Mutation test is a way to put a "mutation" into your code, run the test and then see if the test fails or not. A mutation is a change to production code which should make it behave in a different way. The idea is, if the code is just enough to pass the test, any mutation should make the test failed due to the behavior change. Although this statement is not always true, it should be correct in most cases. By TDD, we should get just enough and the simplest code that passing all the test. But, how do we know this? Sometimes, since writing test and code is so interactive in TDD, we may not choose the smallest baby step to pass the test. This means we may write some code which is not driven by current test. On the other side, regardless doing TDD or not, we hope those unit tests can avoid future regression when we changing the code, right? To make it more clear, our expectation is that any "bad" code change (aka. mutation) can be detected by those tests.

1. What Test Coverage
Mean to us?
Joseph Yao
Tuesday, July 9, 13

2. Who I am?
n Agile Coach at Odd-e
n Father, Husband...
n Software Craftsman
n Basketball, Board Game...
Tuesday, July 9, 13

3. Agenda
n Bias and Truth of Test Coverage
n Classical Test Coverage
n Mutation Test Coverage
n TDD and Test Coverage
Tuesday, July 9, 13

4. Bias of Test Coverage
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5. Truth of Test Coverage
I expect a high level of coverage. Sometimes
managers require one. There's a subtle difference.
-- Brian Marick
http://martinfowler.com/bliki/TestCoverage.html
Tuesday, July 9, 13

6. Truth of Test Coverage
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7. Classical Test Coverage
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8. Function Coverage
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>0)) {
z = x;
}
return z;
}
Satisfied Test: foo(2, 2)
Tuesday, July 9, 13

9. Function Coverage
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>0)) {
z = x;
}
return z;
}
Tuesday, July 9, 13

10. Statement Coverage
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>0)) {
z = x;
}
return z;
}
Satisfied Test: foo(2, 2)
Tuesday, July 9, 13

11. Statement Coverage
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>0)) {
z = x;
}
return z;
}
Unsatisfied Test: foo(2, -1)
Tuesday, July 9, 13

12. Decision Coverage
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>0)) {
z = x;
}
return z;
}
T/F
Satisfied Test: foo(2, 2), foo(-1, 2)
Tuesday, July 9, 13

13. Decision Coverage
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>0)) {
z = x;
}
return z;
}
T/F
Unsatisfied Test: foo(2, 2)
Tuesday, July 9, 13

14. Condition Coverage
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>0)) {
z = x;
}
return z;
}
T/F
Satisfied Test: foo(2, 2), foo(2, -1), foo(-1, -1)
T/F
Tuesday, July 9, 13

15. Condition Coverage
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>0)) {
z = x;
}
return z;
}
T/F
Unsatisfied Test: foo(2, 2), foo(-1, 2)
T/F
Tuesday, July 9, 13

16. Classical Test Coverage
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17. Classical Test Coverage
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18. Expectation of Automated Test
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19. Expectation of Automated Test
Any Possible “Bad” Code
Change can be Detected
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20. Mutation Test Coverage
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21. Why Mutation Test
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22. Redundant Code?
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>0)) {
z = x;
}
sideEffect(z);
return z;
}
Tuesday, July 9, 13

23. Redundant Code?
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>0)) {
z = x;
}
sideEffect(z);
return z;
}
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>0)) {
z = x;
}
sideEffect(z);
return z;
}
Tuesday, July 9, 13

24. Redundant Code?
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>0)) {
z = x;
}
sideEffect(z);
return z;
}
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>0)) {
z = x;
}
sideEffect(z);
return z;
}
n Missing test to prove the sideEffect call is needed
n If code is redundant, it can’t be learnt via any
classical test coverage
Tuesday, July 9, 13

25. Conditional Boundary Mutator
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>0)) {
z = x;
}
return z;
}
Condition Coverage Satisfied Test:
assertEquals(2, foo(2, 2))
assertEquals(0, foo(2, -1))
assertEquals(0, foo(-1, 2))
Tuesday, July 9, 13

26. Conditional Boundary Mutator
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>0)) {
z = x;
}
return z;
}
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>=0)) {
z = x;
}
return z;
}
Condition Coverage Satisfied Test:
assertEquals(2, foo(2, 2))
assertEquals(0, foo(2, -1))
assertEquals(0, foo(-1, 2))
Tuesday, July 9, 13

27. Conditional Boundary Mutator
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>0)) {
z = x;
}
return z;
}
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>=0)) {
z = x;
}
return z;
}
Condition Coverage Satisfied Test:
assertEquals(2, foo(2, 2))
assertEquals(0, foo(2, -1))
assertEquals(0, foo(-1, 2))
Test to cover this Mutator:
assertEquals(2, foo(2, 2))
assertEquals(0, foo(2, 0))
assertEquals(0, foo(-1, 2))
Tuesday, July 9, 13

28. Conditional Boundary Mutator
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>0)) {
z = x;
}
return z;
}
int foo (int x, int y)
{
int z = 0;
if ((x>0) && (y>=0)) {
z = x;
}
return z;
}
Condition Coverage Satisfied Test:
assertEquals(2, foo(2, 2))
assertEquals(0, foo(2, -1))
assertEquals(0, foo(-1, 2))
n Test not prevent “bad” code change to happen
Test to cover this Mutator:
assertEquals(2, foo(2, 2))
assertEquals(0, foo(2, 0))
assertEquals(0, foo(-1, 2))
Tuesday, July 9, 13

29. Other Mutators
n Negate Conditionals Mutator
n Math Mutator
n Increments Mutator
n Invert Negatives Mutator
n Inline Constant Mutator
n Return Values Mutator
n Void Method Calls Mutator
n Non Void Method Calls Mutator
n Constructor Calls Mutator
Tuesday, July 9, 13

30. Test Driven Development
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31. TDD and Test Coverage
n You can most likely ignore Classical Test Coverage
n You can’t ignore Mutation Test Coverage
n If not TDD, then you can learn a lot from both
Classical and Mutation Test Coverage
Tuesday, July 9, 13

32. PokerHands Kata
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33. True Mutation
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34. Problematic Test
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35. False Mutation
Tuesday, July 9, 13

36. Reference
n Martin Fowler’s blog http://martinfowler.com/bliki/
TestCoverage.html
n Mutation Test Wikipedia http://en.wikipedia.org/
wiki/Mutation_testing
n Mutation Test Analysis Report http://
crestweb.cs.ucl.ac.uk/resources/
mutation_testing_repository/TR-09-06.pdf
Tuesday, July 9, 13

37. Q & A
新浪微博：姚若舟
微信：yaoruozhou
土豆（Kata视频）：姚若舟
Github: JosephYao
Tuesday, July 9, 13