1. Slide 7.1 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Object-Oriented Software Engineering WCB/McGraw-Hill, 2008 Stephen R. Schach srs@vuse.vanderbilt.edu

2. Slide 7.2 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. CHAPTER 7 FROM MODULES TO OBJECTS

3. Slide 7.3 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Overview l What is a module? l Cohesion l Coupling l Data encapsulation l Abstract data types l Information hiding l Objects l Inheritance, polymorphism, and dynamic binding l The object-oriented paradigm

4. Slide 7.4 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 7.1 What Is a Module? l 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

5. Slide 7.5 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Design of Computer l 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

6. Slide 7.6 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Design of Computer (contd) l The architect designs three silicon chips Figure 7.2

7. Slide 7.7 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Design of Computer (contd) l Redesign with one gate type per chip l Resulting “masterpiece” Figure 7.3

8. Slide 7.8 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Computer Design (contd) l 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 l Modules must be like the first design – Maximal relationships within modules, and – Minimal relationships between modules

9. Slide 7.9 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Composite/Structured Design A method for breaking up a product into modules to achieve – Maximal interaction within a module, and – Minimal interaction between modules l Module cohesion – Degree of interaction within a module l Module coupling – Degree of interaction between modules

10. Slide 7.10 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Function, Logic, and Context of a Module l In C/SD, the name of a module is its function l Example: – A module computes the square root of double precision integers using Newton’s algorithm. The module is named computeSquareRoot

11. Slide 7.11 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 7.2 Cohesion l The degree of interaction within a module l Seven categories or levels of cohesion (non-linear scale) Figure 7.4

12. Slide 7.12 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 7.2.1 Coincidental Cohesion l A module has coincidental cohesion if it performs multiple, completely unrelated actions l Example: – printTheNextLine, reverseStringOfCharactersComprisingSecondparameter, add7ToFifthParameter, convertFourthParameterTofloatingPoint l Such modules arise from rules like – “Every module will consist of between 35 and 50 statements”

13. Slide 7.13 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Why Is Coincidental Cohesion So Bad? l It degrades maintainability l A module with coincidental cohesion is not reusable l The problem is easy to fix – Break the module into separate modules, each performing one task

14. Slide 7.14 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 7.2.2 Logical Cohesion l A module has logical cohesion when it performs a series of related actions, one of which is selected by the calling module

15. Slide 7.15 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Logical Cohesion (contd) l Example 1: functionCode = 7; newOperation (functionCode, dummy1, dummy2, dummy3); // dummy1, dummy2, and dummy3 are dummy variables, // not used if functionCode is equal to 7 l Example 2: – An object performing all input and output l Example 3: – One version of OS/VS2 contained a module with logical cohesion performing 13 different actions. The interface contains 21 pieces of data

16. Slide 7.16 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Why Is Logical Cohesion So Bad? l The interface is difficult to understand l Code for more than one action may be intertwined l Difficult to reuse

17. Slide 7.17 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Why Is Logical Cohesion So Bad? (contd) l A new tape unit is installed – What is the effect on the laser printer? Figure 7.5

18. Slide 7.18 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 7.2.3 Temporal Cohesion l A module has temporal cohesion when it performs a series of actions related in time l Example: – openOldMasterFile, newMasterFile, transactionFile, and printFile; initializeSalesDistrictTable, readFirstTransactionRecord, readFirstOldMasterRecord (a.k.a. performInitialization)

19. Slide 7.19 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Why Is Temporal Cohesion So Bad? l The actions of this module are weakly related to one another, but strongly related to actions in other modules – Consider salesDistrictTable l Not reusable

20. Slide 7.20 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 7.2.4 Procedural Cohesion l A module has procedural cohesion if it performs a series of actions related by the procedure to be followed by the product l Example: – readPartNumberAndUpdateRepairRecordOnMasterFile

21. Slide 7.21 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Why Is Procedural Cohesion So Bad? l The actions are still weakly connected, so the module is not reusable

22. Slide 7.22 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 7.2.5 Communicational Cohesion l 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 l Example 1: updateRecordInDatabaseAndWriteItToAuditTrail l Example 2: calculateNewCoordinatesAndSendThemToTerminal

23. Slide 7.23 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Why Is Communicational Cohesion So Bad? l Still lack of reusability

24. Slide 7.24 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 7.2.6 Functional Cohesion l A module with functional cohesion performs exactly one action

