Module 3: UML In Action - Design Patterns

1. Module 3: UML In Action - Design Patterns

2. Overview • Books • Design Patterns – Basics • Creational Design Patterns • Structural Design Patterns • Behavioral Design Patterns

3. Book • Design Patterns : Elements of Reusable Object-Oriented Software (1995) • (The-Gang-of-Four Book) • The-Gang-of-Four (GoF) - Gamma, Helm, Johnson , Vlissides

4. Life Challenges Software Design!!!

5. Design Patterns “Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice”. --- Christopher Alexander, 1977 This was in describing patterns in buildings and towns. In SE, design patterns are in terms of objects and interfaces, not walls and doors. The manner in which a collection of interacting objects collaborate to accomplish a specific task or provide some specific functionality.

6. Architecture vs. Design Patterns Architecture • High-level framework for structuring an application • “client-server based on remote procedure calls” • “abstraction layering” • “distributed object-oriented system based on CORBA” • Defines the system in terms of computational components & their interactions Design Patterns • Lower level than architectures (Sometimes, called micro-architecture) • Reusable collaborations that solve subproblems within an application • how can I decouple subsystem X from subsystem Y? Why Design Patterns? • Design patterns support object-oriented reuse at a high level of abstraction • Design patterns provide a “framework” that guides and constrains object-oriented implementation

7. 4 Essential Elements of Design Patterns • Name: identifies a pattern • Problem: describes when to apply the pattern in terms of the problem and context • Solution: describes elements that make up the design, their relationships, responsibilities, and collaborations • Consequences: results and trade-offs of applying the pattern

8. Design Patterns in MVC • Model: the application object • View: its screen presentation • Controller : defines the way the user interface reacts to user input • MVC decouples views and models by establishing a subscribe/notify protocol between them

9. How to Describe Design Patterns more fully This is critical because the information has to be conveyed to peer developers in order for them to be able to evaluate, select and utilize patterns. • A format for design patterns • Pattern Name and Classification • Intent • Also Known As • Motivation • Applicability • Structure • Participants • Collaborations • Consequences • Implementation • Sample Code • Known Uses • Related Patterns

10. Organizing Design Patterns • By Purpose (reflects what a pattern does): • Creational Patterns • Structural Patterns • Behavioral Patterns • By Scope: specifies whether the pattern applies primarily to • classes or to • objects.

11. Design Patterns Space Purpose Creational Structural Behavioral Scope Class Factory Method Adapter Interpreter Template Object Abstract Factory Builder Prototype Singleton Adapter Bridge Composite Decorator Façade Flyweight Proxy Chain of Responsibility Command Iterator Mediator Memento Observer State Strategy Visitor

12. How Design Patterns Solve Design Problems (1) • Finding Appropriate Objects • encapsulation, granularity, dependency, flexibility, performance, evolution, reusability, … • Determining Object Granularity • Specifying Object Interfaces • Signature • Interface • Type • Dynamic Binding • Polymorphism • Specifying Object Implementations

13. How Design Patterns Solve Design Problems (2) • Specifying Object Implementations

14. Principles of Object-Oriented Design • Programming to an Interface, not an Implementation • Design for Reuse • White-box reuse • Black-box reuse • Favor object composition over class inheritance • Delegation • Inheritance versus Parameterized Types • e.g., Templates in C++

15. Software Design • Big problems in practice • Big opportunities in research Have Fun!!

16. Why modularization? • Managerial • Changeability • Comprehensibility

17. What is a “Modularization”? • Making design decisions beforethe work on independent modules can begin. • Quite different decisions are included for each alternative • In all cases, the intention is to describe all "system level" decisions (i.e. decisions which affect more than one module).

18. A Canonical Topic “On the Criteria To Be Used in Decomposing Systems into Modules” • David Parnas, 1972

19. A Canonical Example “The KWIC index system accepts an ordered set of lines, each line is an ordered set of words, and each word is an ordered set of characters. Any line may be "circularly shifted" by repeatedly removing the first word and appending it at the end of the line. The KWIC index system outputs a listing of all circular shifts of all lines in alphabetical order. “ Key Word in Context [Parnas72]

20. A Canonical Example • Input • Circular Shift • Alphabetizing • Output Key Word in Context [Parnas72] Software Architecture Sense and Sensibility Crouching Tiger Hidden Dragon Architecture Software and Sensibility Sense Sensibility Sense and Tiger Hidden Dragon Crouching Hidden Dragon Crouching Tiger Dragon Crouching Tiger Hidden

