se7_ch07_v07

8/14/2019 se7_ch07_v07

1/93

Slide 7.1

The McGraw-Hill Companies, 2007

Object-Oriented andClassical Software

Engineering

Seventh Edition, WCB/McGraw-Hill, 2007

Stephen R. [email protected]

8/14/2019 se7_ch07_v07

2/93

Slide 7.2


CHAPTER 7

FROM MODULES

TO OBJECTS

8/14/2019 se7_ch07_v07

3/93

Slide 7.3


Overview

What is a module?

Cohesion

Coupling

Data encapsulation

Abstract data types

Information hiding

Objects

Inheritance, polymorphism, and dynamic binding

The object-oriented paradigm

8/14/2019 se7_ch07_v07

4/93

Slide 7.4


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

8/14/2019 se7_ch07_v07

5/93

Slide 7.5


Design of Computer

A highly incompetent

computer architectdecides to build anALU, shifter, and 16registers with AND,

OR, and NOT gates,rather than NAND orNOR gates

Figure 7.1

8/14/2019 se7_ch07_v07

6/93

Slide 7.6


Design of Computer (contd)

The architect

designs three

silicon chips

Figure 7.2

8/14/2019 se7_ch07_v07

7/93

Slide 7.7


Design of Computer (contd)

Redesign with

one gate typeper chip

Resultingmasterpiece

Figure 7.3

8/14/2019 se7_ch07_v07

8/93

Slide 7.8


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, andMinimal relationships between modules

8/14/2019 se7_ch07_v07

9/93

Slide 7.9


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

8/14/2019 se7_ch07_v07

10/93

Slide 7.10


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 isnamed compute_square_root

The underscores denote that the classical

paradigm is used here

8/14/2019 se7_ch07_v07

11/93

Slide 7.11


7.2 Cohesion

The degree of interaction within a module

Seven categories or levels of cohesion (non-linear

scale)

Figure 7.4

8/14/2019 se7_ch07_v07

12/93

Slide 7.12


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 likeEvery module will consist of between 35 and 50

statements

8/14/2019 se7_ch07_v07

13/93

Slide 7.13


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

8/14/2019 se7_ch07_v07

14/93

Slide 7.14


7.2.2 Logical Cohesion

A module has logical cohesion when it performs a

series of related actions, one of which is selectedby the calling module

8/14/2019 se7_ch07_v07

15/93

Slide 7.15


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 interfacecontains 21 pieces of data

8/14/2019 se7_ch07_v07

16/93

8/14/2019 se7_ch07_v07

17/93

Slide 7.17


Why Is Logical Cohesion So Bad? (contd)

A new tape unit is installed

What is the effect on the laser printer?

Figure 7.5

8/14/2019 se7_ch07_v07

18/93

Slide 7.18


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)

8/14/2019 se7_ch07_v07

19/93

Slide 7.19


Why Is Temporal Cohesion So Bad?

The actions of this module are weakly related to

one another, but strongly related to actions in

other modulesConsider sales_district_table

Not reusable

8/14/2019 se7_ch07_v07

20/93

Slide 7.20


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

8/14/2019 se7_ch07_v07

21/93

Slide 7.21


Why Is Procedural Cohesion So Bad?

The actions are still weakly connected, so the

module is not reusable

8/14/2019 se7_ch07_v07

22/93

Slide 7.22


7.2.5 Communicational Cohesion

A module has communicational cohesion if it

performs a series of actions related by theprocedure 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

8/14/2019 se7_ch07_v07

23/93

Slide 7.23


Why Is Communicational Cohesion So Bad?

Still lack of reusability

8/14/2019 se7_ch07_v07

24/93

Slide 7.24


7.2.6 Functional Cohesion

A module with functional cohesion performs

exactly one action

8/14/2019 se7_ch07_v07

25/93

Slide 7.25


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

8/14/2019 se7_ch07_v07

26/93

Slide 7.26


Why Is Functional Cohesion So Good?

More reusable

Corrective maintenance is easier

Fault isolation

Fewer regression faults

Easier to extend a product

8/14/2019 se7_ch07_v07

27/93

Slide 7.27


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

8/14/2019 se7_ch07_v07

28/93

Slide 7.28


Why Is Informational Cohesion So Good?

