Seventh Edition - Chương 7 pptx

Slide 7.1 © The McGraw-Hill Companies, 2007 Object-Oriented and Classical Software Engineering Seventh Edition, WCB/McGraw-Hill, 2007 Stephen R. Schach srs@vuse.vanderbilt.edu Slide 7.2 © The McGraw-Hill Companies, 2007 CHAPTER 7 FROM MODULES TO OBJECTS Slide 7.3 © The McGraw-Hill Companies, 2007 Overview  What is a module?  Cohesion  Coupling  Data encapsulation  Abstract data types  Information hiding  Objects  Inheritance, polymorphism, and dynamic binding  The object-oriented paradigm Slide 7.4 © The McGraw-Hill Companies, 2007 7.1 What Is a Module?  A lexically contiguous sequence of program statements, bounded by boundary elements, with an aggregate identifier  “Lexically contiguous”  Adjoining in the code  “Boundary elements”  { }  begin end  “Aggregate identifier”  A name for the entire module Slide 7.5 © The McGraw-Hill Companies, 2007 Design of Computer  A highly incompetent computer architect decides to build an ALU, shifter, and 16 registers with AND, OR, and NOT gates, rather than NAND or NOR gates Figure 7.1 Slide 7.6 © The McGraw-Hill Companies, 2007 Design of Computer (contd)  The architect designs three silicon chips Figure 7.2 Slide 7.7 © The McGraw-Hill Companies, 2007 Design of Computer (contd)  Redesign with one gate type per chip  Resulting “masterpiec e” Figure 7.3 Slide 7.8 © The McGraw-Hill Companies, 2007 Computer Design (contd)  The two designs are functionally equivalent  The second design is  Hard to understand  Hard to locate faults  Difficult to extend or enhance  Cannot be reused in another product  Modules must be like the first design  Maximal relationships within modules, and  Minimal relationships between modules Slide 7.9 © The McGraw-Hill Companies, 2007 Composite/Structured Design  A method for breaking up a product into modules to achieve  Maximal interaction within a module, and  Minimal interaction between modules  Module cohesion  Degree of interaction within a module  Module coupling  Degree of interaction between modules Slide 7.10 © The McGraw-Hill Companies, 2007 Function, Logic, and Context of a Module  In C/SD, the name of a module is its function  Example:  A module computes the square root of double precision integers using Newton’s algorithm. The module is named compute_square_root  The underscores denote that the classical paradigm is used here [...]... McGraw-Hill Companies, 20 07 Why Is Informational Cohesion So Good? Slide 7. 28 Figure 7. 6  Essentially, this is an abstract data type (see later) © The McGraw-Hill Companies, 20 07 7.2.8 Cohesion Example © The McGraw-Hill Companies, 20 07 Slide 7. 29 Figure 7. 7 7. 3 Coupling  Slide 7. 30 The degree of interaction between two modules Five categories or levels of coupling (non-linear scale) Figure 7. 8 ©... The McGraw-Hill Companies, 20 07 Why Is Logical Cohesion So Bad? Slide 7. 16  The interface is difficult to understand  Code for more than one action may be intertwined  Difficult to reuse © The McGraw-Hill Companies, 20 07 Why Is Logical Cohesion So Bad? (contd) Slide 7. 17  A new tape unit is installed What is the effect on the laser printer? Figure 7. 5 © The McGraw-Hill Companies, 20 07 7.2.3 Temporal... write_to_diskette  Example 4:  calculate_sales_commission © The McGraw-Hill Companies, 20 07 Slide 7. 25 Why Is Functional Cohesion So Good? Slide 7. 26  More reusable  Corrective maintenance is easier  Fault isolation  Fewer regression faults  Easier to extend a product © The McGraw-Hill Companies, 20 07 7.2 .7 Informational Cohesion  Slide 7. 27 A module has informational cohesion if it performs a number... assembler, requires a change to module © The McGraw-Hill Companies, 20 07 Slide 7. 32 p 7. 3.2 Common Coupling Slide 7. 33  Two modules are common coupled if they have write access to global data  Example 1 Figure 7. 9  Modules cca and ccb can access and change the value of global_variable © The McGraw-Hill Companies, 20 07 7.3.2 Common Coupling (contd)  Slide 7. 34 Example 2:  Modules cca and ccb both have... calculate_new_coordinates_and_send_them_to_terminal © The McGraw-Hill Companies, 20 07 Why Is Communicational Cohesion So Bad? Slide 7. 23  Still lack of reusability © The McGraw-Hill Companies, 20 07 7.2.6 Functional Cohesion  Slide 7. 24 A module with functional cohesion performs exactly one action © The McGraw-Hill Companies, 20 07 7.2.6 Functional Cohesion  Example 1:  get_temperature_of_furnace  Example... COBOL common (nonstandard)  COBOL-80 global © The McGraw-Hill Companies, 20 07 Why Is Common Coupling So Bad?  It contradicts the spirit of structured programming The resulting code is virtually unreadable What causes this loop to terminate? Figure 7. 10 © The McGraw-Hill Companies, 20 07 Slide 7. 35 Why Is Common Coupling So Bad? (contd) Slide 7. 36  Modules can have side-effects  This affects their... sales_district_table  Not reusable © The McGraw-Hill Companies, 20 07 Slide 7. 19 7. 2.4 Procedural Cohesion Slide 7. 20  A module has procedural cohesion if it performs a series of actions related by the procedure to be followed by the product  Example:  read_part_number_and_update_repair_record_on_ master_file © The McGraw-Hill Companies, 20 07 Why Is Procedural Cohesion So Bad?  Slide 7. 21 The actions are still weakly.. .7. 2 Cohesion Slide 7. 11  The degree of interaction within a module  Seven categories or levels of cohesion (nonlinear scale) Figure 7. 4 © The McGraw-Hill Companies, 20 07 7.2.1 Coincidental Cohesion Slide 7. 12  A module has coincidental cohesion if it performs multiple, completely unrelated actions  Example:  print_next_line, reverse_string_of_characters_comprising_second_ parameter, add _7_ to_fifth_parameter,... of between 35 and 50 statements” © The McGraw-Hill Companies, 20 07 Why Is Coincidental Cohesion So Bad? Slide 7. 13  It degrades maintainability  A module with coincidental cohesion is not reusable  The problem is easy to fix  Break the module into separate modules, each performing one task © The McGraw-Hill Companies, 20 07 7.2.2 Logical Cohesion  Slide 7. 14 A module has logical cohesion when it... McGraw-Hill Companies, 20 07 7.3.1 Content Coupling Slide 7. 31  Two modules are content coupled if one directly references contents of the other  Example 1:  Module p modifies a statement of module q  Example 2:  Module p refers to local data of module q in terms of some numerical displacement within q  Example 3:  Module p branches into a local label of module q © The McGraw-Hill Companies, 2007 . Slide 7. 1 © The McGraw-Hill Companies, 20 07 Object-Oriented and Classical Software Engineering Seventh Edition, WCB/McGraw-Hill, 20 07 Stephen R. Schach srs@vuse.vanderbilt.edu Slide 7. 2 ©. gates Figure 7. 1 Slide 7. 6 © The McGraw-Hill Companies, 20 07 Design of Computer (contd)  The architect designs three silicon chips Figure 7. 2 Slide 7. 7 © The McGraw-Hill Companies, 20 07 Design. here Slide 7. 11 © The McGraw-Hill Companies, 20 07 7.2 Cohesion  The degree of interaction within a module  Seven categories or levels of cohesion (non- linear scale) Figure 7. 4 Slide 7. 12 © The