21. Making your First Decision How should such a system be designed? Modularized? What are the factors to consider?

22. Thinking in the perspective of Software Design! • What does it mean? 1. Elements 2. Relations

23. Describe the Design: 1. Plain English 2. Elements and Relations

24. Main Program/Subroutine

25. Module1: Input “This module reads the data lines from the input medium and stores them in core for processing by the remaining modules. The characters are packed four to a word, and an otherwise unused character is used to indicate the end of a word. An index is kept to show the starting address of each line. “

26. Build KWIC Architecture Input Characters Line Index Input Medium System I/O Data Repr in Memory Direct Memory Access

27. Module 2: Circular Shift “This module is called after the input module has completed its work. It prepares an index which gives the address of the first character of each circular shift, and the original index of the line in the array made up by module 1. It leaves its output in core with words in pairs (original line number, starting address)“

28. Build KWIC Architecture Input Circular Shifter Characters Line Index Shifts Index Input Medium

29. Module 3: Alphabetizing “This module takes as input the arrays produced by modules 1 and 2. It produces an array in the same format as that produced by module 2. In this case, however, the circular shifts are listed in another order (alphabetically).“

30. Build KWIC Architecture Input Circular Shifter Alphabetizer Characters Line Index Shifts Index Sorted Shifts Index Input Medium

31. Module 4: Output “Using the arrays produced by module 3 and module 1, this module produces a nicely formatted output listing all of the circular shifts. In a sophisticated system the actual start of each line will be marked, pointers to further information may be inserted, and the start of the circular shift may actually not be the first word in the line, etc.“

32. Build KWIC Architecture Input Circular Shifter Alphabetizer Output Characters Line Index Shifts Index Sorted Shifts Index Output Medium Input Medium

33. Module 5: Master Control “This module does little more than control the sequencing among the other four modules. It may also handle error messages, space allocation, etc.“

34. Build KWIC Architecture

35. Is this a good design? • What if: • Memory Size change? • Input Size change? • Input Format change? • Alphabetizing Policy change?

36. Consider the Following Changes: 1. Input Format Module 1

37. 2. The decision to have all lines stored in core. For large jobs it may prove inconvenient or impractical to keep all of the lines in core at any one time. All Modules!!!

38. 3. The decision to pack the characters four to a word. In cases where we are working with small amounts of data it may prove undesirable to pack the characters; time will be saved by a character per word layout. In other cases we may pack, but in different formats. All Modules!!!

39. 4. The decision to make an index for the circular shifts rather than actually store them as such. Again, for a small index or a large core, writing them out may be the preferable approach. Module 2, 3

40. 5. The decision to alphabetize the list once, rather than either (a) search for each item when needed, or (b) partially alphabetize as is done in Hoare's FIND [2]. In a number of circumstances it would be advantageous to distribute the computation involved in alphabetization over the time required to produce the index.. Module 3

41. In Summary • A Canonical Example • Describe its Architecture • Evaluate its Changeability

42. Data Abstraction • Elements • Objects • Interfaces • I/O Medium • Relations • Method Invocation • System I/O

43. Parnas’s Modularization 2 • Module 1: Line Storage • Module 2: Input • Module 3: Circular Shift • Module 4: Alphabetizer • Module 5: Output • Module 6: Master Control

44. Our 3-Step Exercise • Read module description • Identify architecture elements and relations • Analyze changeability

45. Module 1: Line Storage “This module consists of a number of functions or subroutines which provide the means by which the user of the module may call on it. The function call CHAR(r,w,c) will have as value an integer representing the cth character in the rth line, wth word. A call such as SETCHAR(rpv,c,d) will cause the cth character in the wth word of the rth line to be the character represented by d (i.e. CHAR(r,w,c) = d). WORDS(r) returns as value the number of words in line r. …“

46. Build OO Architecture Module 1: Line Storage Public Interface addWord addLine getWord getLine Modules (Objects) 1.LineStorage

47. Module 2: Input “This module reads the original lines from the input media and calls the line storage module to have them stored internally. “

48. Build OO Architecture Module 2: Input 2. Input System I/O Method Invocation addLine addWord getLine getWord Public Interface 1.LineStorage I/O Medium Modules (Objects) Input Medium

49. Module 3: Circular Shifter “The principal functions provided by this module are analogs of functions provided in module 1. The module creates the impression that we have created a line holder containing not all of the lines but all of the circular shifts of the lines. Thus the function call CSCHAR(I,w,c) provides the value representing the cth character in the wth word of the Ith circular shift. It is specified that (1) if i < j then the shifts of line i precede the shifts of line j, and (2) for each line the first shift is the original line, the second shift is obtained by making a one-word rotation to the first shift, etc. A function CSSETUP is provided which must be called before the other functions have their specified values. “

50. Build OO Architecture Module 3: Circular Shifter 2. Input addLine addWord getLine getWord setup getLine 1. LineStorage 3. Circular Shifter Input Medium