Comp1004: Building Better Classes I

Comp1004: Building Better Classes I Testing 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 Problem The 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(newDog(“Eric”)); kennel.add(newDog(“Binky”)); for(inti = 0; i <= kennel.size(); i == i + 1) { kennel.get(i).bark() }

Syntax Errors? Forgot to call the Constructor ArrayList<Dog> kennel = new ArrayList<Dog>(); kennel.add(new Dog(“Rover”)); kennel.add(new Dog(“Fido”)); kennel.add(new Dog(“Patch”)); kennel.add(newDog(“Eric”)); kennel.add(newDog(“Binky”)); for(inti = 0; i <= kennel.size(); i == i + 1) { kennel.get(i).bark(); } Forgot to terminate the statement with a semi-colon

Logical Errors? Forgot to call the Constructor ArrayList<Dog> kennel = new ArrayList<Dog>(); kennel.add(new Dog(“Rover”)); kennel.add(new Dog(“Fido”)); kennel.add(new Dog(“Patch”)); kennel.add(newDog(“Eric”)); kennel.add(newDog(“Binky”)); for(inti = 0; i <= kennel.size(); i == i + 1) { kennel.get(i).bark(); } Forgot to terminate the statement with a semi-colon

Logical Errors? Forgot to call the Constructor ArrayList<Dog> kennel = new ArrayList<Dog>(); kennel.add(new Dog(“Rover”)); kennel.add(new Dog(“Fido”)); kennel.add(new Dog(“Patch”)); kennel.add(newDog(“Eric”)); kennel.add(newDog(“Binky”)); for(inti = 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 == Forgot to terminate the statement with a semi-colon

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) Member Age 18+ £10

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) Member Age 18+ Non-Member Age 18+ £20 £10 Member Age 6 - 17 Non-Member Age 6 - 17 £12 £6 Member or Non-Member Age 0 - 5 FREE

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 boundary values? (the points at which the classes change) Member Age 18+ Non-Member Age 18+ £20 £10 Member Age 6 - 17 Non-Member Age 6 - 17 £12 £6 Member or Non-Member Age 0 - 5 FREE

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 boundary values? (the points at which the classes change) Member? true/false Member Age 18+ Non-Member Age 18+ £20 £10 Age? 17/18 Member Age 6 - 17 Non-Member Age 6 - 17 £12 £6 Age? 5/6 Member or Non-Member Age 0 - 5 FREE

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.

Test Harness What might our test harness look like? Member? true/false Member Age 18+ Non-Member Age 18+ £20 £10 Age? 17/18 Member Age 6 - 17 Non-Member Age 6 - 17 £12 £6 Age? 5/6 Member or Non-Member Age 0 - 5 FREE

Test Harness public testCosts() { //Test Harness System.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”); } What might our test harness look like? Member? true/false Member Age 18+ Non-Member Age 18+ £20 £10 Age? 17/18 Member Age 6 - 17 Non-Member Age 6 - 17 £12 £6 Age? 5/6 Member or Non-Member Age 0 - 5 FREE

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 call.

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 public intsumAllCosts() { int costs[] = getAllCosts(); inttotalcost = 0; for(intn : costs){ • totalcost += n; } • return totalcost; } • 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.

Method 4: Print statements public intsumAllCosts() { 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()); } inttotalcost = 0; for(intn : costs){ • System.out.println(totalcost); totalcost += n; System.out.println(totalcost); } • System.out.println(“Finished”); • System.out.println(totalcost); return totalcost; } • 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.

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.

Debuggers Call stack • public void markWork(Workw) { • int mark = allocateMark(w); • if(w.late()) • mark = mark * w.getPenalty(); • w.setMark(mark); • } • public void markStudent(Students) { • Work work[] = s.getWork(); • if(work != null) { • for(Workw : work){ • markWork(w); • } • } • } • public void markAllStudents() { • Students students[] = getStudents(); • if(students != null) { • for(Students : students){ • markStudent(s); • } • } • } • 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.

Debuggers Call stack • public void markWork(Workw) { • int mark = allocateMark(w); • if(w.late()) • mark = mark * w.getPenalty(); • w.setMark(mark); • } • public void markStudent(Students) { • Work work[] = s.getWork(); • if(work != null) { • for(Workw : work){ • markWork(w); • } • } • } • public void markAllStudents() { • Students students[] = getStudents(); • if(students != null) { • for(Students : students){ • markStudent(s); • } • } • } • 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. markAllStudents

Debuggers Call stack • public void markWork(Workw) { • int mark = allocateMark(w); • if(w.late()) • mark = mark * w.getPenalty(); • w.setMark(mark); • } • public void markStudent(Students) { • Work work[] = s.getWork(); • if(work != null) { • for(Workw : work){ • markWork(w); • } • } • } • public void markAllStudents() { • Students students[] = getStudents(); • if(students != null) { • for(Students : students){ • markStudent(s); • } • } • } • 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. markStudent markAllStudents Call

Debuggers Call stack markWork • public void markWork(Workw) { • int mark = allocateMark(w); • if(w.late()) • mark = mark * w.getPenalty(); • w.setMark(mark); • } • public void markStudent(Students) { • Work work[] = s.getWork(); • if(work != null) { • for(Workw : work){ • markWork(w); • } • } • } • public void markAllStudents() { • Students students[] = getStudents(); • if(students != null) { • for(Students : students){ • markStudent(s); • } • } • } • 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. markStudent Call markAllStudents Call

Debuggers Call stack markWork • public void markWork(Workw) { • int mark = allocateMark(w); • if(w.late()) • mark = mark * w.getPenalty(); • w.setMark(mark); • } • public void markStudent(Students) { • Work work[] = s.getWork(); • if(work != null) { • for(Workw : work){ • markWork(w); • } • } • } • public void markAllStudents() { • Students students[] = getStudents(); • if(students != null) { • for(Students : students){ • markStudent(s); • } • } • } • 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. Return markStudent Call markAllStudents Call

Debuggers Call stack • public void markWork(Workw) { • int mark = allocateMark(w); • if(w.late()) • mark = mark * w.getPenalty(); • w.setMark(mark); • } • public void markStudent(Students) { • Work work[] = s.getWork(); • if(work != null) { • for(Workw : work){ • markWork(w); • } • } • } • public void markAllStudents() { • Students students[] = getStudents(); • if(students != null) { • for(Students : students){ • markStudent(s); • } • } • } • 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. markStudent Return markAllStudents Call

Debuggers Call stack • public void markWork(Workw) { • int mark = allocateMark(w); • if(w.late()) • mark = mark * w.getPenalty(); • w.setMark(mark); • } • public void markStudent(Students) { • Work work[] = s.getWork(); • if(work != null) { • for(Workw : work){ • markWork(w); • } • } • } • public void markAllStudents() { • Students students[] = getStudents(); • if(students != null) { • for(Students : students){ • markStudent(s); • } • } • } • 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. markAllStudents

Debuggers Call stack • public void markWork(Workw) { • int mark = allocateMark(w); • if(w.late()) • mark = mark * w.getPenalty(); • w.setMark(mark); • } • public void markStudent(Students) { • Work work[] = s.getWork(); • if(work != null) { • for(Workw : work){ • markWork(w); • } • } • } • public void markAllStudents() { • Students students[] = getStudents(); • if(students != null) { • for(Students : students){ • markStudent(s); • } • } • } • 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. Etc… markStudent markAllStudents Call

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