For this recipe, our system under test will be a TaxTransferer class that will transfer tax for a non-null person. If the passed person value is null, then an error report is sent:

public class TaxTransferer {

    private final TaxService taxService;

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

    public void transferTaxFor(Person person) {
        if (person == null) {
            taxService.sendErrorReport();
            return;
        }
        taxService.transferTaxFor(person);
    }

}

To verify that the only method executed on a mock is the one provided by us, you have to call Mockito.verify(mock, VerificationMode.only()).methodToVerify(...).

Let's check the JUnit test that verifies whether the web service's method has been called at most...