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Component-Level Design - PowerPoint PPT Presentation
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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
Requirements gathering
Analysis/Specification
Architectural design
Component-level design
Coding
Testing
CS427
“Design”: Noun
- Refinement
- Requirements
- High-level design
- Details
- Information-hiding
- Don’t show everything
- Reveal bit by bit
- Modularity
CS427
“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
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Component Design
Structure techniques in book
- flow charts
- Nasi-Schneiderman diagrams
- decision tables
- pseudo-code
- state machines
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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
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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.
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
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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
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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;
}
...
}
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Pseudo-code
- Works well with refinement
- Write pseudo-code
- as comments
- with stubs
- Gradually implement it all
- Execute and test as you go
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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
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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
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State diagram for sockets
uninitialized
connect()
listen()
passive
active
disconnect()
newconn1()
newconn1()
isconnecting()
q0len!=0
CONNECTING
isconnected()
isconnected()
qlen!=0
isconnected()
CONNECTED
accept()
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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”
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State Pattern
- From “Design Patterns”
ConnectingState
SocketState
listen()
connect()
newconn1()
...
Socket
ConnectedState
...
PassiveState
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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
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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
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Design in XP
Much of the design gets created during refactoring.
- Simple design
- Code smells
- Coding standards
- Code communicates design
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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
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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.”
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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
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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
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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
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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
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