1. WARNING
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2. Famous Software Disaster
This shutdown occurred 36.7 seconds after launch, when the guidance system's own computer tried to convert one piece of data -- the sideways velocity of the rocket -- from a 64-bit format to a 16-bit format.
Ariane 5, June 4, 1996
[ TE3201 AY1516S2 Lecture 4a ]

3. Navigation instructions?
Here’s an error message

4. Navigation instructions?
Here’s an error message
Your code
Your teammate’s code

5. CS2103/T, Lecture 7, Part 2, [March, 2017]
Putting up defences to protect our code.
Your code
Your teammate’s code
CS2103/T, Lecture 7, Part 2, [March, 2017]

6. Putting up defences to protect our code.
Your code
Your teammate’s code

7. Putting up defences to protect our code.
Lecture 7:
Putting up defences to protect our code.

8. MISUNDERSTANDINGS
GO!

9. int size = Config.getSize(); if( 0 < size < 5) setSizeAsSmall();
MISUNDERSTANDINGS …
int size = Config.getSize();
if( 0 < size < 5)
setSizeAsSmall();
else
setSizeAsBig(); …
if(sizeFound())
return readSize();
else
return 0;
// size > 0

10. int size = Config.getSize(); if( 0 < size < 5) setSizeAsSmall();
MISUNDERSTANDINGS …
int size = Config.getSize();
if( 0 < size < 5)
setSizeAsSmall();
else
setSizeAsBig(); …
if(sizeFound())
return readSize();
else
return 0;
assert (size > 0)

11. These are different from JUnit assertions!
MISUNDERSTANDINGS
These are different from JUnit assertions! …
int size = Config.getSize();
if( 0 < size < 5)
setSizeAsSmall();
else
setSizeAsBig(); …
if(sizeFound())
return readSize();
else
return 0;
assert (size > 0)
Make your assumptions explicit. Get the runtime to confirm them.

12. MISUNDERSTANDINGS

13. MISUNDERSTANDINGS
GO!

14. MISUNDERSTANDINGS
GO!

15. MISUNDERSTANDINGS MISHAPS
GO!
GO!

16. void processAge(int age) { //do stuff with age }
MISUNDERSTANDINGS
MISHAPS
What if age is invalid?
void processAge(int age) {
//do stuff with age }
File corrupted
bug?
void foo(String fileName) {
openFile(fileName); }
Handle or raise an Exception
Verify using assertion e.g. assert(isValid(age))

17. Handle or raise an Exception
MISUNDERSTANDINGS
MISHAPS
Developer
User/environment
Programmer mistake?
Environment mishap?
Handle or raise an Exception
Verify using assertion e.g. assert(isValid(age))

18. MISUNDERSTANDINGS MISHAPS
GO!
GO!

19. MISUNDERSTANDINGS MISHAPS
GO!
GO!

20. Assertions Exceptions It depends
Your ToDo manager software tries to interact with Google calendar, but finds Google servers are down. This should be handled using,
Assertions
Exceptions
It depends

21. MISUNDERSTANDINGS MISHAPS MYSTERIES
GO!
GO!
GO!

22. My computer crashes when I flush the toilet!
MISUNDERSTANDINGS
MISHAPS
MYSTERIES
My computer crashes when I flush the toilet!

23. int size = Config.getSize(); // size > 0 if( 0 < size < 5)
MISUNDERSTANDINGS
MISHAPS
MYSTERIES …
int size = Config.getSize();
// size > 0
if( 0 < size < 5)
log(LEVEL_INFO, “Setting size to small”);
setSizeAsSmall();
else if( size > 5)
setSizeAsBig();
else
log(LEVEL_WARN, “Size not recognized”);

24. MISUNDERSTANDINGS
MISHAPS
MYSTERIES

25. MISUNDERSTANDINGS MISHAPS MYSTERIES
GO!
GO!
GO!

26. MISUNDERSTANDINGS MISHAPS MYSTERIES
GO!
GO!
GO!

27. MISUNDERSTANDINGS MISHAPS MYSTERIES MISUSE
GO!
GO!
GO!
GO!

28. void setGirlfriend(Woman w){ girlfriend = w; } … class Woman{
MISUNDERSTANDINGS
MISHAPS
MYSTERIES
MISUSE
class Man{
Woman girlfriend;
void setGirlfriend(Woman w){
girlfriend = w; } …
class Woman{
Man boyfriend;
void setBoyfriend(Man m){
boyfriend = m; } …
girlfriend
Man
Woman
boyfriend

29. void setGirlfriend(Woman w){ girlfriend = w; } … class Woman{
MISUNDERSTANDINGS
MISHAPS
MYSTERIES
MISUSE
class Man{
Woman girlfriend;
void setGirlfriend(Woman w){
girlfriend = w; } …
class Woman{
Man boyfriend;
void setBoyfriend(Man m){
boyfriend = m; } …
Man harry, ron;
Woman hermione;
ron.setGirlfriend(hermoine);
hermione.setBoyfriend(harry); …
ron:Man
hermione :Woman
harry:Man
girlfriend
boyfriend

30. What do you do when it is green light?

31. Defensive driving can save your life
Assume other drivers are idiots

32. Defensive driving can save your life
coding
career
Assume other drivers are idiots
coders

33. void setGirlFriend(Woman w){ if(girlfriend!=null)
MISUNDERSTANDINGS
MISHAPS
MYSTERIES
MISUSE
class Man{
Woman girlfriend;
void setGirlFriend(Woman w){
if(girlfriend!=null)
girlfriend.breakupWith(this);
girlfriend = w;
girlfriend.setBoyFriend(this); } …

34. MISUNDERSTANDINGS
MISHAPS
MYSTERIES
MISUSE

35. MISUNDERSTANDINGS
MISHAPS
MYSTERIES
MISUSE

36. Bertrand Meyer, Design by Contract and the Eiffel Language
MISUNDERSTANDINGS
MISHAPS
MYSTERIES
MISUSE
Bertrand Meyer, Design by Contract and the Eiffel Language

37. MISUNDERSTANDINGS
MISHAPS
MYSTERIES
MISUSE

38. MISUNDERSTANDINGS MISHAPS MYSTERIES MISUSE
GO!
GO!
GO!
GO!

39. All 4 used in Sample Project. Use them more in your project.
MISUNDERSTANDINGS
MISHAPS
MYSTERIES
MISUSE
GO!
GO!
GO!
GO!
Assertions
Exceptions
Logging
Defensive
All 4 used in Sample Project. Use them more in your project.