In this recipe, we will stub a method that returns a value so that it returns our desired result.
For this recipe, our system under test will be MeanTaxFactorCalculator, which calls TaxFactorFetcher twice to get a tax factor for the given person and then calculates a mean value for those two results as follows:
public class MeanTaxFactorCalculator {
private final TaxFactorFetcher taxFactorFetcher;
public MeanTaxFactorCalculator(TaxFactorFetcher taxFactorFetcher) {
this.taxFactorFetcher = taxFactorFetcher;
}
public double calculateMeanTaxFactorFor(Person person) {
double taxFactor = taxFactorFetcher.getTaxFactorFor(person);
double anotherTaxFactor = taxFactorFetcher.getTaxFactorFor(person);
return (taxFactor + anotherTaxFactor) / 2;
}
}To stub nonvoid methods so they return a given value, you have to perform the following steps:
- For the BDD approach, call
BDDMockito.given(mock.methodToStub()).willReturn(value),or in the standard way, callMockito.when(mock.methodToStub()).thenReturn(value). - Regardless of the chosen approach in the
given(...)orwhen(...)methods, you have to provide the mock's method call, and in thewillReturn(...)orthenReturn(...)methods, you have to provide the desired output. - Remember that the last passed value during the stubbing will be for each stubbed method call. In other words, say that you stub the mock as follows:
given(taxFetcher.getTax()).willReturn(50, 100);
Then, regardless of the number of
taxFetcher.getTax()method executions, you will first return50and then, you will always receive100(until it's stubbed again).
Let's check the JUnit test. Refer to Chapter 1, Getting Started with Mockito, for the TestNG configuration (remember that I'm using BDDMockito.given(...) and AssertJ's BDDAssertions.then(...) static methods. Check out Chapter 7, Verifying Behavior with Object Matchers, for more details on how to work with AssertJ or how to do the same with Hamcrest's assertThat(...)):
@RunWith(MockitoJUnitRunner.class)
public class MeanTaxFactorCalculatorTest {
@Mock TaxFactorFetcher taxFactorFetcher;
@InjectMocks MeanTaxFactorCalculator systemUnderTest;
@Test
public void should_calculate_mean_tax_factor() {
// given
double expectedTaxFactor = 10;
given(taxFactorFetcher.getTaxFactorFor(any(Person.class))).willReturn(expectedTaxFactor);
// when
double meanTaxFactor = systemUnderTest.calculateMeanTaxFactorFor(new Person());
// then
then(meanTaxFactor).isEqualTo(expectedTaxFactor);
}
}What Mockito does internally when you stub methods is that it executes two main actions, validation and answer construction. When you call the given(...) or when(...) methods, the validation takes place for the following situations:
- Stubbing is not complete (when you forget to write
thenReturn(...)orwillReturn(...)) - Argument matchers are misplaced (you can't use them outside of verification or stubbing)
- Stubbing is performed on an object that is not a mock
- Invalid checked exception is being thrown
As for the answer construction phase, it takes place on the execution of the willReturn(...) or thenReturn(...) method calls. Eventually, Mockito constructs the Returns answer with the passed value and then delegates the execution to it.
Mockito allows for providing a series of possible stubbed results either by using the fluent interface API, or by means of varargs. The following snippet shows you how to stub a method and provide subsequent results by means of fluent API using JUnit (see Chapter 1, Getting Started with Mockito, for the TestNG configuration):
@RunWith(MockitoJUnitRunner.class)
public class MeanTaxFactorCalculatorTest {
@Mock TaxFactorFetcher taxFactorFetcher;
@InjectMocks MeanTaxFactorCalculator systemUnderTest;
@Test
public void should_calculate_mean_tax_factor_for_two_different_tax_factors() {
// given
double taxFactor = 10;
double anotherTaxFactor = 20;
double expectedMeanTaxFactor = (taxFactor + anotherTaxFactor) / 2;
given(taxFactorFetcher.getTaxFactorFor(any(Person.class))).willReturn(taxFactor).willReturn(anotherTaxFactor);
// when
double meanTaxFactor = systemUnderTest.calculateMeanTaxFactorFor(new Person());
// then
then(meanTaxFactor).isEqualTo(expectedMeanTaxFactor);
}
}To achieve the same result using varargs, take the following code:
given(taxFactorFetcher.getTaxFactorFor(any(Person.class))).willReturn(taxFactor).willReturn(anotherTaxFactor);
Then, change it to the following code:
given(taxFactorFetcher.getTaxFactorFor(any(Person.class))).willReturn(taxFactor, anotherTaxFactor);
- Refer to the Mockito documentation (especially, check argument matchers and stubbing consecutive calls) at http://docs.mockito.googlecode.com/hg/1.9.5/org/mockito/Mockito.html