUM_ELEMENTS, because you know that they re both equal to the same value 15 15

16 Be wary of semantic violations of encapsulation The problem is that the client code depends not on the class s public interface, but on its private implementation Program to the interface Anytime you find yourself looking at a class s implementation to figure out how to use the class, you re not programming to the interface You re programming through the interface to the implementation If you re programming through the interface, encapsulation is broken Once encapsulation starts to break down, abstraction won t be far behind 16 16

17 Be wary of semantic violations of encapsulation Make interfaces programmatic rather than semantic Each interface consists of a programmatic part and a semantic part The programmatic part consists of the data types and other attributes of the interface that can be enforced by the compiler The semantic part of the interface consists of the assumptions about how the interface will be used, which cannot be enforced by the compiler Look for ways to convert semantic interface elements to programmatic interface elements by using Asserts or other techniques The semantic interface can be documented in comments, but try to keep interfaces minimally dependent on documentation 17 17

18 Don t make assumptions about the class s users A class should be designed and implemented to adhere to the contract implied by the class interface It shouldn t make any assumptions about how that interface will or won t be used, other than what s documented in the interface Comments like the following one are an indication that a class is more aware of its users than it should be: -- initialize x, y, and z to 1.0 because DerivedClass blows -- up if they're initialized to

19 Containment ( has a Relationships) Implement has a through containment Implement has a through private inheritance as a last resort With private inheritance, public and protected member of the base class become private members of the derived class A class does inherit the implementation with private inheritance "Private inheritance means nothing during software design, only during software implementation, by Scott Meyers The private inheritance can introduce unnecessary multiple inheritance 19 19

20 Inheritance ( is a Relationships) Implement is a through public inheritance When a programmer decides to create a new class by inheriting from an existing class, that programmer is saying that the new class is a more specialized version of the older class Design and document for inheritance or prohibit it Inheritance adds complexity to a program, and, as such, it s a dangerous technique Adhere to the Liskov Substitution Principle (LSP) 20 20

21 Prefer polymorphism to extensive type checking The calls to shape.drawcircle() and shape.drawsquare() should be replaced by a single routine named shape.draw(), which can be called regardless of whether the shape is a circle or a square 21 21

22 Prefer polymorphism to extensive type checking On the other hand, sometimes case statements are used to separate truly different kinds of objects or behavior 22 22

23 Multiple inheritance Multiple inheritance can be incredibly useful when used with care, but it s dangerous in the hands of someone who doesn t observe proper precautions Multiple inheritance is useful primarily for defining mixins, simple classes that are used to add a set of properties to an object Mixins allow properties to be mixed in to derived classes 23 23

24 More guidelines Be critical of classes that contain more than about seven data members The number 7±2 has been found to be a number of discrete items a person can remember while performing other tasks (Miller 1956) If a class contains more than about seven data members, consider whether the class should be decomposed into multiple smaller classes (Riel 1996) Move unrelated information to another class In some cases, you ll find that half a class s routines work with half the class s data and half the routines work with the other half of the data In such a case, you really have two classes masquerading as one Break them up! 24 24

25 More guidelines Favor read-time convenience to write-time convenience Be suspicious of classes of which there is only one instance Be suspicious of base classes of which there is only one derived class Avoid deep inheritance trees Provide services in pairs with their opposites 25 25

26 High-Quality Routines

27 What is a routine? Routine A routine is an individual method or procedure invocable for a single purpose A function returns a value A procedure does not return a value, but usually performs an operation on an object Examples include a function in C++, a method in Java, a function or sub procedure in Microsoft Visual Basic What is a high-quality routine? That s a harder question Perhaps the easiest answer is to show what a high-quality routine is not 27 27

28 Example of a low-quality code 28 28

29 Example of a low-quality code The routine has a bad name HandleStuff() tells you nothing about what the routine does The routine isn t documented The routine has a bad layout It gives few hints about its logical organization The routine s input variable is changed (inputrec) If it s an input variable, its value should not be modified (and in C++ it should be declared const) The routine reads and writes global variables it reads from corpexpense and writes to profit 29 29

30 Example of a low-quality code The routine doesn t have a single purpose It initializes some variables, writes to a database, does some calculations none of which seem to be related to each other in any way The routine doesn t defend itself against bad data If crntqtr equals 0, the expression ytdrevenue * 4.0 / (double) crntqtr causes a divide-by-zero error The routine uses several magic numbers: 100, 4.0, 12, 2, and 3 Some of the routine s parameters are unused screenx and screeny are not referenced within the routine One of the routine s parameters is passed incorrectly prevcolor is labeled as a reference parameter (&) even though it isn t assigned a value within the routine 30 30

31 Example of a low-quality code The routine has too many parameters The upper limit for an understandable number of parameters is about 7; this routine has 11 The parameters are laid out in such an unreadable way that most people wouldn t try to examine them closely or even count them The routine s parameters are poorly ordered and are not documented 31 31

32 Good routine names Avoid meaningless, vague, or wishy-washy verbs Some verbs are elastic, stretched to cover just about any meaning Routine names like HandleCalculation(), PerformServices(), OutputUser(), ProcessInput(), and DealWithOutput() don t tell you what the routines do The exception would be when the verb handle was used in the specific technical sense of handling an event Don t differentiate routine names solely by number 32 32

