Mutation testing helps improve test quality by automatically generating small source code changes, called mutations, and determining whether tests can detect, or kill, those mutations. While unit tests may achieve high code coverage, mutation testing helps identify weaknesses by ensuring tests fail when source code is changed. It works by finding mutations in code, like changing conditionals from AND to OR, covering affected cases to kill mutants, and calculating a mutation score based on the number of mutants killed over the total. Mutation testing tools can help automate the process of finding and running mutants on a package or class without running all tests on the full codebase each time.

1. Mutation Testing
THE WAY TO IMPROVE YOUR UNIT TESTS QUALITY
"Quis custodiet ipsos custodes?“
["Who will guard the guards?"]

2. Why do we need unit tests?
Ensure quality of code
Ensure safety of code changes

3. How good are our unit tests?
Coverage – 100% ?

4. Coverage
public class Positive {
public boolean isPositive(int number) {
return number > 0;
}
public boolean isNegative(int number){
return number < 0;
}
}
@Test
public void testPositive()
throws Exception {
assertTrue(Positive.isPositive( 2 ));
}
@Test
public void testNegative()
throws Exception {
assertTrue(Positive.isNegative( -2 ));
}

5. What if…
public class Positive {
public boolean isPositive(int number) {
return number >= 0;
}
public boolean isNegative(int number) {
return number <= 0;
}
}
@Test
public void testPositive()
throws Exception {
assertTrue(Positive.isPositive( 2 ));
}
@Test
public void testNegative()
throws Exception {
assertTrue(Positive.isNegative( -2 ));
}

6. 100% coverage !
public class Positive {
public boolean isPositive(int number) {
return number >= 0;
}
public boolean isNegative(int number) {
return number <= 0;
}
}
@Test
public void testPositive()
throws Exception {
assertTrue(Positive.isPositive( 2 ));
}
@Test
public void testNegative()
throws Exception {
assertTrue(Positive.isNegative( -2 ));
}

7. Changing source code should
change test results!
Tests should fail if source code
changes
Changes of source code are
known as … MUTATIONS!

8. What kind of mutations?
Conditionals ( AND -> OR)
Boundary ( < -> <= )
Void method calls
Increments ( i++, i--)
Negative ( -1*x)
etc

9. How to kill mutants?
Find mutations
Cover affected cases
mutation score =
number of mutants killed /
total number of mutants

10. How to find mutants?
Use tools
(e.g. Pitest – www.pitest.org )
Configure
(Maven, Ant, Gradle, Idea plugins)
No need to run every time for whole
codebase
Per package, per class
For altered code

11. PROs & CONs
Critical code
Find weaknesses in
test suites
TDD
Build time
Development efforts
Not suitable for
integration or blackbox
testing

12. Good luck!