Essentially, this is an abstract data type (see later)

Figure 7.6

8/14/2019 se7_ch07_v07

29/93

Slide 7.29


7.2.8 Cohesion Example

Figure 7.7

8/14/2019 se7_ch07_v07

30/93

Slide 7.30

The McGraw-Hill Companies, 2007Figure 7.8

7.3 Coupling

The degree of interaction between two modules

Five categories or levels of coupling (non-linear scale)

8/14/2019 se7_ch07_v07

31/93

Slide 7.31


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

8/14/2019 se7_ch07_v07

32/93

Slide 7.32


Why Is Content Coupling So Bad?

Almost any change to module q, even recompilingq with a new compiler or assembler, requires achange to module p

8/14/2019 se7_ch07_v07

33/93

Slide 7.33


7.3.2 Common Coupling

Two modules are common coupled if they have

write access to global data

Example 1Modulesccaandccbcan access and change the

value ofglobal_variable

Figure 7.9

3 2 C C ( )

8/14/2019 se7_ch07_v07

34/93

Slide 7.34


7.3.2 Common Coupling (contd)

Example 2:

Modulesccaandccbboth have access to the same

database, and can both read andwrite the same record

Example 3:FORTRANcommon

COBOLcommon (nonstandard)

COBOL-80global

Wh I C C li S B d?

8/14/2019 se7_ch07_v07

35/93

Slide 7.35


Why Is Common Coupling So Bad?

It contradicts the spirit of structured

programmingThe resulting code is virtually unreadable

What causes this loop to terminate?

Figure 7.10

Wh I C C li S B d? ( td)

8/14/2019 se7_ch07_v07

36/93

Slide 7.36


Why Is Common Coupling So Bad? (contd)

Modules can have side-effects

This affects their readabilityExample:edit_this_transaction (record_7)

The entire module must be read to find out what it does

A change during maintenance to the declaration ofa global variable in one module necessitatescorresponding changes in other modules

Common-coupled modules are difficult to reuse

Wh I C C li S B d? ( td)

8/14/2019 se7_ch07_v07

37/93

Slide 7.37


Why Is Common Coupling So Bad? (contd)

Common coupling between a modulepand the

rest of the product can change without changingpin any wayClandestine common coupling

Example: The Linux kernel

A module is exposed to more data than necessaryThis can lead to computer crime

7 3 3 C t l C li

8/14/2019 se7_ch07_v07

38/93

Slide 7.38


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

C t l C li ( td)

8/14/2019 se7_ch07_v07

39/93

Slide 7.39


Control Coupling (contd)

Modulep calls module q

Message: I have failed data

Message: I have failed, so write error message ABC123 control

Wh I C t l C li S B d?

8/14/2019 se7_ch07_v07

40/93

Slide 7.40


Why Is Control Coupling So Bad?

The modules are not independent

Module q (the called module) must know the internalstructure and logic of module p

This affects reusability

Associated with modules of logical cohesion

7 3 4 St C li

8/14/2019 se7_ch07_v07

41/93

Slide 7.41


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 datastructures part_record

satellite_coordinates

segment_table

St C li ( td)

8/14/2019 se7_ch07_v07

42/93

Slide 7.42


Stamp Coupling (contd)

Two modules are stamp coupled if a data structure

is passed as a parameter, but the called moduleoperates on some but not all of the individual

components of the data structure

Wh Is Stamp Co pling So Bad?

8/14/2019 se7_ch07_v07

43/93

Slide 7.43


Why Is Stamp Coupling So Bad?

It is not clear, without reading the entire module,

which fields of a record are accessed or changedExample

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)

8/14/2019 se7_ch07_v07

44/93

Slide 7.44


Why Is Stamp Coupling So Bad? (contd)

However, there is nothing wrong with passing a

data structure as a parameter, provided that allthecomponents of the data structure are accessed

and/or changed

Examples:invert_matrix (original_matrix, inverted_matrix);

print_inventory_record (warehouse_record);

8/14/2019 se7_ch07_v07

45/93

Why Is Data Coupling So Good?

