软件工程第7版课件.ppt

Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,Slide 7.,*,The McGraw-Hill Companies,2007,Object-Oriented and Classical Software Engineering,Seventh Edition,WCB/McGraw-Hill,2007,Stephen R.Schach,srsvuse.vanderbilt.edu,CHAPTER 7,FROM MODULES,TO OBJECTS,Overview,What is a module?,Cohesion,Coupling,Data encapsulation,Abstract data types,Information hiding,Objects,Inheritance,polymorphism,and dynamic binding,The object-oriented paradigm,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,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,Design of Computer(contd),The architect designs three silicon chips,Figure 7.2,Design of Computer(contd),Redesign with one gate type per chip,Resulting“masterpiece”,Figure 7.3,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,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,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 Newtons algorithm.The module is named,compute_square_root,The underscores denote that the classical paradigm is used here,7.2 Cohesion,The degree of interaction within a module,Seven categories or levels of cohesion(non-linear scale),Figure 7.4,7.2.1 Coincidental Cohesion,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,convert_fourth_parameter_to_ floating_point,Such modules arise from rules like,“Every module will consist of between 35 and 50 statements”,Why Is Coincidental Cohesion So Bad?,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,7.2.2 Logical Cohesion,A module has logical cohesion when it performs a series of related actions,one of which is selected by the calling module,Logical Cohesion(contd),Example 1:,function_code=7;,new_operation(op code,dummy_1,dummy_2,dummy_3);,/dummy_1,dummy_2,and dummy_3 are dummy variables,/not used if function code is equal to 7,Example 2:,An object performing all input and output,Example 3:,One version of OS/VS2 contained a module with logical cohesion performing 13 different actions.The interface contains 21 pieces of data,Why Is Logical Cohesion So Bad?,The interface is difficult to understand,Code for more than one action may be intertwined,Difficult to reuse,Why Is Logical Cohesion So Bad?(contd),A new tape unit is installed,What is the effect on the laser printer?,Figure 7.5,7.2.3 Temporal Cohesion,A module has temporal cohesion when it performs a series of actions related in time,Example:,open_old_master_file,new_master_file,transaction_file,and print_file;initialize_sales_district_table,read_first_transaction_record,read_first_old_master_record(a.k.a.perform_initialization),Why Is Temporal Cohesion So Bad?,The actions of this module are weakly related to one another,but strongly related to actions in other modules,Consider,sales_district_table,Not reusable,7.2.4 Procedural Cohesion,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,Why Is Procedural Cohesion So Bad?,The actions are still weakly connected,so the module is not reusable,7.2.5 Communicational Cohesion,A module has communicational cohesion if it performs a series of actions related by the procedure to be followed by the product,but in addition all the actions operate on the same data,Example 1:,update_record_in_database_and_write_it_to_audit_trail,Example 2:,calculate_new_coordinates_and_send_them_to_terminal,Why Is Communicational Cohesion So Bad?,Still lack of reusability,7.2.6 Functional Cohesion,A module with functional cohesion performs exactly one action,7.2.6 Functional Cohesion,Example 1:,get_temperature_of_furnace,Example 2:,compute_orbital_of_electron,Example 3:,write_to_diskette,Example 4:,calculate_sales_commission,Why Is Functional Cohesion So Good?,More reusable,Corrective maintenance is easier,Fault isolation,Fewer regression faults,Easier to extend a product,7.2.7 Informational Cohesion,A module has informational cohesion if it performs a number of actions,each with its own entry point,with independent code for each action,all performed on the same data structure,Why Is Informational Cohesion So Good?,Essentially,this is an abstract data type(see later),Figure 7.6,7.2.8 Cohesion Example,Figure 7.7,Figure 7.8,7.3 Coupling,The degree of interaction between two modules,Five categories or levels of coupling(non-linear scale),7.3.1 Content Coupling,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,Why Is Content Coupling So Bad?,Almost any change to module,q,even recompiling,q,with a new compiler or assembler,requires a change to module,p,7.3.2 Common Coupling,Two modules are common coupled if they have write access to global data,Example 1,Modules,cca,and,ccb,can access,and change,the value of,global_variable,Figure 7.9,7.3.2 Common Coupling(contd),Example 2:,Modules,cca,and,ccb,both have access to the same database,and can both read,and,write,the same record,Example 3:,FORTRAN,common,COBOL,common,(nonstandard),COBOL-80,global,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,Why Is Common Coupling So Bad?