01.mutation-testing.md

Mutation Testing

• In java we can use pitest as mutation testing library :

<properties>
    <pitest.version>1.8.0</pitest.version>
    <pitest.junit5.version>0.15</pitest.junit5.version>
</properties>
    
<plugin>
    <groupId>org.pitest</groupId>
    <artifactId>pitest-maven</artifactId>
    <version>${pitest.version}</version>
</plugin>

<dependency>
    <groupId>org.pitest</groupId>
    <artifactId>pitest-junit5-plugin</artifactId>
    <version>${pitest.junit5.version}</version>
    <scope>test</scope>
</dependency>

• After that we can run pitest:report
  • It mutates your code and run mutation coverage analysis
• Check the report inside target/pit-reports

• We can see that we have a line coverage of 95% and a mutation coverage of 88%
  • Let's analyze which mutants survived

MoneyCalculator

• We have an issue with our tests on the MoneyCalculator on the add method :
  • We just assert that the result is not null...

@Test
@DisplayName("5 USD + 10 USD = 15 USD")
void shouldAddInUsd() {
	 // Missing assertion
    assertThat(MoneyCalculator.add(5, USD, 10))
            .isNotNull();
}

The fix

• Let's fix the test by improving our assertion :

@Test
@DisplayName("5 USD + 10 USD = 15 USD")
void shouldAddInUsd() {
    assertThat(MoneyCalculator.add(5, USD, 10))
            .isEqualTo(15);
}

• When implemented, verify your test pass and run the pitest analysis again

Mutants eradicated!

• Congratulations you have killed your first mutant !!!

• Why you should focus on good assertion?
  • Take a look at the Test Desiderata