Chapter 1 Programming Review and Introduction to Software Design

1. Chapter 1Programming Review and Introduction to Software Design

2. Process Phase Introduced in This Chapter Requirements Analysis Design Framework Architecture Detailed Design Implementation Key: = main emphasis = secondary emphasis x x Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

3. 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.

4. Coding Practices Used in This Book • Instance variables may be referred to with “this.” • Example: class Car { int milesDriven; … } May use this.milesDrivenwithin methods of Carto clarify • Static variables may be referred to with class name. • Example: class Car { static int numCarsSold; … } May use Car.numCarsSoldwithin methods of Carto clarify • Parameters are given prefix “a” or “an” • Example: public … getVolume( int aLength ) {…} Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

5. Programming Conventions: Method Documentation 1 of 2 • Preconditions: conditions on non-local variables that the method’s code assumes • Includes parameters • Verification of these conditions not promised in method itself • Postconditions: value of non-local variables after execution • Includes parameters • 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.) • Equivalent to inclusion in both pre- and post-conditions • There may also be invariants among local variables Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

6. Programming Conventions: Method Documentation 2 of 2 • Return: • What the method returns • Known issues: • Honest statement of what has to be done, defects that have not been repaired etc. • (Obviously) limited to what’s known! Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

7. Key Concept: Specifying Methods We specify each method in its comment section with preconditions, postconditions, return, invariants and known issues. Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

8. Flowchart Example else Parameter & settings make sense Set name to “defaultName" Nominal path Output notification to console else Parameter name too long 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 ); } Set name to parameter Truncate name Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

9. Pseuodocode Example For an X-ray Controller • 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 • ENDIF • IF power level exceeds critical value • abort with "power level exceeded" message • ENDIF • IF ( patient properly aligned & shield properly placed • & machine self-test passed ) • Apply X-ray at power level p • ENDIF • ENDFOR Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

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

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

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

13. Good Habits for Writing Functions 1 of 3 • Use expressive naming: the names of the function, the parameters and the variables should indicate their purpose • … manipulate( float aFloat, int anInt ) poor • … getBaseRaisedToExponent( float aBase, int anExponent ) • Avoid global variables: consider passing parameters instead • … extract( int anEntry ) { …… table = …. } replace? • … extract( int anEntry, EmployeeTable anEmployeeTable ) But not when the number of parameters exceeds  7 • Defend against bad data • Check parameter and other input values • Use exceptions – or – • Use defaults -- or – • Return special values (less desirable) Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

14. Good Habits for Writing Functions 2 of 3 • Don’t use parameters as method variables • Give names to numbers for( i = 0; i < 8927; ++i )  poor: why 8927? • 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 Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

15. Good Habits for Writing Functions 3 of 3 • Initialize all variables • re-initialize where necessary to “reset” • Check loop counters, especially for range correctness • Avoid nesting loops more than 3 levels • introduce auxiliary methods to avoid • Ensure loop termination • a proof is ideal – in any case, be convinced • Inspect before compiling • be convinced of correctness first Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

16. Requirements for Command Line Calculator Example • CommandLineCalculator begins by asking the user how many accounts he wants to open. It then establishes the desired number, each with zero balance. • CommandLineCalculator asks the user which of these accounts he wants to deal with. • 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. • When the user is done with an account, he is permitted to quit, or to pick another account to process. Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

17. Typical I/O For Command Line Calculator Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

18. Problems With CommandLineCalculator Implementation* 1 of 2 • 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 crashes • Invalid number of accounts • Invalid account • Invalid amount to add (not an integer) • Invalid string (not “stop” or “Quit application”) • Not clear what some of the method are meant to do (documentation) * See appendix to this chapter Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

19. Problems With CommandLineCalculator Implementation* 2 of 2 • Hard to modify, add or remove parts. (flexibility) • Executes fast enough? (speed efficiency) • Satisfies memory requirements? (space efficiency) • Class usable for other applications? (reusability) * See appendix to this chapter Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

20. Key Concept: Ensure Correctness We are primarily responsible for ensuring that our code does what it’s intended to. Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

21. I/O For Robust Command Line Calculator Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

22. Better Design forinteractWithUser() Prompt for account number and get userRequest Thick line is nominal path userRequest != “Quit application” else Exception return Try to make integer accountNum from userRequest Handle integer exception else accountNum within range Notify user of bad value do executeAdditions on accountNum Prompt for account number and get userRequest Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

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

24. Key Concept: Write Robust Code Good designs withstand anomalous treatment. Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

25. Aspects of Flexibility • Obtaining more or less of what’s already present • Example: handle more kinds of accounts without needing to change the existing design or code • Adding new kinds of functionality • Example: add withdraw to existing deposit function • Changing functionality • Example: allow withdrawals to create an overdraft Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

26. Types of Reuse We can reuse …. • Object code (or equivalent) • Example: sharing dll’s between word processor and spreadsheet • To be covered in the Components chapters xx - xx • Classes – in source code form • Example: Customer class used by several applications • Thus, we write generic code whenever possible • Assemblies of Related Classes • Example: the java.awt package • Patterns of Class Assemblies • To be covered in Design Pattern chapters xx - xx Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

27. Key Concept: Design for Flexibility and Reuse Good designs are more easily modified and reused. Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

28. Remaining Problems With CommandLineCalculator • Insufficient flexibility • To add subtraction, multiplication etc. • To change the nature of the project • Speed efficiency not explored • Space efficiency not explored • Limit to number of accounts? • Reusability doubtful • OO not leveraged • No visualization of design provided Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.