comp1004: building better classes i

Comp1004: Building Better Classes ITesting and Debugging

Partly based on BlueJ Book – Chapter 6

Software Errors Are A Problem• Mars Climate Orbiter

– $125 million dollar probe– Communications relay for future

NASA Mars missions

• Launched successfully on Dec 11, 1998 and travelled 669 million km in 286 days

• And finally fired its engines, causing it to plunge into the atmosphere and never be seen again :-(

Software Errors Are A ProblemThe Problem?

• NASA built the engines to take navigational instructions in metric units (m/sec)

• Lockhead Martin provided them in imperial units (ft/sec)

• So the probe dropped too far, and its engines overheated in the atmosphere

Coming upwe aim to save $125 million dollars

• Debugging– Syntax and Logical Errors

• Testing Strategies– Equivalence Classes and Boundary Values– Test Automation

• Debuggers

We have to deal with errors

• Early errors are usually syntax errors.– The compiler will spot these.

• Later errors are usually logic errors.– The compiler cannot help with these.– Also known as bugs.

• Some logical errors have no immediately obvious manifestation.– Commercial software is rarely error free.

Syntax Errors?ArrayList<Dog> kennel = new ArrayList<Dog>;

kennel.add(new Dog(“Rover”));kennel.add(new Dog(“Fido”));kennel.add(new Dog(“Patch”));kennel.add(new Dog(“Eric”));kennel.add(new Dog(“Binky”));

for(int i = 0; i <= kennel.size(); i == i + 1) {kennel.get(i).bark()

Syntax Errors?ArrayList<Dog> kennel = new ArrayList<Dog>();

for(int i = 0; i <= kennel.size(); i == i + 1) {kennel.get(i).bark();

Forgot to call the Constructor

Forgot to terminate the statement with a semi-colon

Logical Errors?ArrayList<Dog> kennel = new ArrayList<Dog>();

for(int i = 0; i <= kennel.size(); i == i + 1) {kennel.get(i).bark();

Logical Errors?ArrayList<Dog> kennel = new ArrayList<Dog>();

for(int i = 0; i < kennel.size(); i = i + 1) {kennel.get(i).bark();

Need to use < or the loop will overrun the arraylist

Need to use assignment operator = rather than the equality operator ==

Prevention vs Detection(Developer vs Maintainer)

• We can lessen the likelihood of errors.– Use software engineering techniques, like

encapsulation.

• We can improve the chances of detection.– Use software engineering practices, like

modularization and documentation.

• We can develop detection skills.

Testing and debugging

• These are crucial skills.• Testing searches for the presence of errors.• Debugging searches for the source of errors.– The manifestation of an error may well occur

some ‘distance’ from its source.

Method 1: Unit testing

• Each unit of an application may be tested.– Method, class, module (package in Java).

• Can (should) be done during development.– Finding and fixing early lowers development costs (e.g.

programmer time).– A test suite is built up.

Testing fundamentals

• Understand what the unit should do – its contract.– You will be looking for violations.– Use positive tests and negative tests.

• Test boundaries.– Zero, One, Full.• Search an empty collection.• Add to a full collection.

Equivalence Classes and Boundary Values

“A theme park is testing a new ticketing system. Adults aged 18 or over pay £20, children age 5 years or younger get in free, and other children pay £12. In all cases if the person is a member of the theme park fan club the cost is reduced by 50%.”

What are the equivalence classes? (the sets of values that have the same result)

MemberAge 18+

What are the equivalence classes? (the sets of values that have the same result)

MemberAge 18+

Non-MemberAge 18+

MemberAge 6 - 17

Non-MemberAge 6 - 17

Member or Non-MemberAge 0 - 5

£10 £20

£6 £12

MemberAge 18+

Non-MemberAge 18+

MemberAge 6 - 17

£10 £20

£6 £12

What are the boundary values? (the points at which the classes change)

Age?5/6

What are the boundary values? (the points at which the classes change)

MemberAge 18+

Non-MemberAge 18+

MemberAge 6 - 17

£10 £20

£6 £12

Member? true/false

Age?17/18

Test automation

• Good testing is a creative process, but ...• ... thorough testing is time consuming and repetitive.• Regression testing involves re-running tests to see if

new changes have caused old code to break• Use of a test rig or test harness can relieve some of

the burden.– Classes are written to perform the testing.

Age?5/6

Test Harness

What might our test harness look like?

MemberAge 18+

Non-MemberAge 18+

MemberAge 6 - 17

£10 £20

£6 £12

Member? true/false

Age?17/18

Age?5/6

Test Harness

What might our test harness look like?