(contd),Modules can have side-effects,This affects their readability,Example:,edit_this_transaction(record_7),The entire module must be read to find out what it does,A change during maintenance to the declaration of a global variable in one module necessitates corresponding changes in other modules,Common-coupled modules are difficult to reuse,Why Is Common Coupling So Bad?(contd),Common coupling between a module,p,and the rest of the product can change without changing,p,in any way,Clandestine common coupling,Example:The Linux kernel,A module is exposed to more data than necessary,This can lead to computer crime,7.3.3 Control Coupling,Two modules are control coupled if one passes an element of control to the other,Example 1:,An operation code is passed to a module with logical cohesion,Example 2:,A control switch passed as an argument,Control Coupling(contd),Module,p,calls module,q,Message:,I have failed,data,Message:,I have failed,so write error message ABC123,control,Why Is Control Coupling So Bad?,The modules are not independent,Module,q,(the called module)must know the internal structure and logic of module,p,This affects reusability,Associated with modules of logical cohesion,7.3.4 Stamp Coupling,Some languages allow only simple variables as parameters,part_number,satellite_altitude,degree_of_multiprogramming,Many languages also support the passing of data structures,part_record,satellite_coordinates,segment_table,Stamp Coupling,(contd),Two modules are stamp coupled if a data structure is passed as a parameter,but the called module operates on some but not all of the individual components of the data structure,Why Is Stamp Coupling So Bad?,It is not clear,without reading the entire module,which fields of a record are accessed or changed,Example,calculate_withholding(employee_record),Difficult to understand,Unlikely to be reusable,More data than necessary is passed,Uncontrolled data access can lead to computer crime,Why Is Stamp Coupling So Bad?(contd),However,there is nothing wrong with passing a data structure as a parameter,provided that,all,the components of the data structure are accessed and/or changed,Examples:,invert_matrix(original_matrix,inverted_matrix);,print_inventory_record(warehouse_record);,7.3.5 Data Coupling,Two modules are data coupled if all parameters are homogeneous data items(simple parameters,or data structures all of whose elements are used by called module),Examples:,display_time_of_arrival(flight_number);,compute_product(first_number,second_number);,get_job_with_highest_priority(job_queue);,Why Is Data Coupling So Good?,The difficulties of content,common,control,and stamp coupling are not present,Maintenance is easier,7.3.6.Coupling Example,Figure 7.11,Coupling Example(contd),Interface description,Figure 7.12,Coupling Example(contd),Coupling between all pairs of modules,Figure 7.13,7.3.7 The Importance of Coupling,As a result of tight coupling,A change to module,p,can require a corresponding change to module,q,If the corresponding change is not made,this leads to faults,Good design has high cohesion and low coupling,What else characterizes good design?(see over),Key Definitions,Figure 7.14,7.4 Data Encapsulation,Example,Design an operating system for a large mainframe computer.Batch jobs submitted to the computer will be classified as high priority,medium priority,or low priority.There must be three queues for incoming batch jobs,one for each job type.When a job is submitted by a user,the job is added to the appropriate queue,and when the operating system decides that a job is ready to be run,it is removed from its queue and memory is allocated to it,Design 1(Next slide),Low cohesion operations on job queues are spread all over the product,Data Encapsulation Design 1,Figure 7.15,Data Encapsulation Design 2,Figure 7.16,Data Encapsulation(contd),m_encapsulation,has informational cohesion,m_encapsulation,is an implementation