8/14/2019 se7_ch07_v07

46/93

Slide 7.46


Why Is Data Coupling So Good?

The difficulties of content, common, control, and

stamp coupling are not present

Maintenance is easier

8/14/2019 se7_ch07_v07

47/93

Coupling Example (contd)

8/14/2019 se7_ch07_v07

48/93

Slide 7.48



Interface description

Figure 7.12


8/14/2019 se7_ch07_v07

49/93

Slide 7.49



Coupling between all pairs of modules

Figure 7.13

7 3 7 The Importance of Coupling

8/14/2019 se7_ch07_v07

50/93

Slide 7.50


7.3.7 The Importance of Coupling

As a result of tight coupling

A change to modulepcan require a correspondingchange to moduleq

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

8/14/2019 se7_ch07_v07

51/93

Slide 7.51


Key Definitions

Figure 7.14

7 4 Data Encapsulation

8/14/2019 se7_ch07_v07

52/93

Slide 7.52


7.4 Data Encapsulation

Example

Design an operating system for a large mainframecomputer. 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 bya 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

8/14/2019 se7_ch07_v07

53/93

Slide 7.53



Figure 7.15


8/14/2019 se7_ch07_v07

54/93

Slide 7.54



Figure 7.16

Data Encapsulation (contd)

8/14/2019 se7_ch07_v07

55/93

Slide 7.55


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

DevelopmentMaintenance

Data Encapsulation and Development

8/14/2019 se7_ch07_v07

56/93

Slide 7.56


Data Encapsulation and Development

Data encapsulation is an example ofabstraction

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

8/14/2019 se7_ch07_v07

57/93

Slide 7.57


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

8/14/2019 se7_ch07_v07

58/93

Slide 7.58


Stepwise Refinement

1. Design the product in terms of higher level

conceptsIt is irrelevant how job queues are implemented

2. Then design the lower level componentsTotally ignore what use will be made of them

Stepwise Refinement (contd)

8/14/2019 se7_ch07_v07

59/93

Slide 7.59


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

8/14/2019 se7_ch07_v07

60/93

Slide 7.60


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

changeData structures are unlikely to change

Implementation details may change

Data encapsulation provides a way to cope with

change

Implementation of JobQueueClass

8/14/2019 se7_ch07_v07

61/93

Slide 7.61


Implementation ofJobQueueClass

C++

JavaFigure 7.17 Figure 7.18

Implementation of queueHandler

8/14/2019 se7_ch07_v07

62/93

Slide 7.62


Implementation ofqueueHandler

C++ Java

Figure 7.19 Figure 7.20

Data Encapsulation and Maintenance (contd)

8/14/2019 se7_ch07_v07

63/93

Slide 7.63


Data Encapsulation and Maintenance (contd)

What happens if the queue is now implemented

as a two-way linked list ofJobRecordClass?A module that usesJobRecordClassneed not be

changed at all, merely recompiled

Figure 7.22

Figure 7.21

C++

Java

8/14/2019 se7_ch07_v07

64/93

7.5 Abstract Data Types

8/14/2019 se7_ch07_v07

65/93

Slide 7.65


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 ofthat data type

Abstract data type

Abstract Data Type Example

8/14/2019 se7_ch07_v07

66/93

Slide 7.66


Abstract Data Type Example

(Problems caused by public attributes solved later)

Figure 7.24

8/14/2019 se7_ch07_v07

67/93

8/14/2019 se7_ch07_v07

68/93

Slid 7 69Information Hiding (contd)

8/14/2019 se7_ch07_v07

69/93

Slide 7.69


g ( )

C++ abstract

data typeimplementation

with information

hiding

Figure 7.26

Slid 7 70Information Hiding (contd)

8/14/2019 se7_ch07_v07

70/93

Slide 7.70


g ( )

Effect of information hiding viaprivateattributes

Figure 7.27

Slide 7 71Major Concepts of Chapter 7

8/14/2019 se7_ch07_v07

71/93

Slide 7.71


j p p

Figure 7.28

Slide 7 727.7 Objects

8/14/2019 se7_ch07_v07

72/93

Slide 7.72


j

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

8/14/2019 se7_ch07_v07

73/93

Slide 7 74Inheritance (contd)