33 Good routine names Describe everything the routine does Describe all the outputs and side effects If a routine computes report totals and opens an output file, ComputeReportTotals() is not an adequate name for the routine ComputeReportTotalsAndOpenOutputFile() is an adequate name but is too long and silly If you have routines with side effects, you ll have many long, silly names The cure is not to use less-descriptive routine names; the cure is to program so that you cause things to happen directly rather than with side effects Make names of routines as long as necessary Research shows that the optimum average length for a variable name is 9 to 15 characters 33 33

34 Good routine names For a function, use a description of the return value For example, cos(), customerid.next(), printer.isready(), and pen.currentcolor() are all good function names that indicate precisely what the functions return For a procedure, use a strong verb followed by an object The name should reflect what the procedure does, and an operation on an object implies a verb-plus object name PrintDocument(), CalcMonthlyRevenues(), CheckOrderlnfo(), and RepaginateDocument() are samples of good procedure names 34 34

35 Use opposites precisely Good routine names Using naming conventions for opposites helps consistency, which helps readability Opposite-pairs like first/last are commonly understood Opposite-pairs like FileOpen() and _lclose() are not symmetrical and are confusing Here are some common opposites: add/remove, increment/decrement, open/close begin/end, insert/delete, show/hide, create/destroy lock/unlock, source/target, first/last, min/max, start/stop get/put, next/previous, up/down, get/set, old/new 35 35

36 Routine parameters Put parameters in input-modify-output order Do not order parameters randomly or alphabetically List the parameters that are input-only first, input-and-output second, and output-only third 36 36

37 Routine parameters Consider creating your own in and out keywords Some modern languages don t support the in and out keywords like Ada does In those languages, you might still be able to use the preprocessor to create your own in and out keywords: 37 37

38 Macro routines Fully parenthesize macro expressions Cube( x+1 ) expands to x+1 * x + 1 * x Cube( x+1 ) expands to ++(x + 1) * (x + 1) * (x + 1) ++Cube( x+1 ) expands to ++((x + 1) * (x + 1) * (x + 1)) 38 38

39 More guidelines The theoretical best maximum length is often described as one screen or one or two pages of program listing, approximately 50 to 150 lines Use all the parameters If you pass a parameter to a routine, use it In one study, 46 percent of routines with no unused variables had no errors, and only 17 to 29 percent of routines with more than one unreferenced variable had no errors (Card, Church, and Agresti 1986) Limit the number of a routine s parameters to about seven Seven is a magic number for people s comprehension Psychological research has found that people generally cannot keep track of more than about seven chunks of information at once (Miller 1956) 39 39

40 Defensive Programming

41 Protecting the program from invalid inputs No garbage in, garbage out A good program never puts out garbage, regardless of what it takes in A good program uses garbage in, nothing out, garbage in, error message out, or no garbage allowed in Three general ways to handle garbage in Check the values of all data from external sources Check the values of all routine input parameters Decide how to handle bad inputs 41 41

42 Assertions An assertion is code that s used during development usually a routine or macro that allows a program to check itself as it runs Assertions are normally compiled into the code at development time and compiled out of the code for production An assertion usually takes two arguments: a boolean expression that describes the assumption that s supposed to be true, and a message to display if it isn t 42 42

43 Building your own assertion mechanism Many languages have built-in support for assertions, including C++, Java, and Microsoft Visual Basic If your language doesn t directly support assertion routines, they are easy to write The standard C++ assert macro doesn t provide for text messages Here s an example of an improved ASSERT implemented as a C++ macro 43 43

44 Guidelines for using assertions Use error-handling code for conditions you expect to occur; use assertions for conditions that should never occur Error-handling code checks for off-nominal circumstances that might not occur very often, but that have been anticipated by the programmer who wrote the code and that need to be handled by the production code Error handling typically checks for bad input data; assertions check for bugs in the code A good way to think of assertions is as executable documentation You can t rely on them to make the code work, but they can document assumptions more actively than program-language comments can 44 44

45 Guidelines for using assertions Use assertions to document and verify preconditions and postconditions Part of an approach to program design and development known as design by contract (Meyer 1997) 45 45

46 Error handling techniques How do you handle errors that you do expect to occur? Options for error handling Return a neutral value A numeric computation might return 0 A string operation might return an empty string, or a pointer operation might return an empty pointer Substitute the next piece of valid data Return the same answer as the previous time Substitute the closest legal value Log a warning message to a file Return an error code Display an error message wherever the error is encountered Shut down 46 46

47 Barricade the program to contain the damage caused by errors 47 47

48 Pseudocode Programming Process

49 Pseudocode for pros The term pseudocode refers to an informal, English-like notation for describing how an algorithm, a routine, a class, or a program will work The Pseudocode Programming Process defines a specific approach to using pseudocode to streamline the creation of code within routines Guidelines Use English-like statements that precisely describe specific operations Avoid syntactic elements from the target programming language Write pseudocode at the level of intent Write pseudocode at a low enough level that generating code from it will be nearly automatic 49 49

50 Bad pseudocode It includes target language coding details, such as *hrsrcptr (in specific C-language pointer notation) and malloc() (a specific C-language function) This pseudocode block focuses on how the code will be written rather than on the meaning of the design It gets into coding details whether the routine returns a 1 or a

51 Good pseudocode It s written entirely in English It is also written at the level of intent 51 51

52 Code the routine 52 52

53 Start with pseudocode 53 53

54 Write the routine declaration Write the routine interface statement the function declaration in C++ or method declaration in Java Turn the original header comment into a programminglanguage comment 54 54

55 Turn the pseudocode into high-level comments 55 55

56 Fill in the code below each comment 56 56