MemberAge 18+

Non-MemberAge 18+

MemberAge 6 - 17

£10 £20

£6 £12

Member? true/false

Age?17/18

public testCosts() {

//Test HarnessSystem.out.println(“Starting Cost Tests”);int cost;

cost = calcCost(18, true);if(cost == 10) System.out.println(“Test 1 Pass”);else System.out.println(“Test 1 Fail”);

cost = calcCost(18, false);if(cost == 20) System.out.println(“Test 2 Pass”);else System.out.println(“Test 2 Fail”);

cost = calcCost(17, true);if(cost == 6) System.out.println(“Test 1 Pass”);else System.out.println(“Test 1 Fail”);

cost = calcCost(17, false);if(cost == 12) System.out.println(“Test 1 Pass”);else System.out.println(“Test 1 Fail”);

//etc – until all boundaries are tested

System.out.println(“Ending Cost Tests”);}

Method 2: Manual walkthroughs

• Relatively underused.– A low-tech approach.– More powerful than appreciated.

• Get away from the computer!• ‘Run’ a program by hand.• High-level (Step) or low-level (Step into) views.

Tabulating object state

• An object’s behavior is usually determined by its state.

• Incorrect behavior is often the result of incorrect state.

• Tabulate the values of all fields.• Document state changes after each method

Method 3: Verbal walkthroughs

• Explain to someone else what the code is doing.– They might spot the error.– The process of explaining might help you to spot it

for yourself.

• Group-based processes exist for conducting formal walkthroughs or inspections.

Method 4: Print statements• The most popular

technique.• No special tools required.• All programming

languages support them.• Only effective if the right

methods are documented.• Output may be

voluminous!• Turning off and on

requires forethought.

public int sumAllCosts() {

int costs[] = getAllCosts();

int totalcost = 0;for(int n : costs){

totalcost += n;}

return totalcost;}

Method 4: Print statements• The most popular

technique.• No special tools required.• All programming

languages support them.• Only effective if the right

methods are documented.• Output may be

voluminous!• Turning off and on

requires forethought.

public int sumAllCosts() {

int costs[] = getAllCosts();

if(costs == null)System.out.println(“Costs array is

null!”);else {

System.out.println(“Costs array returned”);

System.out.println(“Size:” + costs.size());}

int totalcost = 0;for(int n : costs){

System.out.println(totalcost);totalcost += n;System.out.println(totalcost);

System.out.println(“Finished”);System.out.println(totalcost);

return totalcost;}

Choosing a test strategy

• Be aware of the available strategies.• Choose strategies appropriate to the point of

development.• Automate whenever possible.– Reduces tedium.– Reduces human error.– Makes (re)testing more likely.

Debuggers• Debuggers are both

language- and environment-specific.– BlueJ and Eclipse

have integrated debugger.

• Support breakpoints.• Step and Step-into

controlled execution.• Call sequence (stack).• Object state.

public void markWork(Work w) {int mark = allocateMark(w);if(w.late())

mark = mark * w.getPenalty();w.setMark(mark);

public void markStudent(Student s) { Work work[] = s.getWork();if(work != null) {

for(Work w : work){

markWork(w);}

public void markAllStudents() {Students students[] = getStudents();if(students != null) {

for(Student s : students){

markStudent(s);}

Call stack

for(Work w : work){

markWork(w);}

markStudent(s);}

markAllStudents

Call stack

for(Work w : work){

markWork(w);}

markStudent(s);}

markAllStudents

markStudent

Call stack

for(Work w : work){

markWork(w);}

markStudent(s);}

markAllStudents

markStudent

markWork

Call stack

for(Work w : work){

markWork(w);}

markStudent(s);}

markAllStudents

markStudent

markWork

Return

Call stack

for(Work w : work){

markWork(w);}

markStudent(s);}

markAllStudents

markStudent

Call stack

for(Work w : work){

markWork(w);}

markStudent(s);}

markAllStudents

markStudent

markWork

Call stack

for(Work w : work){

markWork(w);}

markStudent(s);}

markAllStudents

markStudent

markWork

Return

Call stack

for(Work w : work){

markWork(w);}

markStudent(s);}

markAllStudents

markStudent

Call stack

for(Work w : work){

markWork(w);}

markStudent(s);}

markAllStudents

markStudent

markWork

Call stack

for(Work w : work){

markWork(w);}

markStudent(s);}

markAllStudents

markStudent

markWork

Return

Call stack

for(Work w : work){

markWork(w);}

markStudent(s);}

markAllStudents

markStudent

Call stack

for(Work w : work){

markWork(w);}

markStudent(s);}

markAllStudents

markStudent

Return

Call stack

for(Work w : work){

markWork(w);}

markStudent(s);}

markAllStudents

Call stack

for(Work w : work){

markWork(w);}

markStudent(s);}

markAllStudents

markStudent

Call stack

Etc…

Summarywe aimed to save $125 million dollars

• Debugging– Syntax and Logical Errors

• Testing Strategies– Equivalence Classes and Boundary Values– Test Automation

• Debuggers

comp1004: building better classes i

software errors

syntax errors

logical errors

later errors

logic errors

errorsearly errors

addnew dogerickennel

addnew dogroverkennel

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