8/14/2019 se7_ch07_v07

74/93

Slide 7.74


( )

DefineParentClassto be a subclass ofHumanBeingClass

An instance ofParentClasshas all the attributes of aninstance ofHumanBeingClass, plus attributes of his/herown nameOfOldestChild, numberOfChildren

An instance ofParentClass inherits all attributes ofHumanBeingClass


8/14/2019 se7_ch07_v07

75/93

Slide 7.75


( )

The property of inheritance is an essential feature

of all object-oriented languagesSuch as Smalltalk, C++, Ada 95, Java

But not of classical languagesSuch as C, COBOL or FORTRAN


8/14/2019 se7_ch07_v07

76/93

Slide 7.76


( )

UML notation

Inheritance is represented by a large open triangle

Figure 7.29

Slide 7 77Java Implementation

8/14/2019 se7_ch07_v07

77/93

Slide 7.77


Figure 7.30

Slide 7 78Aggregation

8/14/2019 se7_ch07_v07

78/93

Slide 7.78


UML notation for aggregation open diamond

Figure 7.31

Slide 7.79Association

8/14/2019 se7_ch07_v07

79/93

Slide 7.79


Figure 7.32

UML notation for association line

Optional navigation triangle

Slide 7.80Equivalence of Data and Action

8/14/2019 se7_ch07_v07

80/93


Classical paradigm

record_1.field_2

Object-oriented paradigm thisObject.attributeB

thisObject.methodC ()

8/14/2019 se7_ch07_v07

81/93

Slide 7.82Inheritance, Polymorphism and Dynamic Binding (contd)

8/14/2019 se7_ch07_v07

82/93


Figure 7.33(b)

Object-oriented paradigm


8/14/2019 se7_ch07_v07

83/93


Classical code to open a file

The correct method is explicitly selected

Figure 7.34(a)


8/14/2019 se7_ch07_v07

84/93


Object-oriented code to open a file

The correct method is invoked at run-time (dynamically)

Method open can be applied to objects of different classes Polymorphic

Figure 7.34(b)

8/14/2019 se7_ch07_v07

85/93


8/14/2019 se7_ch07_v07

86/93


Polymorphism and dynamic binding

Can have a negative impact on maintenance The code is hard to understand if there are multiple possibilities

for a specific method

Polymorphism and dynamic bindingA strength and a weakness of the object-oriented

paradigm

Slide 7.877.9 The Object-Oriented Paradigm

8/14/2019 se7_ch07_v07

87/93


Reasons for the success of the object-oriented

paradigm

The object-oriented paradigm gives overallequal

attention to data and operations

At any one time, data or operations may be favored

A well-designed object (high cohesion, low coupling)

models all the aspects of one physical entity

Implementation details are hidden

Slide 7.88The Object-Oriented Paradigm (contd)

8/14/2019 se7_ch07_v07

88/93


The reason why the structured paradigm worked

well at firstThe alternative was no paradigm at all


8/14/2019 se7_ch07_v07

89/93


How do we know that the object-oriented

paradigm is the best current alternative?

We dont

However, most reports are favorable Experimental data (e.g., IBM [1994])

Survey of programmers (e.g., Johnson [2000])

Slide 7.90Weaknesses of the Object-Oriented Paradigm

8/14/2019 se7_ch07_v07

90/93


Development effort and size can be large

Ones first object-oriented project can be larger

than expected

Even taking the learning curve into accountEspecially if there is a GUI

However, some classes can frequently be reused

in the next project

Especially if there is a GUI

8/14/2019 se7_ch07_v07

91/93

Slide 7.92Weaknesses of the Object-Oriented Paradigm (contd)

8/14/2019 se7_ch07_v07

92/93


As already explained, the use of polymorphism

and dynamic binding can lead to problems

It is easy to write bad code in any language

It is especially easy to write bad object-oriented code


8/14/2019 se7_ch07_v07

93/93

Some day, the object-oriented paradigm will

undoubtedly be replaced by something betterAspect-oriented programming is one possibility

But there are many other possibilities

se7_ch07_v07

Documents

Transcript of se7_ch07_v07