chapter 1 programming review and introduction to software design

Chapter 1Programming Review and Introduction to

Software Design

Process Phase Introduced in This ChapterRequirementsAnalysis

Design

Implementation

ArchitectureFramework Detailed Design

Key: = secondary emphasis= main emphasis xx

Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

Key Concept: Where We’re Headed

In development, we start by thinking about architecture, and end with programming. For learning purposes, this book begins by discussing programming, and ends by explaining architecture.

Coding Practices Used in This Book

Instance variables may be referred to with “this.”

o Example: class Car { int milesDriven; … }

May use this.milesDriven within methods of Car to clarify

Static variables may be referred to with class name.

o Example: class Car { static int numCarsSold; … }

May use Car.numCarsSold within methods of Car to clarify

Parameters are given prefix “a” or “an”

o Example: public … getVolume( int aLength ) {…}


Programming Conventions: Method Documentation 1 of 2

Preconditions: conditions on non-local variables that the method’s code assumeso Includes parameters

o Verification of these conditions not promised in method itself

Postconditions: value of non-local variables after executiono Includes parameters

o Notation: x' denotes the value of variable x after execution

Invariants: relationships among non-local variables that the function’s execution do not change

(The values of the individual variables may change, however.)

o Equivalent to inclusion in both pre- and post-conditions

o There may also be invariants among local variablesAdapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

Programming Conventions: Method Documentation 2 of 2

Return: o What the method returns

Known issues: o Honest statement of what has to be done, defects

that have not been repaired etc.

o (Obviously) limited to what’s known!


Key Concept: Specifying Methods

We specify each method in its comment section with preconditions, postconditions, return, invariants and known issues.


Flowchart Example

protected final void setName( String aName ) { // Check legitimacy of parameter and settings if( ( aName == null ) || ( maxNumCharsInName() <= 0 ) || ( maxNumCharsInName() > alltimeLimitOfNameLength() ) ) { name = new String( "defaultName" ); System.out.println ( "defaultName selected by GameCharacter.setName()"); } else // Truncate if aName too long if( aName.length() > maxNumCharsInName() ) name = new String ( aName.getBytes(), 0, maxNumCharsInName() ); else // assign the parameter name name = new String( aName ); }

Nominal pathSet name to “defaultName"

Truncate name

Set name to parameter

Parameter & settings make sense else

Parametername too long elseOutput notification to console


FOR number of microseconds supplied by operator IF number of microseconds exceeds critical value

Try to get supervisor's approval IF no supervisor's approval

abort with "no supervisor approval for unusual duration" message

ENDIF ENDIFIF power level exceeds critical value

abort with "power level exceeded" message ENDIFIF ( patient properly aligned & shield properly placed & machine self-test passed )

Apply X-ray at power level p ENDIF

ENDFOR

Pseuodocode Example For an X-ray Controller


Advantages of Pseudocode & Flowcharts

Clarify algorithms in many cases

Impose increased discipline on the process of documenting detailed design

Provide additional level at which inspection can be performed

Help to trap defects before they become code

Increases product reliability

May decreases overall costs


Disadvantages of Pseudocode & Flowcharts

Create an additional level of documentation to

maintain

Introduce error possibilities in translating to code

May require tool to extract pseudocode and facilitate

drawing flowcharts


Key Concept: The What vs. the How of Methods

Preconditions etc. specify what a method accomplishes. Activity charts etc. describe how the method accomplishes these.


Good Habits for Writing Functions 1 of 3

Use expressive naming: the names of the function, the parameters and the variables should indicate their purposeo … manipulate( float aFloat, int anInt ) poor

o … getBaseRaisedToExponent( float aBase, int anExponent )

Avoid global variables: consider passing parameters insteado … extract( int anEntry ) { …… table = …. } replace?

o … extract( int anEntry, EmployeeTable anEmployeeTable )

But not when the number of parameters exceeds 7

Defend against bad datao Check parameter and other input values

o Use exceptions – or –

o Use defaults -- or –

o Return special values (less desirable)Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.


Don’t use parameters as method variables Give names to numbers

for( i = 0; i < 8927; ++i ) poor: why 8927?

o Instead:

int NUM_CELLS = 8927;

for( cellCounter = 0; cellCounter < NUM_CELLS; ++cellCounter )

Limit number of parameters to 6 or 7 Introduce variables near their first usage



Initialize all variableso re-initialize where necessary to “reset”

Check loop counters, especially for range correctness

Avoid nesting loops more than 3 levelso introduce auxiliary methods to avoid

Ensure loop terminationo a proof is ideal – in any case, be convinced

Inspect before compilingo be convinced of correctness first


Requirements for Command Line Calculator Example

1. CommandLineCalculator begins by asking the user how many accounts he wants to open. It then establishes the desired number, each with zero balance.

2. CommandLineCalculator asks the user which of these accounts he wants to deal with.

3. When the user has selected an account, CommandLineCalculator allows the user to add whole numbers of dollars to, or subtract them from the account for as long as he requires.

4. When the user is done with an account, he is permitted to quit, or to pick another account to process.


Typical I/O For

Command Line

Calculator


Problems With CommandLineCalculator Implementation*

How do we know that all required functionality has been handled? (correctness)

If the user makes a mistake the system crashes or performs unpredictably (robustness)The following cause crasheso Invalid number of accountso Invalid accounto Invalid amount to add (not an integer)o Invalid string (not “stop” or “Quit application”)

Not clear what some of the method are meant to do (documentation)

1 of 2

* See appendix to this chapterAdapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

Problems With CommandLineCalculator Implementation*

Hard to modify, add or remove parts. (flexibility)

Executes fast enough? (speed efficiency)

Satisfies memory requirements? (space efficiency)

Class usable for other applications? (reusability)

2 of 2

* See appendix to this chapter


Key Concept: Ensure Correctness

We are primarily responsible for ensuring that our code does what it’s intended to.


I/O For Robust Command Line

Calculator


Better Design forinteractWithUser()

Thick line is nominal path

else

accountNum within rangeelse

Exception

Prompt for account number and get userRequest

userRequest != “Quit application”

Try to make integeraccountNum from userRequest

Notify userof bad value

Handle integer

exception

return

do executeAdditions on accountNum

Prompt for account number and get userRequest


Key Concept: Good Code May Not Be Good Design

The code here is more robust, but it does not exploit object-orientation or exhibit a clear design. Consequently, it’s inflexible, not easy to verify, and unlikely to be reused.


Key Concept: Write Robust Code

Good designs withstand anomalous treatment.


Aspects of Flexibility

Obtaining more or less of what’s already present

o Example: handle more kinds of accounts without

needing to change the existing design or code

Adding new kinds of functionality

o Example: add withdraw to existing deposit function

Changing functionality

o Example: allow withdrawals to create an overdraft


We can reuse …. Object code (or equivalent)

o Example: sharing dll’s between word processor and spreadsheet

o To be covered in the Components chapters xx - xx Classes – in source code form

o Example: Customer class used by several applicationso Thus, we write generic code whenever possible

Assemblies of Related Classes o Example: the java.awt package

Patterns of Class Assemblies o To be covered in Design Pattern chapters xx - xx

Types of Reuse


Key Concept: Design for Flexibility and Reuse

Good designs are more easily modified and reused.


Remaining Problems With CommandLineCalculator

Insufficient flexibility

o To add subtraction, multiplication etc.

o To change the nature of the project

Speed efficiency not explored

Space efficiency not explored

o Limit to number of accounts?

Reusability doubtful

o OO not leveraged

No visualization of design providedAdapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

chapter 1 programming review and introduction to software design

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