Getting ready

In the following code, our system under test is a class that calculates a mean value of tax factors retrieved through a web service:

public class MeanTaxFactorCalculator {

    private final TaxService taxService;

    public MeanTaxFactorCalculator(TaxService taxService) {
        this.taxService = taxService;
    }

    public double calculateMeanTaxFactorFor(Person person) {
        double currentTaxFactor = taxService.getCurrentTaxFactorFor(person);
        double anotherTaxFactor = taxService.getCurrentTaxFactorFor(person);
        return (currentTaxFactor + anotherTaxFactor) / 2;
    }

}

Let's now write a test for the system that will check whether it can properly calculate the mean value of the tax factor. We have to create a stub of TaxService and stub its behavior (we don't want it to send any real requests).

How to do it...

To create a mock of a given class using the Mockito static method, you have to call the static Mockito.mock(Class<T> classToMock) method with the type of class to mock.

The following test is written for JUnit. For the TestNG configuration, refer to Chapter 1, Getting Started with Mockito (I'm using the BDDMockito.given(...) and AssertJ's BDDAssertions.then(...) static methods. Refer to Chapter 7, Verifying Behavior with Object Matchers, on how to work with AssertJ or how to do the same with Hamcrest's assertThat(...)).

public class MeanTaxFactorCalculatorTest {

  static final double TAX_FACTOR = 10;
  
  TaxService taxService = mock(TaxService.class);

  MeanTaxFactorCalculator systemUnderTest = new MeanTaxFactorCalculator(taxService);

  @Test
  public void should_calculate_mean_tax_factor() {
    // given
    given(taxService.getCurrentTaxFactorFor(any(Person.class))).willReturn(TAX_FACTOR);

    // when
    double meanTaxFactor = systemUnderTest.calculateMeanTaxFactorFor(new Person());

    // then
    then(meanTaxFactor).isEqualTo(TAX_FACTOR);
  }
  
}

import static org.mockito.Mockito.mock;   
TaxService taxService = mock(TaxService.class);

How it works...

Internally, Mockito calls the overloaded mock method that takes the MockSettings argument and executes it with a default answer set to the RETURNS_DEFAULT value (in other words, returns zeroes, empty collections, null values, and so on.) Next, by means of the MockitoCore class, a custom CGLIB proxy is created and returned to the user.

See also