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Component-Level Design

Component-Level Design. Requirements gathering. Analysis/Specification. Architectural design. Component-level design. Coding. Testing. “Design”: Noun. Refinement Requirements High-level design Details Information-hiding Don’t show everything Reveal bit by bit Modularity.

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Component-Level Design

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  1. Component-Level Design Requirements gathering Analysis/Specification Architectural design Component-level design Coding Testing CS427

  2. “Design”: Noun • Refinement • Requirements • High-level design • Details • Information-hiding • Don’t show everything • Reveal bit by bit • Modularity CS427

  3. “Design”: Verb • Bounce from high-level to low-level • New requirements come after design is created • Design is created incrementally • As requirements are known • As better design ideas are invented • As design flaws are discovered CS427

  4. Component Design Structure techniques in book • flow charts • Nasi-Schneiderman diagrams • decision tables • pseudo-code • state machines CS427

  5. Decision tables • Used to specify program with complex conditions • Makes it easy to see if any cases are missing • Can be implemented with IF statements CS427

  6. The recovery period is 27.5 years for residential real property, 31.5 years for non-residential real property placed in service before May 13, 1993, 39 years for non-residential real property placed in service after May 12, 1993, 50 years for railroad gradings and tunnel bores, except that nonresidential real property on an Indian reservation has a recovery period of 22 years.

  7. Decision table for recovery period Conditions 1 2 3 4 5 Real property Residential Placed before May 13, 1993 Railroad grading or bore On Indian reservation 27.5 years 31.5 years 39 years 50 years 22 years T T T T T T F F F F T F F F T F F F T Actions x x x x x CS427

  8. Pseudo-code • Also known as “Program Design Language” • Advantages: • Expressive and compact • Can use any editor • (Sometimes) can compile it • Disadvantages: • Must know the language CS427

  9. Pseudo-code recoverPeriod(property) { if (isReal(property)) { if (isResidential(property)) return 27.5; if (onReservation(property)) return 22; if (isRailroad(property)) return 50; if (property.date > “May 13, 1993”) return 31.5; else return 39; } ... } CS427

  10. Pseudo-code • Works well with refinement • Write pseudo-code • as comments • with stubs • Gradually implement it all • Execute and test as you go CS427

  11. State Machines (FSM) • Lots of theory • how to minimize number of states • how to merge state machines • how to tell whether two state machines are equal • Can generate code directly from state machines • but usually do not CS427

  12. State diagram for stop light 30 R/G R/Y 4 2 R/R R/R 2 4 Y/R G/R Events are time delays, in seconds. 25 CS427

  13. Pseudocode for stop light Action = Record {integer wait, east, north} Action: Actions[1 .. 6] repeat forever for I = 1 to 6 do setEast(actions[i].east) setNorth(actions[i].north) wait(actions[i].wait) end for CS427

  14. State diagram for sockets uninitialized connect() listen() passive active disconnect() newconn1() newconn1() isconnecting() q0len!=0 CONNECTING isconnected() isconnected() qlen!=0 isconnected() CONNECTED accept() CS427

  15. Implementing socket • Socket is an object • Actions are methods of the socket • State is stored in variable of the object • Each method uses IF statement to make sure socket is in the right state • When IF statements get too complicated, use “State Pattern” CS427

  16. State Pattern • From “Design Patterns” ConnectingState SocketState listen() connect() newconn1() ... Socket ConnectedState ... PassiveState CS427

  17. Detailed design • Lots of different techniques • Most only works in a few places • Pseudocode works most places, but often there is a better technique • Often best to use code and skip detailed design CS427

  18. Design in XP • No required design documents • Developers talk about design, but write test cases and code • Need to design during: • estimating user stories • iteration planning (making engineering tasks) • at start of programming task • when task does not go well CS427

  19. Design in XP Much of the design gets created during refactoring. • Simple design • Code smells • Coding standards • Code communicates design CS427

  20. Keep system simple • Small classes • 7 methods is very nice • 20 methods is a little large • 80 methods is horrible • Small methods • Smalltalk: 20 lines is large • Java: 40 lines is large • C++: 60 lines is large CS427

  21. Keep System Simple • Decompose large classes • Variables that change together should stay together • Methods should be in class of variables that they access • “Ask, don’t tell.” CS427

  22. Keep System Simple • Decompose large methods • Turn repeating code into new methods • Turn loop bodies into new methods • Turn complex conditions into new methods • Turn logical blocks into new methods, hiding temporaries CS427

  23. Code (Design) Smells • Large classes, methods, packages • Duplication • Classes with no methods except accessors • One class with all the code • Complex conditionals and no polymorphism CS427

  24. Good design • Good design is product of many small steps • “Do the right thing” • Each step adds a feature • Do one more thing right • “Do the thing right” • Each step makes design a little simpler • Eliminates one more unnecessary complexity CS427

  25. Next time: http://www.acm.org/classics/may96/ On the Criteria To Be Used in Decomposing Systems into ModulesD.L. Parnas http://www.toa.com/pub/abstraction.txt CS427

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