of data encapsulation,A data structure(,job_queue,)together with operations performed on that data structure,Advantages,Development,Maintenance,Data Encapsulation and Development,Data encapsulation is an example of,abstraction,Job queue example:,Data structure,job_queue,Three new functions,initialize_job_queue,add_job_to_queue,delete_job_from_queue,7.4.1 Data Encapsulation and Development,Abstraction,Conceptualize problem at a higher level,Job queues and operations on job queues,Not a lower level,Records or arrays,Stepwise Refinement,1.Design the product in terms of higher level concepts,It is irrelevant how job queues are implemented,2.Then design the lower level components,Totally ignore what use will be made of them,Stepwise Refinement(contd),In the 1st step,assume the existence of the lower level,Our concern is the behavior of the data structure,job_queue,In the 2nd step,ignore the existence of the higher level,Our concern is the implementation of that behavior,In a larger product,there will be many levels of abstraction,7.4.2 Data Encapsulation and Maintenance,Identify the aspects of the product that are likely to change,Design the product so as to minimize the effects of change,Data structures are unlikely to change,Implementation details may change,Data encapsulation provides a way to cope with change,Implementation of,JobQueueClass,C+,Java,Figure 7.17,Figure 7.18,Implementation of,queueHandler,C+,Java,Figure 7.19,Figure 7.20,Data Encapsulation and Maintenance(contd),What happens if the queue is now implemented as a two-way linked list of,JobRecordClass,?,A module that uses,JobRecordClass,need not be changed at all,merely recompiled,Figure 7.22,Figure 7.21,C+,Java,Data Encapsulation and Maintenance(contd),Only implementation details of,JobQueueClass,have changed,Figure 7.23,7.5 Abstract Data Types,The problem with both implementations,There is only one queue,not three,We need:,Data type+operations performed on instantiations of that data type,Abstract data type,Abstract Data Type Example,(Problems caused by,public,attributes solved later),Figure 7.24,Another Abstract Data Type Example,(Problems caused by,public,attributes solved later),Figure 7.25,7.6 Information Hiding,Data abstraction,The designer thinks at the level of an ADT,Procedural abstraction,Define a procedure extend the language,Both are instances of a more general design concept,information hiding,Design the modules in a way that items likely to change are hidden,Future change is localized,Changes cannot affect other modules,Information Hiding(contd),C+abstract data type implementation with information hiding,Figure 7.26,Information Hiding(contd),Effect of information hiding via,private,attributes,Figure 7.27,Major Concepts of Chapter 7,Figure 7.28,7.7 Objects,First refinement,The product is designed in terms of abstract data types,Variables(“objects”)are instantiations of abstract data types,Second refinement,Class:an abstract data type that supports,inheritance,Objects are instantiations of classes,Inheritance,Define,HumanBeingClass,to be a,class,An instance of,HumanBeingClass,has,attributes,such as,age,height,gender,Assign values to the attributes when describing an object,Inheritance(contd),Define,ParentClass,to be a,subclass,of,HumanBeingClass,An instance of,ParentClass,has all the attributes of an instance of,HumanBeingClass,plus attributes of his/her own,nameOfOldestChild,numberOfChildren,An instance of,ParentClass,inherits all attributes of,HumanBeingClass,Inheritance(contd),The property of inheritance is an essential feature of all object-oriented languages,Such as Smalltalk,C+,Ada 95,Java,But not of classical languages,Such as C,COBOL or FORTRAN,Inheritance(contd),UML notation,Inheritance is represented by a large open triangle,Figure 7.29,Java Implementation,Figure 7.30,Aggregation,UML notation for aggregation open diamond,Figure 7.31,Figure 7.32,Association,UML notation for association line,Optional navigation triangle,Equivalence of Data and Action,Classical paradigm,record_1.field_2,Object-oriented paradigm,thisObject.attributeB,thisObject.methodC(),Figure 7.33a,7.8 Inheritance,Polymorphism and Dynamic Binding,Classical paradigm,We must explicitly invoke the appropriate version,Figure 7.33(b),Inheritance,Polymorphism and Dynamic Binding(contd),Object-oriented paradigm,Inheritance,Polymorphism and Dynamic Binding(contd),Classical code to open a file,The correct met