25. Slide 7.25 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 7.2.6 Functional Cohesion l Example 1: – getTemperatureOfFurnace l Example 2: – computeOrbitalOfElectron l Example 3: – writeToDiskette l Example 4: – calculateSalesCommission

26. Slide 7.26 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Why Is Functional Cohesion So Good? l More reusable l Corrective maintenance is easier – Fault isolation – Fewer regression faults l Easier to extend a product

27. Slide 7.27 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 7.2.7 Informational Cohesion l 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

28. Slide 7.28 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Why Is Informational Cohesion So Good? l Essentially, this is an abstract data type (see later) Figure 7.6

29. Slide 7.29 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 7.2.8 Cohesion Example Figure 7.7

30. Slide 7.30 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Figure 7.8 7.3 Coupling l The degree of interaction between two modules – Five categories or levels of coupling (non-linear scale)

31. Slide 7.31 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 7.3.1 Content Coupling l Two modules are content coupled if one directly references contents of the other l Example 1: – Module p modifies a statement of module q l Example 2: – Module p refers to local data of module q in terms of some numerical displacement within q l Example 3: – Module p branches into a local label of module q

32. Slide 7.32 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 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

33. Slide 7.33 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 7.3.2 Common Coupling l Two modules are common coupled if they have write access to global data l Example 1 – Modules cca and ccb can access and change the value of globalVariable Figure 7.9

34. Slide 7.34 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 7.3.2 Common Coupling (contd) l Example 2: – Modules cca and ccb both have access to the same database, and can both read and write the same record l Example 3: – FORTRAN common – COBOL common (nonstandard) – COBOL-80 global

35. Slide 7.35 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Why Is Common Coupling So Bad? l It contradicts the spirit of structured programming – The resulting code is virtually unreadable – What causes this loop to terminate? Figure 7.10

36. Slide 7.36 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Why Is Common Coupling So Bad? (contd) l Modules can have side-effects – This affects their readability – Example: editThisTransaction (record7) – The entire module must be read to find out what it does l A change during maintenance to the declaration of a global variable in one module necessitates corresponding changes in other modules l Common-coupled modules are difficult to reuse

37. Slide 7.37 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 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 l A module is exposed to more data than necessary – This can lead to computer crime

38. Slide 7.38 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 7.3.3 Control Coupling l Two modules are control coupled if one passes an element of control to the other l Example 1: – An operation code is passed to a module with logical cohesion l Example 2: – A control switch passed as an argument

39. Slide 7.39 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Control Coupling (contd) Module p calls module q l Message: – I have failed — data l Message: – I have failed, so write error message ABC123 — control

40. Slide 7.40 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Why Is Control Coupling So Bad? l The modules are not independent – Module q (the called module) must know the internal structure and logic of module p – This affects reusability l Associated with modules of logical cohesion

41. Slide 7.41 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 7.3.4 Stamp Coupling l Some languages allow only simple variables as parameters – partNumber – satelliteAltitude – degreeOfMultiprogramming l Many languages also support the passing of data structures – partRecord – satelliteCoordinates – segmentTable

42. Slide 7.42 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Stamp Coupling (contd) l 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

43. Slide 7.43 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Why Is Stamp Coupling So Bad? l It is not clear, without reading the entire module, which fields of a record are accessed or changed – Example calculateWithholding (employeeRecord) l Difficult to understand l Unlikely to be reusable l More data than necessary is passed – Uncontrolled data access can lead to computer crime

44. Slide 7.44 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Why Is Stamp Coupling So Bad? (contd) l 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 l Examples: invertMatrix (originalMatrix, invertedMatrix); printInventoryRecord (warehouseRecord);

45. Slide 7.45 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 7.3.5 Data Coupling l 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) l Examples: – displayTimeOfArrival (flightNumber); – computeProduct (firstNumber, secondNumber); – getJobWithHighestPriority (jobQueue);

46. Slide 7.46 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Why Is Data Coupling So Good? l The difficulties of content, common, control, and stamp coupling are not present l Maintenance is easier

47. Slide 7.47 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 7.3.6 Coupling Example Figure 7.11

48. Slide 7.48 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Coupling Example (contd) l Interface description Figure 7.12

49. Slide 7.49 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. Coupling Example (contd) l Coupling between all pairs of modules Figure 7.13

50. Slide 7.50 Copyright © 2008 by The McGraw-Hill Companies, Inc. All rights reserved. 7.3.7 The Importance of Coupling l 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 l Good design has high cohesion and low coupling – What else characterizes good design? (see over)